About Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC

Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is principal of Hutchinson Design Group Ltd., Barrington, Ill., and a member of Roofing’s editorial advisory board.

Ignorance, Lack of Foresight Can Lead to Roof Designs That Defy Common Sense

Photo 1: Really! This is ridiculous. A 24-inch curb should have been specified. Twelve-inch roof curbs in my opinion should be eliminated from manufacturing offerings and 16-inch curbs should become the new norm. Anyone listening here? IMAGES: HUTCHINSON DESIGN GROUP LTD.

As you know, I am not shy about calling out detailing that is not to the standard of care or construction that is not to the standard of workmanship. But this article is set aside for those special instances where those involved showed a great lack of intelligence or common sense. In other words, they were just plain stupid.

We are often requested to perform observation services on roof installations designed (and I use that term loosely) by others. Oh, what one can learn. It is impressive that some of the design firms are actually getting paid for the documents we see.

The following conditions and discussions border on the comical. The truth, though, is that they are sad realities, but at least they can function as a learning tool. Let this be the first step in providing better detailing.

Roof Hatches

After roof drains with the hole in the membrane cut out so small that you wonder if the drain takes any water, the condition the makes me shake my head the most is the roof hatch with a 12-inch curb set on multiple layers of wood blocking. (See Photo 1.) Sometimes I think it’s the architects’ nod to keep the carpenters’ union happy. More often than not, the reason this occurs is because the designer does not calculate the height of insulation (another violation of the standard of care) that would have buried the hatch.

Photo 2: When the carpenters’ union is supported and wood blocking used, it could at least match the size of the curb flange. The roofing contractor had to cut several layers of insulation to fit, resulting in various voids, which, you guessed it, were not filled with spray foam. Corrective work required was not so easy.

To make matters worse, the wood on the interior is exposed and creates the condition that can catch clothing. Wrapping the wood in sheet metal can alleviate this concern, which of course would require forethought and detail. Often the roof ladder ends up short because the height of the insulation is not accounted for, creating another safety condition.

In Photo 2 you can see that the wood blocking is not even the same width at the flange of the hatch, creating a condition in which the insulation may not be tight to the wood and curb. This all could have been eliminated if the designer had known the insulation height and specified a roof hatch with the appropriate curb height.

With the new insulation thermal values required by code, the old 12-inch curb heights, in my opinion, should be eliminated and 16-inch curbs should become the new standard. We specify 16-inch and 18-inch curb heights — with thermally broken curbs — as a matter of course. (See Figure 1.)

While we’re at it, let’s also talk about the roof hatch location. The role of the roof hatch is to provide safe roof access. Why, then, are they so often located at the roof edge? Often it’s so a ladder can be mounted on the exterior wall, but it just doesn’t make sense to step out of a roof hatch just feet — or even inches — from the roof edge. I do not care if there is a ladder up post and a safety rail — it’s not fun walking to a hatch 20 stories up with winds changing direction every few minutes. (See Photo 3.)

Figure 1: A properly detailed roof hatch. Specifying and detailing roof hatches with 16-inch, 18-inch or greater heights is the proper standard of care method. (Sorry, carpenters — no wood blocking needed.) The curb is also thermally broken and has greater insulation than normal. Note the 2-inch-thick rubber pavers that protect the roof insulation.

Let us not forget the protection of the roof cover. Too many roofs are designed and installed without a cover board. The membrane is directly on top of the insulation, which often has a low psi, and after a few years of foot traffic, this condition results in crushed insulation — even if walkway pads are used. (Some walkways are so thin they are almost worthless.) The result is failure of the bond with adhered roof systems and fastener penetration through the membrane with mechanically attached systems. Nothing like designing in failure. We, by the way, specify 2-inch-thick rubber pavers cut to fit.

Level Copings

Photo 3: Gee, let’s make it a bit more unsafe and turn the roof hatch around. Locating roof hatches along or near roof edges is just plain stupid.

Walking historic fortifications, castles, or fortress sites, did you ever wonder why the top of the walls, parapets, crenellations all have sloping stones? Empirical experience taught the builders that water running down the exterior walls was not good, so they sloped the battlements toward the interior. So, why do so many designers have flat copings and/or gravel stop edges? Several local schools are exhibiting the effects of water runoff from the level copings, including staining, moss, brick faces spalling off the brick, and mortar deterioration. (See Photos 4A and 4B.) Sloping copings back toward the interior helps move water and snow to the roof, preventing it from saturating brick and other façade materials. (See Photo 5.) This is a standard of care issue for designers.

Additionally, the coping is often installed on partially covered wood and most certainly underlying conditions where the membrane does not lap over the wood blocking onto the façade. This should be, in my opinion, a code requirement, along with sloping the copings back to the interior.

Lack of Coordination With Mechanical and Plumbing

Photo 4A: Taxpayer dollars at work: Flat copings above the walls on this school allow water to run down and saturate the masonry, resulting in severe deterioration. At the time of captioning this photo, the Chicago area has 18 inches of snow and portions of this wall are coated in ice.

Several years ago, I presented a paper at a Durability of Materials Conference on the need in new construction for coordination by the roof system designer with the HVAC and plumbing engineers. I argued that the same detail on the roofing sheets should also appear on the MEP sheets — the only difference being that the notes pertaining to the roofing be shown, but in a lower opacity. I am saddened that no one has read that paper. Pipe portals are one of the details that can turn my stomach. (See Photo 6.) They are seldom included on the roof drawings, and even if they are, common problems include penetrations through the roof deck that are never sealed; curbs that are never insulated; portals that are never sealed to the curb cap; and nipples that are never sealed or secured. Of course, the HVAC contractor often tries to place as many wires, pipes, and pieces of conduit as possible through one nipple and then “caulk the hell out of it.” In cold weather locations, condensation on the interior of the curb often occurs and drips into the interior creating — you got it —a “roof leak.”

Roof Drains

Photo 4B: Masonry walls that were expected to have a 40–50-year service life are deteriorating and will require substantial repair.

Selecting the correct roof drain, sump pan, extension — whether fixed or adjustable — and the type of drain dome should be a function of the roof designer and plumbing engineer working together. When you have 4 to 5 inches of insulation at the roof drain, an extension is required. The sump pan should be provided by and installed by the plumbing contractor, which results in the drain being flush with the top of the roof deck — not dropped 1.5 inches, as it is when the steel contractor installs the sump pan which is 40 years out of date. A reversible collar, an extension ring, and a cast iron dome should be specified and detailed. But there is often no coordination between the roof system designer and the plumbing engineer. The plumber shows up with just a drain, and the roofing contractor says, “I can’t sump down 4 inches.” The solution is to then set the roof drain up off the roof deck — really? More insidious is when the roof drain is actually designed to be up off the deck. This is absurd. (See Figure 2.) Have you ever tried to re-roof a building, installing a vapor retarder, and find the roof drains set up off the deck? (See Photo 7.) Not cool when it rains and the drains are high — just stupid.

Roof Access Door Sills

Photo 5: As can be seen on this photo, when the coping is properly sloped to the interior the snow, frost melt and rainwater slope onto the roof system and not onto the exterior wall.

Whether the access is for the roof, a rooftop deck, or roof patio, designers love to keep those sills as low as possible. (See Photo 8.) Then they do not detail appropriate transitions into the roof — no sill and jamb end dam waterproofing, no protective sill pans. I have one client in which the sliding glass doors on the second floor are over the first-floor interior, and the third-floor doors are over the second floor interior, and so on. This terraced design eases the height, but the leaks and the doors wreak havoc on the spaces below. Correcting the condition is not a cheap endeavor when you have to remove 12-foot sliders and transoms to install the correct waterproofing.

Is Stupid Taught?

Photo 6: I hate this type of detailing often specified by the mechanical engineer who has no clue. The curbs and/or spun aluminum portals are not insulated, the penetrations are never properly sealed, and more than one penetration is often passed through one nipple.

The conditions above are observed every week. Why do they occur? Most of the designers have little clue as to how to properly detail. There is little or no discussion in most universities on the building envelope and certainly on detailing. Thus, the graduate is forced to learn in the office from someone, and the person teaching them might have little knowledge themselves. I would have to say most architects don’t see the glory in roof system design. Way too many designs reference manufacturer details, which is a guide for detailing and a market-driven minimum to compete with other manufacturers doing the same. Contractors bid what’s on the drawing: “If it’s not shown, you’re not entitled to get it.” Many of the situations described in this article do not immediately create a concern, but the problems are latent. When the extent of the problem becomes clear, everyone has forgotten who installed the work and they call me to fix the problem. No learning takes place, and this is the true tragedy.

Figure 2: The wrong way to do it. Even today, some designers are showing the roof drains held up off the roof deck. The roofing crews, contractors, and plumbers who deal with these on roof replacement projects despise this type of detailing.
Photo 7: Roof drains set up off the deck make tying in the vapor retarder very difficult, and this is a nightmare for contractors during a re-roofing application. If a temporary roof/vapor retarder is installed and left without new roofing so that large areas of a roof can be removed, the drains are high and ponding water is a concern.
Photo 8: Having door interior floors (especially wood), doorsills and balcony or patio decks in relatively the same plane is asking for water entry. Most designers do not properly design the sill, which requires waterproofing the rough opening; sill pan with soldered end dams; and door frames and sill with proper weeping.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Designing Resilient Single-Ply Membrane Roof Systems for Hot Climates

Photo 1: The Temple of Karnak, Luxor, Egypt: The ancients learned by experience that shade in association with ventilation provided comfort.

The growing popularity of increased thermal insulation, in association with code and standard mandates, assists in mitigating exterior ambient temperatures and heat flow migration influences on building interior environments. Some have tried to mitigate these exterior influences on the interior by roof surface color alone — an incorrect precept. Roof color alone, an attribute of a single roof system component, cannot mitigate exterior influence in and by itself. Insulation, roof system design, roof deck, etc., all have a role to play.

To make matters worse, HVAC designers have not been informed as to how roof system design can detrimentally affect HVAC performance. Increased air temperatures above the roof surface, high-temperature heating of rooftop piping, and the heating of rooftop units by reflection of the roof surface have all resulted in HVAC performance well below that for which it was designed.

Photo 2. The Greeks learned to use mass to mitigate high levels of solar radiation and resultant heat flow to maintain interior comfort. Shown here is the architecture on the Greek Island of Santorini.

The roof system is made up of various components, which can include some or all of the following: roof deck; substrate board; vapor and/or air barrier; thermal insulation layers; the insulation adhesive or mechanical fasteners; spray foam insulation sealer; cover board; cover board adhesive or mechanical fasteners; roof membrane; membrane adhesive or mechanical fasteners; and roof cover of ballast or coating. Thus, the function of the roof is not a single component effect, but the sum of the whole — all components working together in association with building type, interior use, and location.

Appropriate roof system design is the result of the architect, engineer and building owner working together, taking into consideration the function of the building and the effects of the climatic and environmental conditions expected to be experienced.

This article explains the effects of roof system design on HVAC design in hot climates from the perspective of a roof system designer. It is based on a paper I delivered at the 2014 ASHRAE International Conference on Energy & Indoor Environment for Hot Climates in Doha, Qatar. Its lessons are even more relevant today, with the increase in ambient temperatures worldwide. Concerns such as heat flow, reflected ultraviolet light effects, rooftop temperatures and their potential detrimental effects on HVAC performance will be reviewed. Design recommendations and detailing suggestions for achieving long-term roof service life performance in hot climates with single-ply membranes will be explored. Proactive design recommendations for HVAC designers on how to deal with roof-borne effects will also be provided.

Environmental Concerns Lead to Changes

Societal concerns for the environment, which led to the development of the Leadership in Energy and Environmental Design (LEED) program under the auspices of the United States Green Building Council (USGBC) promoted the use of “cool roofs” — now referred to as “reflective roofs” — as both a potential energy conservation and urban heat island reduction methodology. This movement led to legislative and code mandates that became drivers for massive changes in the roofing industry. Consequently, the use of reflecive roof membranes, which are defined by the U.S. Environmental Protection Agency’s Energy Star program as roof covers with an initial solar reflectance of 0.65 or greater, have become the code-mandated choice that architects have when designing low-slope roof systems. The specifying of reflective roof membranes — albeit with little forethought in their use and implementation into a roof system — resulted in unintended consequences, such as the formation of moisture below the membrane, excessive heat production to rooftop equipment and building components, and premature failure of some roof systems.

(FIGURE 1A) Figures 1A and 1B: A vented and ballasted roof system not only shades the roof cover, but provides a ventilation layer allowing warm air to rise and dissipate from the roof, thus reducing heat gain to the interior.
(FIGURE 1B)

The goal of cool roofing has moved over the past years from a potential energy-saving roof cover to an urban heat island mitigator. The challenge for the building design community is to realize that if energy savings is the goal, ballasted roofs are the best choice, as research shows that cool roofs actually raise the ambient temperature above the roof surface. Additionally, reflected UV rays are heating rooftop piping. Clearly in hot and sunny climates, reflective roofs are not in the best interest of the HVAC system performance.

As with any roof-cover material, the appropriate design and use of the material is required to achieve long-term success and a truly sustainable roof system. Roof-system design is equal in importance to structural, mechanical, plumbing and electrical design. Therefore, it is imperative that designers who utilize single-ply cool roof systems, especially those which fall within American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Climate Zones 4 through 7 (approximately from the state of Tennessee north), take extra care to achieve a properly functioning and sustainable low-slope roof system. Efforts have recently been made to raise the mandate for reflective roof surfaces to include ASHRAE Climate Zone 4. While this author firmly believes that the selection of a roof system (no matter the climate zone) should be the decision of the architect and owner, raising the mandate to Climate Zone 4 would be imprudent and result in little if any energy savings, with increased potential for roof system failures.

Optimal HVAC Performance in Hot Climates

HVAC cooling equipment design in hot climates often utilizes over-design to compensate for the building’s thermal gain and heat on the roof. Another often overlooked aspect of rooftop equipment is the drop-off in efficiency due to cooling loss in the ductwork and piping; as a result of solar gain, heat and exacerbation from cool roof surfaces that reflect rays back up at the piping can “superheat” the pipe/duct contents.

If the roof cover temperature can be reduced, and the roof’s effects on ducts and pipes can be reduced, the efficiency of roof op equipment will rise, units can be reduced in size, and operating costs will be reduced.

Roof System Requirements

Roof system design should take into consideration the climate and micro-climates in which the roofs are to be located. This is often not the case, with architects simply selecting a roof system by its warranty length and how many LEED credits it can procure. This lack of design methodology has kept many a forensic roof consultant busy, owners frustrated, and manufacturers unsettled, as failures are frequent and mitigation costly.

IMAGE: HUTCHINSON DESIGN GROUP LTD.

The need for climatic considerations is exacerbated when the roof system will be located in geographical areas of extreme weather: high winds, extreme cold, and extreme heat. For the purposes of this paper the climatic parameters to be considered are:

  • Extreme heat
  • Intense ultraviolet radiation
  • Sand erosion

Thus, to be successful, the roof cover (membrane) must resist these forces for the term of the desired service life. This author believes in designing with long-term service life in mind. Long-term service life is the essence of sustainability, and in this author’s opinion is a minimum of 30 years.

Heat aging and deterioration of roof membranes from ultraviolet radiation has been the bane of roof covers for decades. Premature end of service life as a result of these effects has been well documented by professionals, studied by researchers, and experienced by building owners.

The effects of windblown sand across, or accumulation upon, roof membrane is less understood, but as a rough-surfaced material moving across a pliable membrane it is intuitive that this action could be egregious to the long-term performance of the roof membrane.

Consequently, to achieve long-term performance in hot climates, the roof membrane, in addition to meeting all the needs of the building and roof system, must have a history of resisting long periods of high ambient temperatures, and high surface temperatures, and be resistant to the effects of ultraviolet radiation.

Lessons From History

Learning from historical examples from indigenous peoples who had to deal with the climate with fewer tools than are available today is both prudent and wise. Cultures in the Middle East have dealt with extreme heat is several ways. The first is through shade. While exposed to the sun, hot and arid ambient climates are almost unbearable. Indigenous people first protected their skin with “galabeyas,” a traditional garment. For structures, shade became a key design element. This can be observed in many of the ancient Egyptian structures that have been uncovered and are viewable today. The Temple of Karnak along the Nile in Luxor is one fine example. (See Photo 1.)

Photo 4: At Queen Alia International Airport in Jordan, architect Norman Foster utilized ventilation cavities below the metal roof systems to vent out any possible build-up of heat. Photo credit: Markus Mainka – stock.adobe.com

The Temple of Karnak also provides us with a second example of a method used as protection from the heat and sun, which is to cool via ventilation. The tall columns of the various halls provided needed structure, but also induced air movement. This concept was integral in the design of the Jeddah Airport in Saudi Arabia.

Wall and roof construction across the Mediterranean, not only in the European cultures, but also in the Asia Minor, Middle Eastern, and Northern African cultures, utilized thick, massive walls that could absorb the heat of the day and prevent it from moving to the interior — a cave above ground, if you will. (See Photo 2.)

Thus, we learn from history that the following were important design features in providing comfort in extremely hot and arid climates:

1. Shade

2. Ventilation

3. Mass

In translating the historical precedents in regard to roofing to today’s building needs and roof systems, the issue of shading needs to be given more consideration. In the United States, the current roof systems that offer shading are ballasted assemblies with river-washed gravel of approximately 1.5 inches in diameter (3.8 cm). Spread at a minimum of 10 pounds (4 kg) per square foot(30.5 cm2), the stone creates a shading layer over the roof membrane below. The stone ballast also creates a mass element that can absorb the sun’s energy. While the stones lying next to each other create voids and spaces, the ventilation element is small, but present. In order to achieve the ventilation element, a drainage mat (used in garden roof systems) is placed above the roof membrane and below the stone.

To complete the roof system, a roof membrane with a historical in situ record of exposure performance and resistance to UV is needed. EPDM satisfies this requirement with its carbon black component, as well as its proven performance, given this author’s experience with EPDM roofs designed 30 years ago which are still in service today. Thermal insulation layers should be multiple, and in the range of 3 inches (7. 5 cm) each.

This roof assembly can be seen in Photo 4 and is detailed in Figures 1A and 1B. It typically includes the following:

· Ballast to shade the membrane from solar heat gain and prevent reflection back at the walls and mechanical equipment. The aim is to provide a mass to gather the solar energy and not allow it to dissipate to the building interior, rooftop equipment, and/or the atmosphere.

· Drainage mat to provide a ventilation layer.

· EPDM to provide resistance to heat and UV radiation, and to provide a break in potential heat flow.

· Thermal insulation to “keep the hot out, and keep the cold in.”

There are several goals to this system, including:

1. Shade the roof membrane and thus provide a cooling layer.

2. Provide protection from the deleterious effects of heat and UV radiation.

3. Provide a ventilation plan to dissipate heat.

4. Eliminate the reflection on rooftop equipment.

5. Reduce cooling loads.

6. Provide a rooftop environment that will allow for the downsizing of rooftop equipment, and thus increase efficiency and lower energy usage.

7. Achieve a sustainable long-term roof system.

A roof system of similar concept was recently installed at the Queen Alia International Airport in Amman, Jordan, in which metal roof panels were elevated off the roof deck to form a cavity to vent any possible heat build-up. (See Photo 4.)

Design Recommendations

The goal of architects/designers should be to design roof systems to achieve sustainable and resilient long-term service lives. Today’s society is asking that roof systems provide more than just protection from the exterior environment. For extreme climatic areas of the world, the standard of care required to be exercised by the design professional has increased. For dry and hot climates interior comfort is paramount, and the roof system can be designed to assist rooftop HVAC systems in regard to performance, energy conservation, and efficiency, as well as extending the roof system service life.

Many of the required roof system design parameters apply, but for hot climates there are several key design elements that should be given consideration. Following are the design considerations that will provide a greater opportunity for successful roof systems in hot and dry climates:

1. Ensure collaboration and coordination with the HVAC system designer. The association between potential heat flow, resultant interior heat gain and cooling demand is so closely related that it would appear obvious that the coordination of the two building system designers should be a given. Unfortunately, this is far from reality.

2. Gain an understanding that heat energy is first and foremost transmitted by solar energy, and protecting the roof membrane’s surface from the “sun’s rays” will result in diminished heat gains. Use indigenous concepts to your benefit.

3. Use thermal insulation to provide a formidable barrier between the interior and exterior environments. It is not only about the cost of cooling that should be dictating the amount of insulation, but the loss of cool air and preventing heating. This author feels that the insulation amounts used on roofs of hot climates should be equal to those in cold climates.

4. Shading 1: Protecting the roof surface from direct contact by the solar radiation will provide enormous benefits.

5. Shading 2: The shading element typically will absorb (to the extent the solar radiation is not deflected), thus minimizing and/or absolving the effects of heat flow to the interior.

6. Specify roof membranes (roof covers) that have a history of in situ long-term performance in hot climates.

7. Specify roof membranes (roof covers) that have high resistance to ultraviolet radiation.

8. Specify roof membranes (roof covers) that have high resistance to heat aging.

9. Understand that the high base flashings are part of the roof system and will need to be designed appropriately. The should be protected with double layers of flashings.

10. Specify robust and durable materials: Increase the thickness of roof membranes and covers. If the membrane is reinforced, the thickness of material protection above the scrim is the critical dimension.

11. Design roof system components with the same care for the effects of the sun, solar radiation, and heat as you would the roof. For example, the use of no-hub couplings on roof drains will see the sun for several hours each day and will deteriorate over time, and will become attributable to one of those “hidden, mystery” leaks.

12. Use the historically proven method of heat disbursement: Provide a ventilation layer above the roof membrane (roof cover).

13. Design to protect rooftop HVAC equipment and walls from deflected solar radiation. Remember how you started a fire as a kid with a magnifying glass? This is the same concept.

14. When using metal components such as roof edge copings, realize that the temperature of the metal during daylight periods will work to heat age and deteriorate the roofing below. Try to incorporate a ventilation layer below the metal.

The Importance of the Roof

The design of roof systems has historically been given little forethought, and was often regulated to junior designers with little or no empirical experience, and armed with little more than a “canned” master specification that provided little more than a market-driven minimum of a roof system. Today’s buildings are much too expensive and sophisticated to allow poorly conceived and designed roof systems to prevail. With an increase in detrimental “climactic events,” roof systems demand the same level of consideration and design as do all other building systems: structural, mechanical, plumbing, communications, and building envelopes.

Hot climates are special and unique climatic environments, and as such, have special environmental conditions that need to be designed for. Using empirical and historical information, proven materials, and designing to particular in situ environmental conditions can produce roof systems that will reach sustainable levels of performance. With proper coordination with HVAC designers, the roof can rise above just a protection layer, and provide both raised interior comfort and greater HVAC cooling efficiencies. Greater emphasis on education on proper, innovative, and sustainable roof system design can be achieved if all stakeholders (manufacturers, contractors, architects, engineers and consultants) work together.

It is well past the time to move roofing system design to the forefront of building design and have it become a system that is appreciated for its crucial role in energy conservation and resilient construction.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Detailing for Resilience, Part 3

The Resilient Parapet Roof Edge

Photo 1. Installing roof edge blocking over existing roofing and the unknown conditions below comes with risk. Note the wood blocking and sheet metal lifted off the roof. Nothing you see here is good practice. Images: Hutchinson Design Group Ltd.

Experts cite — and codes and standards reflect — that most roof damage done by winds starts at the roof edge. From coping blow-offs, which often take the wood blocking on top the parapet wall with it, to removal of the membrane and more, evidence shows the roof edge is a key element in attaining a high-performance, resilient roof edge. (See Photo 1.)

When I was president of RCI (now IIBEC), Reid Ribble was the president of NRCA (he is now its CEO). He and I felt that the parapet was the way to go in regard to providing safety on the roof for roofing crews, HVAC crews, and maintenance crews alike. Great idea, right? You won’t believe the fight we received, and you wouldn’t believe the greatest argument came from the firefighters who might have to drop over a 42-inch parapet. When was the last time you saw a fire where the firefighters accessed the roof? I only see them pouring water on it from a hose from a lift. But I digress. Despite this argument, more and more parapets are gaining height.

What Makes a Parapet Resilient?

A parapet whose height deflects some of the straight-line winds is a great start. For our discussion, let’s start by defining what we mean by parapet: roof edge, part of the perimeter exterior wall system that extends above the roof. For this article, we will concern ourselves with those rising to a height of 30 inches or more above the roof, where the roof membrane/base flashing extends up and over the top of the parapet.

Photo 2. It is always a precarious situation when the roof membrane is left flapping in the wind. This structural steel and metal decking with IMP panels is a wind event waiting to happen.

Well, since we are talking roofing, it’s the combination of parapet wall construction and the integration of the roof cover that is crucial. Let’s take a quick look at what the parameters might be. Types of parapet construction include:

· Precast panels

· Brick – concrete masonry units

· Brick on structural metal studs (Egh — I hate this type of construction.)

· Metal panels on structural metal studs (Egh — see Photo 2. Point proven.)

· Anything on structural metal studs (Egh.)

The parapet height places several outward forces on the roofing (base flashing) that are not experienced by lower roof edges, and thus certain enhancements need to be considered:

· Proper substrate. (OK you roofers, how often do you see a wall substrate board specified where membrane will be adhered?)

Sealing the roof base flashing to the exterior wall face prevents wind from moving up behind the membrane, where I have observed coping “popping off.” A specific detail like this should be included in the drawing so that there can be no confusion as to what is required.

· For metal stud walls, heavy gauge metal plate to secure the roof base anchor attachment. (See The Hutchinson Files article “The Stud Wall and the Roof” in the January/February 2019 issue of Roofing.)

· Stoppage of air transport into the parapet construction. (This topic requires its own article on the requirements and detailing.)

· Proper anchorage of any wood blocking on top of the parapet.

Photo 3. A litany of design errors resulted in this roof failure, requiring replacement. I make no secret of my disgust with stud wall parapet construction, as I have seen way too many failures.

· Enhanced membrane anchorage.

· Membrane peel stops.

· Mid-wall securement: wall peel stops.

· Base flashing that extends up and over the parapet and adheres to the exterior cladding. (Under no circumstances should the base flashing be installed loose; it must always be adhered.)

· And for those designers out there, a positive securement of the air barrier to the roof vapor barrier or membrane.

· ANSI-ES1-compliant copings

These enhancements will also protect delaminating base flashing from pulling off the parapet wall and taking with it the roof cover. This condition is the result of poor roof edge design, air transport and condensation.

Detail Drawings

The resilient parapet should incorporate several enhancements and be specifically detailed and specified. Shop drawings and mockups are required.

Below are five examples of the types of enhancements that I suggest would help make the parapet more resilient. Details that I suggest being incorporated into the drawings include:

Photo 4. Four-foot parapet wall on metal studs run up the exterior outside the concrete floor. We were asked to observe the construction and advised to install a wall peel stop over a 16-gauge steel plate, but only this anchor strip was funded. Note the superior anchoring of the anchor strip into the 1/2-inch substrate board and a few studs.

1. Wood Blocking atop the parapet: If wood is incorporated atop the parapet (and it’s nice if you can detail it without it), it needs to be adequately anchored to the building structure. Nails are never used, and drive-in and Tapcon anchors are not sufficient. For masonry walls and pre-cast walls, I suggest expansion anchors. I like them at least at 2 feet on center (O.C.), staggered to prevent warping. Atop structural metal stud walls, first the top plate needs to be secured, the wood installed with self-tapping screws at 1 foot O.C. and then the wood blocking strapped to the studs with heavy-gauge metal —20 gauge or heavier. Joints should be scarfed at 45 degrees and screwed. If a second layer of wood is required, shim it for taper to the interior and secure with coated wood screws at 1 foot O.C. staggered, and stagger the joints. The strapping then would go over both layers.

2. Seal the membrane to the exterior edge: The roof base flashing should be extended up and over the parapet, fully adhered to both the back and top of the parapet, down and onto the face of the exterior wall. It should be adhered and nailed. (See Photo 2 and Figure 1.)

Photo 5. This shot was taken during a punch list inspection. All the base flashing is loose and required removal and replacement.

3. Wall peel stop: Due to a variety of issues (lack of air seal, condensation, inadequate adhesive application, weight of the material, and air pressure, to name a few), the base flashing can delaminate. Delaminated base flashing create a condition that I like to refer to as “pulling tape off the floor.” With each flutter of the membrane, a little bit more is detached until the force is great enough to pull the membrane anchorage out of the plates through the membrane. Thus, a wall peel stop is required: A batten bar installed on the wall and flashed in. The location of the batten bar is dependent on the parapet height and construction; I like to start out with the mid-point and no more than 3 feet from the coping and the base anchor. (See Photos 3-5 and Figure 2 for an example of a wall peel stop.)

4. Enhanced membrane base anchorage: Manufacturers (whose requirements are often market-driven minimums) require a base anchor (for single plies) of 12 inches O.C. Not good enough. We enhance that spacing to 6 inches or 9 inches O.C., depending on the location. Very seldom do we specify 12 inches. (See Figure 2.)

Including a wall peel stop as part of the roof edge design is both prudent and, on metal stud walls of good height, a standard of care design.

5. Roof peel stop: Like the wall peel stop, I suggest a roof peel stop so that in the chance that the roof cover detaches, only a small portion of the perimeter is in jeopardy. This peel stop should be located along the perimeters. Depending on the construction, I like to locate them every 2 feet (half an insulation board). The insulation is stopped at this location and the membrane taken down to the vapor retarder/roof deck, sealed with water block, and anchored with a batten bar and screw fasteners. The remaining insulation is then set, the void spray foamed with insulation, and the new membrane taken over and adhered/welded to the main roof cover. (See Figure 3.)

A roof peel stop is another quality assurance detail that should be included as part of your resilient roof system design.

Included here are several examples of enhanced and resilient parapet roof edge detailing. (See Figures 4 and 5.) Showing the detail in depth or by exploded location will help you as the designer to know what is going on and will communicate to the contractor what is required. While the design of roof edge parapets will more often than not be different from project to project, I hope the concepts here provide the impetus for enhanced detailing.

Parapets that are tall and hollow can be particularly difficult to properly detail, given consideration of air transport, potential condensation, and membrane delamination. Including wall and roof peel stops and enhanced perimeter anchoring will at least allow a failed condition to not result in losing your roof.

It All Starts at the Edge

Resilient roof system design is needed for our clients, whether they know it or not. The survival of the roof system during enhanced climatic events starts at the roof edge, and thoughtful design and detailing of the parapet will help protect the roof — and help you sleep at night.

Noting and detailing the enhancement is required to properly communicate to the contractor what is required on masonry/precast parapets. Noting a second coat of adhesive on such porous surfaces is always a good idea.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

The Resilient Roof Curb

Photo 1: Roof damage after a storm. Thank goodness the conduit is still attached to the RTU so it didn’t blow off the roof. Images: Hutchinson Design Group Ltd.

Resiliency is the buzzword for this decade. Designing resilient roof systems, in my estimation, will become a standard and make its way into the codes by 2030 or before. This is the second in a series of articles based on experience and observations following extreme climatic events on how I have designed resilient roofs and/or how I would suggest various components of the roof be designed for resiliency. In this article we will look at roof exhaust curbs, typically used to support mechanical equipment. The goal is to prevent the units and/or curb from being blown out of place and across the roof. (See Photos 1 and 2.)

What are the qualities that make a resilient roof curb? This is the first question you are now thinking, so I will tell you. Resilient roof curbs should:

  • Be tall enough to be at least 4 inches above the top of the highest point of overflow drainage.
  • Be of solid and robust construction.
  • Be anchored to the roof structure.
  • Secure the unit to the curb.

There you go, go to it.

For those of you who wish a little more information, let explain.

Appropriate Height

The reason for the height is based on experience. The best way to explain this is by example. A client remained in the building during Hurricane Maria. During the storm, she opened the roof hatch and took a photo of the roof, which she sent to me. Upon viewing the photo, I thought it was the ocean. There was water as far as I could see, and there were waves and whitecaps. The drains and small roof edge scuppers had clogged with palm fronds and other debris. The water was over 10 inches in depth. Seeing that visual, I couldn’t believe the roof structure didn’t collapse. (The building was designed for Class 5 hurricanes and was very robust.) Perhaps it would have collapsed had it not been for the low roof curb height and the fact that all the curbs acted as drains once the water gained enough height. The water damaged high-value products in the building’s interior.

Photo 2: Units that leave the curb not only allow water into the interior, but units cartwheeling across the roof will damage the roof with every corner.

The scuppers should have been much larger to prevent blockage, but if the curbs had been higher than the roof edge, the millions of dollars of destroyed goods could have been saved.

Note: With so much damage to surrounding buildings, there is some thought that the water depth on this particular roof provided ballast weight to the roof and prevented wind-related roof damage from occurring. Something to ponder as a defensive option to storms with high winds.

Robust Construction

The construction of the curb is important, in that it not only needs to support the equipment on top but also to take the loads imposed on it by wind, water, snow, sliding ice, etc. The curb is recommended to be of 16-gauge metal, of fully welded construction. It should be insulated and have a metal liner of the same gauge as the exterior of the curb. For long curbs, internal reinforcing is recommended. We recently stopped specifying curbs with wood blocking at the top, an apparent holdover from BUR that needed to be nailed off. The advancement in self-tapping screws make deleting this weak link possible.

Anchorage to the Roof Structure

Keeping the curb attached to the building during storms seems like an obvious goal. The height of the equipment on the curb will determine its overturning potential; the taller the unit, the greater the overturning moment. Thus, I suggest that the curb opening be framed in steel (on steel roof structure with steel decks) as designed by the structural engineer. Coordination with other professionals involved with the building’s design is critical. The curb should be bolted to the steel framing and nuts and washers used; I suggest 16 inches on center. If a linear void exists between the steel framing and the steel deck, it should be infilled with solid dimensional lumber and sandwiched when bolted.

Securing the Unit to the Curb

Rooftop equipment blows off curbs all the time, and often part of the units, typically hoods, blow off. Sharp metal objects blowing across the roof possess a threat to the integrity of the roof and those who may be on the roof. When it is carried over the roof edge, it becomes a life safety threat!

To prevent these failures, the units need to be well secured to the curb and often strapped down. This is a major reason for the robust curb. Exhaust fans typically arrive at the construction site with predrilled pilot holes in the side flanges — often only one per side. When the curbs are 2 feet or greater in length, additional pilot holes should be drilled so that the fasteners are approximately 10 inches on center. The screws should be self–tapping stainless steel, 1/4 inch with stainless steel-clad EPDM washers.

In very high wind conditions such as hurricane-prone regions, it might also be prudent to strap the unit with 1/4-inch stainless steel stranded/twisted aircraft control cable and secure to the unit and curb with stainless steel through bolts, lock washers and bolts with the interior threads deformed to prevent harmonic vibration from loosening the nuts. (See Figure 1.)

Figure 1: At a minimum, roof curbs should be bolted to structural steel and the units above strapped to the heavy-gauge metal curb.

It is hoped that in the near future, manufacturers of the curbs will have these additional support items available as an option.

Achieving Resiliency

Roofs are holistic and their surface is the sum of all their parts. Keeping the roof equipment in place during climatic events is needed to prevent the roof’s failure and interior damage. Roof system designers are encouraged to detail roof curbs and unit attachment — and then specify the correct materials and execution.

This is one more step as we build the resilient roof.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Designing Roof Drains to Survive and Perform in Severe Storms

A well-designed and well-installed roof drain should not allow water to pond at the clamping ring and should be secured to the roof deck structure. Images: Hutchinson Design Group LTD.

The storms have become repetitive and the damage to infrastructure, buildings and life safety has reached historic proportions. From these catastrophes has arisen the concept of resiliency. If you haven’t heard of this movement, you’re not in sync with the current governmental building mindset. Sustainability is virtually passé; it was almost 25 years ago my co-chair Keith Roberts, Roberts Consulting, Abingdon, England, and I headed a group of international experts in roofing, under the auspices of CIB on the topic of “Sustainable Low Slope Roofs.” The resulting report included the “Tenets of Sustainable Low Slope Roofing,” and it is still available on the CIB website, www.cibworld.nl.

CIB determined several years ago that sustainability was no longer the crucial goal of the built environment: Why? Because the building industry was so good in educating clients in regards that sustainability is no longer a goal to be discussed but is a client assumption to be provided.

So, what is this resiliency?

In regard to roofing, the essence of resiliency is to design a roof that can weather the storm(s) with minimal damage and be quickly repaired so that the building in question can be operational.

A resilient roof design is not one designed to membrane manufacturers’ minimal standards and installed to current practice. A resilient roof cannot be summed up in a prescriptive specification.

A resilient roof design is:

  • Supported by the client.
  • One designed by a competent person, knowledgeable about the effects storms have on buildings.
  • One in which all the conditions on the roof are specifically detailed to the project. (OMG: Architects, engineers and consultants — you will actually have to understand construction and do what you’re being paid to do.)
  • A team effort involving the owner, designer, contractor and material suppliers.

There has been a great deal written about sustainability, and many of my colleagues are still confused as to what it all means. I don’t want the concept of resiliency to suffer the same fate. Thus, I would like to bring to you my ideas of how resilient detailing may look.

Over the next several articles, I will review how I detail for resilient roof systems in the hope that it may assist your understanding of what resiliency is and how you might design and detail for it.

The Roof Drain

It is amazing how many roof drains are pulled up and out of the roof deck when the membrane becomes loose in a storm. I guess with the drain gone it leaves a nice large drain. The challenge is I have some clients with hundreds of millions of dollars in product or equipment in the building below, where water is not appreciated. So, the first resilient detail I have chosen to explore is theroof drain.

The roof drain detail for new construction requires coordination with the structural engineer who will be specifying the roof deck and structural framing around the drain. Getting the engineer to place it in the low spot is a discussion for another day. This coordination is also required with the plumbing engineer so that the correct drain system and components are specified. Hint: I tell the plumbing engineer what to specify, give them the detail and provide specification information. It’s just so much easier than to try and get them to change it later. (We will discuss the 12-inch roof curb specification in a later article. Can’t the manufacturers just eliminate the 12-inch roof curb?)

Part of the coordination, and maybe the most difficult, is getting the structural engineer not to specify the drain sump that was for level decks with built-up roofs; we haven’t used these in 30 years. The other half of that is the plumbing engineer needs to specify the sump pan as part of the drain system. Now you see why I provide the spec.

Once this is all coordinated and you’ve spent the weekend exhausted and drank to excess, you’re ready to detail — the fun part.

The sump pan provided by the drain manufacturer allows the drain flange to set in the same pan as the top if the roof deck. This sump pan should be screw fastened, anchored to the deck. For steel decks I suggest a pan head self-tapping screw into each flute and 6-inch O.C. parallel to the flutes.

Securing the Drain to the Structure

After the sump pan is set, the roof drain can be set and secured in place. To do that, an underdeck clamp should be used. Typically, the roof drain has threaded receivers to which the under-deck clamp can be bolted and clamped to the sump pan receiver. But in a blow-off you would be relying on those pan head screws into the steel deck to prevent uplift. Sometimes that will be enough, sometimes not. To guarantee that the roof drain stays in place, 1-5/8-inch Unistrut should be extended from steel angle framing to steel angle framing that the structural engineer has designed. The under-deck clamp should then be placed on the underside of the Unistrut and bolted to the drain. You now have the roof drain compressed to the steel roof deck and the building’s structure. (See Figure 1.)

Figure 1. The goal of a resilient roof drain is to prevent water damage in high-wind events. While an underdeck clamp should always be specified, to enhance the roof drain securement steel braces should be installed from deck framing angle to deck framing angle and the clamping ring placed below that to clamp the roof drain down and prevent disengagement with the drain pipe.

Roof-to-Drain Detailing

With assurance that the roof drain will remain in place during a major storm event, the roofing can now be detailed. The vapor retarder, which will act as a temporary roof in the event of a roof blow-off, needs to be specifically detailed to extend over the roof drain flange and then be secured in place with the reversible collar that will hold the extension ring. The vapor retarder should be adhered to the drain flange prior to the installation of the reversible collar which is bolted to the roof drain bowl.

With increased insulation values, an extension ring is required; this is basically 5 inches in Chicago. I highly recommend that a threaded extension ring be used: it offers easy adjustment and positive engagement with the revisable collar. The insulation should be cut and brought into the roof drain. All voids should be filled with spray foam insulation and trimmed flush. We specify a fire seal on the underside as well.

The roofing membrane should be set over the roof extension drain flange in a full tube of water block and the clamping ring set and bolted to the roof drain. The membrane should right then and there be trimmed back to within 1/2 inch of the clamping ring. Don’t wait to do this later or cut a hole the exact diameter of the drainpipe. Failure to trim the membrane back to within 1/2 inch of the clamping ring prevents that drain from properly functioning, and in a roof collapse situation, I will be hunting you down.

Coping With Severe Damage

Many buildings sustain wind damage associated with heavy rains. When the membrane is blown off the substrate and the drain is high, buildings can experience high levels of water damage. As shown in Figure 1 and as typically is installed, the roof drain is above the roof deck surface. Thus, for resilient roof systems, a roof drain should be installed at the level of the vapor retarder to drain the roof should the membrane be compromised. (See Figure 2.)

This is accomplished by inserting a baffle in the drain downspout that will prevent air vapor from moving into the roof system or water backup. The drain is hidden, ready for use in an emergency.

You’re now started on your resilient roof system design.

Author’s note: Thanks to John Ryan of DeFranco Plumbing in Palatine, Illinois, who has shared his decades of knowledge with me to assist in the detailing of roof drains systems.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Case Study Reveals Key Lessons in Roof Design

Photo 1. In order to manufacture materials inside the facility, humidity had to be added to the space in the form of hanging moisture dispensaries. This increased the relative humidity to 90 percent with interior temperatures reaching 90 degrees. Images: Hutchinson Design Group Ltd.

The client said, “The roof leaks in the dead of winter.” Interesting when the exterior ambient temperature is below zero. The client’s firm had purchased the metal building several years earlier and water had come in every winter. A key piece of evidence was that the original building was used for storage. The new entity purchased the building for manufacturing. Not just manufacturing, but manufacturing of medical-grade textiles that require the use of humidity to reduce static electricity. Not just humidity, but 90 percent relative humidity (RH), where visible water is sprayed into the air. (See Photo 1.) Interior temperatures routinely reached 90 degrees. Now, let’s see: 90 degrees with 90 percent RH inside, zero degrees outside, and 6 inches of vinyl face batt insulation compressed at the purlins with aged, open lap seams. Not good. The mission, if I chose to accept it, was to eliminate the leaking on a roof that was watertight.

Proposed Solutions

During my first meeting with the client, I was provided with proposals from roofing contractors and, sad to say, several roof consultants. Proposed solutions ranged from coating the metal, to flute filler and cover board with a mechanically attached thermoplastic membrane, to flute filler, 2 inches of insulation and adhered thermoplastic. None of the proposals identified the interior’s relative humidity and heat as a concern, and thus these issues were not addressed. So, a good part of the morning was spent educating the client as to why none of the proposed solutions would work. Imagine spending big bucks on a roof solution that would have only exacerbated that situation.

Photo 2. Rising humidity passed breaches in the vinyl vapor barrier in the insulation and condensed on the underside of the metal roof panels. The water would then drip down on to the insulation, and eventually the vapor retarder seams would open and water would pour in.

The existing building was a metal building by Kirby (similar to a Butler Building if that helps). The roof was a trapezoidal seam metal roof panel 24 inches wide on a low slope to an offset ridge. The panel runs from ridge to eave were 200 feet to the north and 100 feet to the south. The roof drained to a gutter on the north and lower metal roof on the south. The east and west roof edges were standard metal building rake metal. The walls were vertical metal siding with exposed screws. The roof and metal wall panels were set over vinyl faced insulation draped over purlins. While there was exhausting of the interior air — typically used in the summer — and some provisions for adding exterior air in the winter and summer, there was no overall mechanical control of the interior.

The Key Issue

While a freezer building has extreme energy low trying with every ounce of its being to pull hot humid air in, this structure has extreme energy high trying to “get out.” The warm, humid air is seeking every crack, split in the insulation facer, and open lap seam to move toward equalization. This warm, humid air was making its way to the underside of the metal roof panels, condensing, and running down the underside of the panel until it dropped off.

Figure 1. Typical Roof System Section

After time, the accumulation of the water in the batt insulation created a large belly until the adjacent lap seam broke and large amounts of water came cascading down. Soiled product had to be discarded. With rolls 5 feet wide and a 6 feet diameter, the losses could be substantial. Water on the floor was also a safety issue. Additionally, when this damage occurred the insulation layer now had an opening which sucked in even more interior air; the condensation increased and water dripping to the floor was an even bigger problem. This was occurring in numerous conditions, with the greatest accumulation of water in the insulation near the ridge. (See Photo 2.)

Determining the Solution

Prior to delving into a potential solution, a parti, or overall concept of architectural design, had to be developed. In this case I decided that the metal roof would become the vapor retarder for the new roof. Simple enough. If the metal roof is to become the new vapor retarder, the key was to keep it warm enough so that even if it came in contact with the interior air, it would not result in condensation. We did this by determining the dew point for several insulation scenarios and found that with the batt insulation still in place and 90 percent RH with a temperature of 90 degrees inside, with an exterior design temperature of minus 10 degrees, 6 inches of insulation above the 2.5-inch flute filler was required.

Photo 3. For budget reasons, the liquid flashing seal of the trapezoidal seams was eliminated. As the metal roof had to act as the vapor retarder, a self-adhered membrane was installed first over the trapezoidal seam and fit into the articulated seam with “finger” rollers, and then placed over the center of the panels and fit around the panel striations.

All roof system designs need to be thought of holistically, as the success depends on the sum of all the components working together. So, let’s start with the roof panel and structural system. Designers of metal buildings are notorious for minimizing each and every structural member to lower costs. A structural check found that the existing structure was able to handle the weight of a new roof system. Prior to proceeding, a mechanical fastener pull-out test was performed by Pro-Fastening Systems of Buffalo Grove, Illinois, an Olympic Distributor (need a special or standard anchor, these guys have it). The tests showed that the 22-gauge metal panel was able to engage the buttress thread screw.

To be effective, a vapor retarder needs to be airtight — or for you purists, have extremely low or no permeability at all — and this metal roof had to function as a vapor retarder. The steel roof panel itself is impermeable, but the seams, though mechanically locked, have the potential under interior pressure to allow air to pass through. The seams had to be sealed. The mechanical fasteners penetrating the metal roof panel needed to be sealed as well. The roof transitions at the vertical rake walls, gutter and low roof also needed to be sealed. After looking at the standing seams, it was decided that they could not be assumed to be airtight, so we selected to seal them with a liquid flashing. As the mechanical fasteners would penetrate the panel, a bituminous self-adhering and hopefully self-sealing vapor retarder was placed on the panel. (See Figure 1 and Photo 3.) Transverse laps, removing the ridge cap and infilling the opening were all addressed. The rakes presented unique challenges which took some good thinking on how to seal. Ultimately, it was decided that a combination of removing the rake metal and installing a prefabricated roof curb and membrane vapor retarder would do the trick. (See Figure 2.)

Figure 2. Rake Edge Detail

My initial thought was to begin the thermal layer with a layer of expanded polystyrene (EPS) on the deck designed to fit the trapezoidal seam profile. This left a void at the seams, so the void between the seam and EPS was sealed with spray foam insulation. (See Photo 4.) The insulation was then mechanically fastened. The thickness of the EPS was 3/8 of an inch greater in height than the standing seam to compensate for varying seam heights. Over the EPS, one layer of 2.6-inch fiberglass-coated faced, 25 psi polyisocyanurate insulation was designed to be mechanically fastened to the roof panel. The top layer of insulation was a 2.6-inch fiberglass-coated faced, 25 psi polyisocyanurate insulation, which was designed to be set in full spatter cover flexible polyurethane foam adhesive. A cover board with the receiver facer to which the membrane would be attached was designed to be set in a full coverage of splatter applied polyurethane insulation. (See Photo 5.)

Photo 4. The EPS was cut to fit the panel profile. The joint at the seam was sealed with spray foam insulation.

As the installation was to take place in late fall and during the winter, adhesive use was determined to be challenging if not impossible, so the roof cover selected was a 90-mil Carlisle FleeceBACK black EPDM. The fleece on the membrane would engage with a unique hook and loop facer and reduced by 95 percent the amount of adhesive required. (See Photo 6.) Six-inch seam taped end lap seams with self-adhering cover strips were designed, while the butt seams were also double sealed with 6-inch and 12-inch cover strips.

The rake edge was designed to be sealed at the top of the metal panels and raised with an insulated metal curb. (See Photos 7 and 8.) The wall panels’ reverse batten seams and bowed inward panel were designed to be sealed with a foam closure set in sealant. The architectural sheet metal on the rakes was a four-piece system of fascia and coping. The roof edge gutter was enlarged and reinforced to hold up to solid ice.

Construction

Photo 5. Once the EPS “flute filler” was in place, two layers of 2.6-inch coated fiberglass insulation were mechanically fastened into the standing seam panels. The high-density cover board was then installed in spray foam adhesive.

The project was bid out and AR Commercial of Aurora, Illinois, was selected. Work on the project began in October 2018 and was completed in April 2019. (See Photo 9.) Like any project, various miscellaneous items not anticipated arose, such as extreme cold early in the fall that precipitated the decision to mechanically attach the insulation in lieu of cold adhesive application. I also forgot about the residual water in the existing batt insulation. While we designed for 90 percent RH and temperatures of 90 degrees, we didn’t anticipate the 100 percent RH condition where the soaked batt insulation was located, which resulted in condensation occurring during the deep freeze. You’re never too old to learn something new. The batts were cut open, dried and all is good.

Sometimes you need a good roof over your head to keep you dry, even when it doesn’t rain.

Photo 6. The FleeceBACK 90-mil EPDM sheets were aligned, rolled out, broomed in and rolled.
Photo 7. Following the removal of the existing rake metal, the roof vapor retarder was extended down and over the existing construction and onto the metal roof panel. The contractor came up with an innovative two-piece roof edge curb that allowed for ease of installation.
Photo 8. Following the installation of the interior curb side, which was accomplished with a jig for continuous alignment, the curb was insulated and the exterior cap piece installed.
Photo 9. The finished roof prevented condensation from occurring even when the ambient temperature dropped to minus 28 degrees with wind chills near minus 50 degrees.

About the author: Thomas W. Hutchinson, AIA, CSI, Fellow-IIBEC, RRC, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com

The 7 Commandments of Roofing

If I were the Roofing God for a day, what would I change? Oh, where do I start? First of all, there would be none of this “you should,” “can,” “may” or “it is recommended” nomenclature. I would have commands: Thou shall do the following.

Freezer Buildings and Block Ice Insulation

Photos 1 and 2. When moist exterior air is pulled into the roof systems of freezer buildings, the moisture condenses and freezes. Here gaps in the insulation are filled with ice. On the interior there are icicles more than 10 feet long. The cause? Air intrusion at the roof edge under the membrane and wood blocking. Images: Hutchinson Design Group Ltd.

I have never opened up a roof over a freezer building that wasn’t solid ice between the insulation joints. How does this travesty occur? Ignorance? In part. Naiveté? Yes. Who is guilty? Whoever is the roof system designer. Most designers should know that there is an enormous moisture drive from the exterior to the interior. This drive is not a passive movement, but a huge, sucking pressure. It’s like there is a shop vac in the interior trying to pull in outside air. But designers fail to realize that the first sources of interior moisture intrusion into the roof system are moisture migrating out from exposed soil until the concrete slab is poured; moisture coming from the interior concrete floor slab; and latent air moisture (relative humidity) in the interior air before the freezer is operational.

We in the roofing industry are very good at keeping water out of the building. It’s the influx of air that is destroying these roofs shortly after bringing the freezer online. So how is the air getting in? Oh, let me count the ways: (1) though the unsealed membrane at the roof edge; (2) past beveled precast concrete joints at the roof edge; (3) below perimeter wood blocking at the roof edge; and (4) up through metal wall panel joints.

Photo 2.

Stopping air transport to the interior is key. Most designers believe that the roof membrane performs as the air/vapor barrier. In the field of the roof, perhaps, but their lack of knowledge about roof material characteristics and proper installation methods often leads designers astray. The perimeter becomes the weak link.

Let’s look at some common design mistakes:

1. In recent years, designers have revised roof membrane selection to reflective roof membranes, in part to garner a LEED point. The trouble is that these membranes are substantially ridged/stiff and can be difficult to turn over the roof edge, adhere and seal, so they are often barely turned over the edge and nailed off. The lack of a positive seal (that would be achieved by adhering the membrane to the perimeter wood blocking and wall) allows air to move up below the membrane.

2. When precast concrete panels are used at the walls, the joints are often beveled. What happens at the roof edge? The bevel extends right up to the perimeter wood of the coping that is straight and parallel to the outside wall face. The bevel becomes a gutter to channel wind up the wall to the underside of the gutter, gravel stop or coping. In a situation like the one outlined in No. 1 above, the wind can move in below the roof system.

3. When perimeter wood blocking is placed in a horizontal position at the roof edge, the underside of the wood blocking needs to be sealed. A non-curing, gun-grade butyl, applied in several rows, works well, such that when the blocking is secured to the wall, the underside of the blocking is sealed. Be aware of uneven substrates that will require additional sealant.

4. Metal wall panel joints are another potential problem spot. Ask a metal wall panel installer why they are only sealing one of the two exterior male–female joints and you are likely to hear, “because the exterior joint completes the vapor retarder” (which is on the exterior of the building when perfect). Technically they are correct. However, getting a perfect sealant joint to create a complete vapor retarder is not so easy. Think of how sealant is applied. The installer squeezes the caulk gun handle and the sealant oozes out in a thick bead, which can vary in thickness as the gun is drawn along. As the trigger is squeezed and the gun moves, the sealant bead decreases in diameter, and then the gun handle is squeezed again and a thick bead oozes out, and so on. At the end of the sealant application, the thinned-out bead is often not sufficient to properly seal the panels where they are engaged. Condensing water weeps out of the joints in the interior in cold storage areas and results in interior ice on freezer buildings. The sealant, whether factory applied or field applied, is not located at the exterior plane of the panel, but recessed in the outer tongue and groove joint, leaving the potential (almost a guarantee) that there will be a vertical “chimney” of about 1/16 of an inch that can channel air up under the membrane turned over the wall panel.

A quality vapor retarder (those of you thinking polyethylene, think again) placed on the roof deck will protect the thermal layers from vapor intrusion from the interior humidity, latent construction moisture, and ground moisture that accumulates before freezer draws down. It also prevents exterior air infiltration, which can result in interior “snow” and the huge icicle formations. (See Photos 1 and 2.)

Commandment #1: Thou shall place a vapor barrier at the roof deck on freezer/cold storage buildings and seal roof edge perimeters, drains and penetrations through the vapor retarder and all perimeter conditions to be airtight.

The Roof Drain Conspiracy

I am convinced that there is an international conspiracy to drive me nuts. It’s called the ‘how small can we cut out the membrane at the roof drain’ contest. (See Photo 3.)

Photo 3. Believe it or not, this is not even close to the winner of “who can cut the smallest hole in the roof membrane at the drain” contest. The membrane should be cut back to within 1/2 inch of the clamping ring to allow the drain to function as designed.

When I am called in as an expert on a building collapse, the first thing I tell the attorney is, “Save the roof drains and attached roof membrane!” Why, you ask? Because I want to see if the roofing contractor competed in the contest and if the installer and the consultant/architect will be party to the repair costs. Drains are designed to create a vortex to drain water most efficiently from the roof. (Watch how a toilet flushes to gain an understanding on how a drain works with the water swirling into the drainpipe.) The shape of the water flow from the roof surface to the drain bowl to the downspout is critical. When the hole cut in the membrane is too small, it can restrict drainage. Costs often drive projects, and it is not uncommon for a roof’s structural elements to be value engineered down to the bone. With intense rainfalls (you know, the 100-year rains that are occurring two or three times per year) and on larger roof areas where large outlet pipes are used, restricted water drainage can and has resulted in structural roof collapse.

So, I’m on a roof and observe the roofing crew cutting out a small hole at the drain. Being the conscientious consultant that I am, I ask, “Can you please cut out the membrane to within 1/2-inch of the clamping ring?” The answer is almost universal: “I’ll do it later.” Usually my blood pressure rises and face turns red as I explain the importance of making sure this detail is not overlooked.

Our details call out the proper way to cut out the membrane and our field observation reports call this out to be corrected, but I am forced to remind contractors again and again — sometimes even when it’s on the punch list. So, what’s a consultant to do? I reject the pay request.

Commandment #2: Call out on your roof drain details to cut back the membrane to within 1/2-inch of the clamping ring (a cloverleaf pattern around the bolts is best), and drive home the importance of this detail to the crew members in the field.

The 12-Inch Roof Curb

Photo 4. Roof insulation thicknesses now required by code make 12-inch roof curbs obsolete. Specify 18-inch curbs. Raising this curb with 16-gauge steel was very expensive. I suggested sending the bill to the engineer.

When energy was cheap, insulation was an inch or two in thickness, and the roof was built up, 12-inch-high roof curbs worked. With the new insulation requirements and tapered insulation, 12-inch curbs can be buried. Furthermore, future code mandates may increase insulation R-value, increasing insulation heights. So, consider this a public announcement to all mechanical engineers and curb manufacturers: Eliminate 12-inch curbs and specify curbs that are 18 inches or higher. (See Photo 4.)

Commandment #3: Specify only 18-inch and above roof curbs and rails.

Flapping in the Breeze

Photos 5 and 6. The membrane left unsealed at the roof perimeter has placed this roof in great jeopardy of wind damage. It is also allowing water to flow back into the insulation.

Driving around Chicago it’s not hard to see roof edges — gutters, gravel stop, and parapets — where the roof membrane is just flapping in the wind. (See Photos 5 and 6.) This is especially a concern when the roof system is mechanically attached and the air can move directly below the membrane. The roof typically is installed prior to the installation of the windows and doors, and while the building is open, airflow in the interior can create upward pressure on the roof system from below. This force, in association with the air getting below the membrane at the roof edge and with uplift above the membrane, drastically raises the risk of wind damage. Furthermore, when the membrane is not secured at the gutter roof edge, water draining off the roof will return back to the roof edge and move into the building and insulation.

Photo 6.

Wrap the membrane over the roof edge, adhere it in place and nail it off. This will save you during the installation and prevent air infiltration once the roof is complete. The designer should also delineate the area where the air barrier meets the roof vapor retarder and/or roof membrane and define who is responsible for what. Detail this explicitly.

Commandment #4: Roof membranes shall be extended down over the edge wood blocking a minimum of 1.5 inches onto the wall substrate, fully adhered and nailed off on the day it is installed. Where applicable, seal to the wall air barriers.

Holding Roof Drains Off the Roof Deck

Photo 7. Drains held up off the deck make re-roofing difficult when a vapor retarder is called for. I have seen roofs covered with 1.5 inches of water due to high drains, with the water just waiting to relieve itself to the interior at the first vapor retarder deficiency.

Nothing is more frustrating to a roofing contractor during a re-roof than removing the old roof to install a vapor retarder and finding that the roof drain has been held up off the roof deck. (See Photo 7.) This goes back to the design when the engineer and architect have no clue as to the use of proper sump pans and roof drains with extension rings — preferably threaded.

Commandment #5: Design, detail and draw the roof drain detail showing the roof deck with a sump pan provided by the roof drain manufacturer, installed by the plumbing contractor not the guys installing the roof deck), with the roof drain now flush to the roof deck, with a reversible collar (to which the extension ring threads engage), the threaded extension ring and dome.

Fill the Void, Bury the Screw, Save the Energy

Photo 8. Often a roofing contractor will leave voids like this around penetrations. Imagine the energy loss.

With the push over the past decade for energy savings/conservation, it is amazing to me that the code bodies have ignored two very highly energy consumptive or energy loss conditions: (1) voids in the thermal layer at penetrations and perimeter conditions; and (2) mechanical fasteners with plates below the roof cover. (See Photos 8-10.)

Photo 9. This photo shows multiple problems, beginning with a stud wall and a large gap at the deck. Warm air coming up the wall will cause deterioration of the water-based adhesives on the base flashing. The insulation panels are not tight to the wall or to each other. The metal strip looks pretty thin, is not a proper vapor retarder termination and will not hold the screws of the base anchor. This is a project that will continue giving work to us expert witnesses.

Some crews work to fit insulation tight to conditions. Others don’t. Eyeballing the circular cutout at vent pipes is common, resulting in fairly large voids at vent pipes. Roof edge conditions vary and significant voids can occur there, too. All of these voids need to be sealed with spray foam insulation, which should be allowed to rise and then trimmed flush to the insulation. I recommend that the spray foam be installed at each layer as subsequent insulation layers can shift the void. We have been requiring this for years without much blowback from contractors. The only issue that arose was when a contractor wanted to use polyurethane adhesive to fill voids; that was a no-go, as the polyurethane adhesive collapses down after it rises.

Photo 10. The screws and plates seen here are costing the building owner a fortune in lost energy.

Mechanical fasteners used to positively secure the insulation and membrane have become commonplace. But as I’ve noted before, we have seen roofs covered in frost with hundreds, if not thousands, of little spots of melted frost. The heat transfer through the fasteners is substantial. Research has found that on a mechanically attached roof cover, the energy loss can be over 40 percent above that of a system without exposed fasteners. As energy requirements are defined by R-value and with the potential for thermal loss due to the fasteners, I propose an R-value penalty for exposed fasteners. For example, in Chicago where the R-value requirement is 30, if you have a mechanically attached roof cover, the R-value required would be 42. That way the thermal efficiency would be equivalent and building owners wouldn’t pay the price for the designer’s lack of knowledge. Thus, as the Roofing God, I would implement this penalty and require all adhered roofs to have fasteners buried below insulation or cover board layers.

Commandment # 6: Show and note on your details the installation of spray foam insulation at penetrations, roof drains and perimeters.

Commandment # 7: All mechanical fasteners should be covered with insulation or a cover board; if not, 40 percent more R-value needs to be added to the thermal layer to compensate for the energy loss.

So, there you have the new roofing commandments that I would bestow if I were the Roofing God for a day. Let’s all work together though to bring about positive change and increase the sustainability and resiliency of our roofs. Together we can do it.

About the author: Thomas W. Hutchinson, AIA, FRCI, RRC, CRP, CSI, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Designing Thermally Efficient Roof Systems

Photo 1. Designing and installing thermal insulation in two layers with offset and staggered joints prevents vertical heat loss through the insulation butt joints. Images: Hutchinson Design Group Ltd.

“Energy efficiency,” “energy conservation,” and “reduction of energy use” are terms that are often used interchangeably, but do they mean the same thing? Let’s look at some definitions courtesy of Messrs. Merriam and Webster, along with my interpretation and comment:

· Energy efficiency: Preventing the wasteful use of a particular resource. (Funny thing, though — when you type in “energy efficiency” in search engines, you sometimes get the definition for “energy conservation.”

· Energy conservation: The total energy of an isolated system remains constant irrespective of whatever internal changes may take place, with energy disappearing in one form reappearing in another. (Think internal condensation due to air leaking, reducing thermal R-value of the system.)

· Reduction: The action of making a specific item (in this case energy use) smaller or less in amount. (Think cost savings.)

· Conservation: Prevention of the wasteful use of a resource.

So, looking at this article’s title, what does “designing a thermally efficient roof system” imply?

Photo 2. Rigid insulation is often cut short of penetrations, in this case the roof curb. To prevent heat loss around the perimeter of the curb, the void has been sprayed with spray polyurethane foam insulation. Open joints in the insulation have also been filled with spray foam insulation. Note too, the vapor retarder beyond the insulation.

I conducted an informal survey of architects, building managers, roof consultants and building owners in Chicago, and they revealed that the goals of a thermally efficient roof system include:

  • Ensuring energy efficiency, thus preventing the wasteful use of energy.
  • Reducing energy use, thus conserving a resource.
  • Being energy conservative so that outside forces do not reduce the energy-saving capabilities of the roof system.

Unfortunately, I would hazard a guess and say that most new roof systems being designed do not achieve energy conservation.

Why is this important? The past decade has seen the world building committee strive to ensure the energy efficiency of our built environment.

A building’s roof is often the most effective part of the envelope in conserving energy. The roof system, if designed properly, can mitigate energy loss or gain and allow the building’s mechanical systems to function properly for occupant comfort.

Photo 3. Rigid insulation is often not tight to perimeter walls or roof edges. Here the roofing crew is spraying polyurethane foam insulation into the void to seal it from air and heat transfer. Once the foam rises it will be trimmed flush with the surface of the insulation.

Energy conservation is increasingly being viewed as an important performance objective for governmental, educational, commercial and industrial construction. Interest in the conservation of energy is high and is being actively discussed at all levels of the building industry, including federal and local governments; bodies that govern codes and standards; and trade organizations.

As with many systems, it is the details that are the difference between success and failure on the roof. This article will be based on the author’s 35 years of roof system design and in-field empirical experience and will review key design elements in the detailing of energy-conserving roof systems. Best design and detail practices for roofing to achieve energy conservation will be delineated, in-field examples reviewed and details provided.  

Advocacy for Improvement

In the past decade, American codes and standard associations have increased the required thermal values every updating cycle. They have realized the importance of energy conservation and the value of an effective thermal layer at the roof plane. They have done this by prescribing thermal R-values by various climatic zones defined by the American Society of Heating and Air-Conditioning Engineers, now better known by its acronym ASHRAE. Additionally, two layers of insulation with offset joints are now prescribed in the IECC (International Energy Conservation Code). Furthermore, the American Institute of Architects (AIA) has also realized the importance of conserving energy and defined an energy conservation goal called the 2030 Challenge, in which they challenge architects, owners and builders to achieve “zero energy” consuming buildings by 2030.

These codes, standards and laudable goals have gone a long way to improving energy conservation, but they are short on the details that are needed to achieve the vision.

Energy Conservation Is More Than Insulation

Roofs are systems and act as a whole. Thus, a holistic view of the system needs to be undertaken to achieve a greater good. Roof system parameters such as the following need to be considered:

  • Air and/or vapor barriers and their transitions at walls, penetrations and various roof edges.
  • Multiple layers of insulation with offset joints.
  • Preventing open voids in the thermal layers at perimeters and penetrations.
  • Protection of the thermal layer from physical damage above and warm moist air from below.
Photo 4. The mechanical fasteners below the roof membrane used to secure the insulation conduct heat through them to the fastening plate. The resultant heat loss can be observed in heavy frost and snowfall.

Air intrusion into the roof system from the interior can have extremely detrimental consequences. In fact, Oak Ridge National Laboratory research has found that air leakage is the most important aspect in reducing energy consumption. Interior air is most often conditioned, and when it moves into a roof system, especially in the northern two-thirds of the country where the potential for condensation exists, the results can include wet insulation, deteriorating insulation facers, mold growth and rendering the roof system vulnerable to wind uplift. Preventing air intrusion into the roof system from the interior of the building needs to be considered in the design when energy efficiency is a goal. Thus, vapor retarders should be considered for many reasons, as they add quality and resiliency to the roof system (refer to my September/October 2014 Roofing article, “Vapor Retarders: You Must Prevent Air and Vapor Transport from a Building’s Interior into the Roof System”). The transition of the roof vapor/air barrier and the wall air barrier should be detailed and the contractors responsible for sealing and terminations noted on the details.

One layer of insulation results in joints that are often open or could open over time, allowing heat to move from the interior to the exterior — a thermal short. Energy high to energy low is a law of physics that can be severe. Thus, the International Code Council now prescribes two layers of insulation with offset joints. (See Photo 1.)

When rigid insulation is cut to conform around penetrations, roof edges and rooftop items, the cuts in the insulation are often rough. This results in voids, often from the top surface of the roof down to the roof deck. With the penetration at the roof deck also being rough, heat loss can be substantial. Thus, we specify and require that these gaps be filled with spray foam insulation. (See Photos 2 and 3.)

Insulation Material Characteristics and Energy Conservation

In addition to the system components’ influence on energy loss, the insulation material characteristics should also be considered. The main insulation type in the United States is polyisocyanurate. Specifiers need to know the various material characteristics in order to specify the correct material. Characteristics to consider are:

Photo 5. Heat loss through the single layer insulation and the mechanical fasteners was so great that it melted the snow, and when temperatures dropped to well below freezing, the melted snow froze. This is a great visual to understand the high loss of heat through mechanical fasteners.
  • Density: 18, 20, 22 or 25 psi; nominal or minimum.
  • Facer type: Fiber reinforced paper or coated fiberglass.
  • Dimensional stability: Will the material change with influences from moisture, heat or foot traffic.
  • Thermal R-value.

In Europe, a popular insulation is mineral wool, which is high in fire resistance, but as with polyisocyanurate, knowledge of physical characteristic is required:

  • Density: If you don’t specify the density of the insulation board, you get 18 psi nominal. Options include 18, 20 and 25 psi; the higher number is more dimensionally stable. We specify 25 psi minimum.
  • Protection required: Cover board or integral cover board.
  • Thermal R-value.

Protecting the Thermal Layer

It is not uncommon for unknowledgeable roof system designers or builders looking to reduce costs to omit or remove the cover board. The cover board, in addition to providing an enhanced surface for the roof cover adhesion, provides a protective layer on the top of the insulation, preventing physical damage to the insulation from construction activities, owner foot traffic and acts of God.

The underside of the thermal layers should be protected as well from the effects of interior building air infiltration. An effective air barrier or vapor retarder, in which all the penetrations, terminations, transitions and material laps are detailed and sealed, performs this feat. If a fire rating is required, the use of gypsum and gypsum-based boards on roof decks such as steel, wood, cementitious wood fiber can help achieve the rating required.

Insulation Attachment and Energy Efficiency

The method in which the insulation is attached to the roof deck can influence the energy-saving potential of the roof system in a major way. This fact is just not acknowledged, as I see some mechanically attached systems being described as energy efficient when they are far from it. Attaching the insulation with asphalt and/or full cover spray polyurethane adhesive can — when properly installed — provide a nearly monolithic thermal layer from roof deck to roof membrane as intended by the codes.

Figure 1. Roof details should be drawn large with all components delineated. Air and vapor retarders should be clearly shown and noted and any special instructions called out. Project-specific roof assembly details go a long way to moving toward ensuring energy conservation is achieved. Here the air and vapor retarder are highlighted and definitively delineated. Voids at perimeters are called out to be filled with spray foam and methods of attachment are noted.

Another very popular method of attaching insulation to the roof deck and each other is the use of bead polyurethane foam adhesive. The beads are typically applied at 6 inches (15.24 cm), 8 inches (20.32 cm), 9 inches (22.86 cm) or 12 inches (30.48 cm).

The insulation needs to be compressed into the beads and weighted to ensure the board does not rise up off the foam. Even when well compressed and installed, there will be a ±3/16-inch void between the compressed beads, as full compression of the adhesive is not possible. This void allows air transport, which can be very detrimental if the air is laden with moisture in cold regions. The linear void below the insulation also interrupts the vertical thermal insulation section.

The most detrimental method of insulation attachment in regard to energy loss is when the insulation is mechanically fastened with the fasteners below the roof cover. Thermal bridging takes place from the conditioned interior to the exterior along the steel fastener. This can readily be observed on roofs with heavy frost and light snowfall, as the metal stress plates below the roof cover transfer heat from the interior to the membrane, which in turn melts the frost or snow above. (See Photo 4.)

The thermal values of roofs are compromised even more when a mechanically attached roof cover is installed. The volume of mechanical fasteners increases, as does the heat loss, which is not insignificant. Singh, Gulati, Srinivasan, and Bhandari in their study “Three-Dimensional Heat Transfer Analysis of Metal Fasteners in Roofing Assemblies”found an effective drop in thermal value of up to 48 percent when mechanical fasteners are used to attach roof covers. (See Photo 5). This research would suggest that for these types of roof systems, in order to meet the code-required effective thermal R-value, the designer needs to increase the required thermal R-value by 50 percent.

Recommendations to Increase Energy Savings

Code and standard bodies as well as governments around the world all agree that energy conservation is a laudable goal. Energy loss through the roof can be substantial, and an obvious location to focus on to prevent energy loss and thus create energy savings. The thermal layer works 24 hours a day, 7 days a week, 52 weeks a year. Compromises in the thermal layer will affect the performance of the insulation and decrease energy savings for years to come. Attention to installation methods and detailing transitions at roof edges, penetrations, walls and drains needs to be given in order to optimize the energy conservation potential of the roof system.

Based on empirical field observation of roof installations and forensic investigations, the following recommendations are made to increase the energy-saving potential of roof systems.

  • Vapor and air barriers are often required or beneficial and should be specifically detailed at laps, penetrations, terminations and transitions to wall air barriers. (See Figure 1.) Call out on the drawings the contractor responsible for material termination so that this is clearly understood.
  • The thermal layer (consisting of multiple layers of insulation) needs to be continuous without breaks or voids. Seal all voids at penetrations and perimeters with closed cell polyurethane sealant.
  • Design insulation layers to be a minimum of two with offset joints.
  • Select quality insulation materials. For polyisocyanurate, that would mean coated fiberglass facers. For mineral wool, that would mean high density.
  • Attach insulation layers to the roof deck in a manner to eliminate thermal breaks. If mechanically fastening the insulation, the fasteners should be covered with another layer of insulation, cover board or both.
  • Design roof covers that do not require mechanical fasteners below the membrane as an attachment method.
  • Protect the thermal layer on top with cover boards and below with appropriate air and vapor barriers.

Saving limited fossil fuels and reducing carbon emissions is a worldwide goal. Designing and installing roof systems with a well thought out, detailed and executed thermal layer will move the building industry to a higher plane. Are you ready for the challenge?

About the author: Thomas W. Hutchinson, AIA, FRCI, RRC, CRP, CSI, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

The Stud Wall and the Roof

Photo 1. With a stud wall parapet, inappropriate wall substrate and base anchor screws into a material with low pull-out resistance, this roof blew off in what would be considered moderate winds. Images: HUTCHINSON DESIGN GROUP LTD.

How do I start an article on a topic that is so problematic, yet it’s not being addressed by designers, roof system manufacturers, FM, SPRI, NRCA or any other quality assurance standard? Like many transitions in the building industry, the use of metal studs in exterior wall construction and roofing in new construction developed out of the twin concerns of value engineering and cost reduction. It has crept silently forward without any real consideration of the possible effects this less robust construction method would have on roof system performance. 

Photo 2. When the base anchors pull out of the substrate, the membrane becomes unsecured and will lift up. Here the membrane was observed lifting to heights of 3 to 4 feet, at which point it popped the coping off.

You would think that someone along the line would say, “Hmm, I wonder how strong, effective or appropriate a screw fastener through a modified gypsum board sheathing would be?” Let me answer that question: Worthless. (See Photos 1-3.) 

There are many issues with metal stud wall construction as it relates to roofing: air drive, moisture, interior pressures, and membrane adhesion to substrate, just to name a few. This article will address only one concern: The base anchor attachment horizontally into steel stud walls, most often clad with a modified gypsum substrate board. (See Photo 4.)

Why Is This a Concern?

Photo 3. All the base anchor screws pulled out of the substrate except one that was into the stud, which just tore away when the rest of the membrane lifted.

Problems often begin in the design phase when the condition is not detailed appropriately. (See Figures 1 and 2.) The architect/engineer/ designer shows some lines and figures that the roofing contractor or manufacturer will make it work — and specifies a 20-year warranty. The designer’s first mistake is to think that contractors and manufacturers design. They do not.If I were a betting man, I would guess that 99 percent of the specified wall substrate for roof-side metal stud walls is a product that is unacceptable for roofing base flashing application. You’re smiling now, aren’t you? Been there, huh? Designers often have little knowledge as to how a roof system, or even a roof membrane, is installed, and thus don’t even realize the errors of their ways. If they did, they might realize that at the very least a base anchor attachment is at 12 inches on center, and at some time a screw is going to have to go horizontally into the inappropriate sheathing substrate. Concept 1: Architects design. I know this is scary.

Figure 1. This is a common architectural stud wall parapet detail. No base anchor is even being acknowledged, nor is the concern with vertical vapor drive in the stud wall cavity. This type of detailing, in my opinion, is below the standard of care of the architect.

Architects and designers who do not prepare project-specific details seem to love manufacturers’ standard details, which are provided as a baseline for developing appropriate project-specific details. They are not an end all, and thinking they are is a huge mistake. Another common mistake is not realizing that manufacturers do not have a standard detail for base anchor attachment into metal stud walls. This is probably because they never imagined that anyone would really try to anchor into such a poor substrate. Concept 2: Manufacturers produce products that can be assembled in a roof system; they do not design.

Oh, but the contractor will make it work. Yeah, right. Concept 3: Contractors install materials provided by the manufacturer, as specified by the designer; they do not design. Are you starting to see a trend here?

You can now see the conundrum of the blind leading the blind. 

So, to be clear:

  • Architects: Design
  • Manufacturers: Produce products
  • Contractors: Install materials

To say it a bit clearer:

  • Architects: Design
  • Manufacturers: Do Not Design
  • Contractors: Do Not Design

Read it again and see where the responsibility lies. Of course, the manufacturer needs to produce quality materials, which sometimes does not occur, and contractors need to install the materials correctly, which sometimes does not occur.

Pull-Out Strength

So that we can get this detail correct, let’s look at pull-out strengths of various materials. But let’s start with trying to determine what pull-out resistance is required. For our example, let’s use 60-mil TPO, a common roofing membrane on new construction projects. 

Figure 2. This parapet detail has been well thought out in regard to thermal drive and concerns with condensation within the stud wall cavity, but ignores how the roof membrane will be attached to the wall. The insulation thickness will result in an unbraced section of the screw and allow rotation before it pulls out of what is assumed to be a gypsum base sheathing.

Manufacturers report on their data sheets for 60-mil TPO tear strength of around 130 pounds of force (lbf). The test for this isn’t pulling the membrane out from base anchors, but it’s a good start for our discussion. I suspect that if base anchors are attached at 9 inches or 12 inches on center that the series of fasteners will elevate this value.

Given that we know that the tear resistance of TPO with a series of fasteners is greater than the ASTM D751 Tearing Strength test, I will suggest that we need a substrate with a pull resistance greater than 260 lbf, or twice the tear strength value. After that the membrane will tear itself out from around the fastener plate. 

To determine the pull-out resistance of various sheathing materials, I had the pull-out resistance of a base anchor screw tested on several materials by Pro-Fastening Systems, a specialty distributor focusing on commercial roofing in the Midwest that provides certified pull-out testing. Three pull-out tests were performed on each material. (See Photo 5.) The mean resistance values are as follows:

Photo 4. This exterior view gives a good idea of how inadequate gypsum-related products are in regard to providing a pull-out resistance. A 16-, 18- or 20-gauge plate should have been placed at the stud wall from the concrete deck up above the anchor point.

1/2” plywood: 422 pounds

5/8” plywood: 402 pounds

1/2” glass-faced gypsum: 13.3 pounds

1/2” integral fiber reinforced gypsum: 110 pounds

22-gauge steel deck: 646 pounds

22-gauge acoustical steel deck: 675 pounds

18-gauge steel stud: 1,086 pounds

26-gauge metal stud: 646 pounds

16-gauge steel plate: 1,256 pounds

18-gauge steel plate: 978 pounds

20-gauge steel plate:724 pounds

22-gauge steel plate:625 pounds

So as a starter we eliminate all the typical gypsum-based sheathing materials from being used at the base of the roof. I’m not keen on plywood either, as over time, as the plywood dries, the pull-out strength lessens. Additionally, gluing to wet plywood never works well. 

Designing the Base Anchor on Metal Stud Walls

Photo 5. Various materials were tested to determine their pull-out resistance. The results confirmed what intuitively most roofing contractors would know — that gypsum-based products have very little holding power.

The concept is simple — provide a substrate with a pull-out resistance greater than the tear strength of the roofing membrane attached in series. So, let’s pretend you’re drafting. Come on now, get your paper out, a number 2H pencil, a parallel rule and triangle to get the feel of the detail — no CAD for you today. For our example, assume you’re in the Chicago area, minimum R-value of 30, tapered insulation and 24 feet from the drain to the wall. 

First, draft and show the roof deck and your wall, roof edge and studs. Now you’re ready to start your detail. First go to your roof plan, where you have shown all the tapered insulation, and calculate what the thickness will be at your studs. Remember, code requires thickness within 4 feet of the drain. For our detail, you’re near Chicago and thus the height of a tapered insulation layout might be as follows. For the R-30 at the roof drain with a substrate board, insulation and cover board, let say for simplicity it’s 6.5 inches (1/2-inch cover board + 5.4 inches of code-required insulation + 1/2-inch cover board). Now you need to calculate the tapered insulation. For our example, the distance from drain to wall is exactly 24 feet. With a taper of 1/4-inch per foot tapered that is 6.5 inches (1/4 inch x 24 feet = 6 inches, plus the 1/2-inch starting thickness of the tapered). If you plan to use foam adhesive, add 3/8 inch per layer of foam, and be sure you understand all the layers in a tapered system. So, at the wall, the insulation will be approximately 13 inches. With the screw and plate anchor say, 2 inches above the insulation surface, we have a height of 15 inches. So, let’s say we need a substrate capable of pull-outs at least 18 inches in height from the roof deck.

Figure 3. Design of a stud wall parapet includes delineating all the components and tells the contractor what is expected. Burying such information in the specification does no one any good, as the architect most likely will not know to review the shop drawings to those requirements.

Now, I know you are thinking, “OMG, 18 inches — I can cut in a little 6-inch strip at the top of the insulation.” Don’t do it. The strip will not have any continuity or strength and will often buckle under load. Additionally, this continuous substrate piece needs to be placed on the stud. 

Back to your drafting board. Draw in against your stud a continuous 16-, 18- or 20-gauge galvanized steel plate. Depending if the membrane is to be taken up and over the stud wall or terminated some distance above the roofing, the rest of the wall can be clad in less robust materials. Pick any substrate that is roofing membrane compatible and place it over the continuous steel plate and studs above. Tell the contractor how often you want the substrate anchored.

Figure 4. We often find that a simple isometric drawing showing the construction of stud wall parapets is helpful in informing all the related trades how their work interrelates.

Draw in your substrate board, vapor retarder, insulation (and don’t forget to show and call out the spray foam seal between the insulation and wall, as there is often a void). Bring your membrane to the wall, turn it up 3 inches fully adhered to the substrate and show a plate and screw. Call this plate and screw out and note the spacing on the drawing; I’ve never seen a spec up on the roof. The base flashing can now be delineated coming down over the anchors and out onto the flat. Depending on the material, show a weld or seam tape. Now compare your detail to Figures 3 and 4. Who has properly designed the condition?

Remember

There are many issues and concerns with steel stud walls and roofing. This issue with substrate cladding in regard to the interface with the roofing system is only one that I see again and again on projects that have wind damage issues. By carefully designing the roof termination conditions, taking into account all the possible impacts and then detailing the conditions properly, your standard of care can be met and the owner well served.

About the author: Thomas W. Hutchinson, AIA, FRCI, RRC, CSI, RRP, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com

4 Common Causes of Inadequate Drainage on Low-Slope Roofs

Photo 1. Roof decks with poor slope, drains that are up slope and deck defection can result in excessive ponding. Images: Hutchinson Design Group Ltd.

The stone church in rural Portugal was constructed some 700 years ago. The roofs of the transepts are large stone slabs: 5 feet wide, 10 feet to 12 feet long, and 8 inches thick. How they even made it into place is amazing, but to those like us who think in terms of water, what is even more amazing is the carved-out drainage channels. Moving water off the roof was important to builders 700 years ago in Europe, just as it was to the builders of Machu Picchu and Angkor Wat. Along with many indigenous building methods, the movement of water off roofs and away from buildings is becoming a lost design element.

It is not uncommon to walk upon recently installed roofs and see ponding at gutters, roof drains and across the roof. There are many reasons for this degradation of roof system design, including ignorance. A lack of knowledge by designers, a “roofer or builder will figure it out” mentality, and poor installation procedures can all be to blame.

Ponding water provides visual evidence to the owner that something isn’t quite right, and in some instances, it can result in roof structure collapse. If breaches in the roof membrane exist, standing water can result in excess moisture intrusion. (See Photo 1.) Additionally, water on the roof promotes algae growth that can attack some materials. It also allows for ice to form in winter, creating life safety issues as well as external forces affecting the roof cover.

So, what can you do?

In this article we’ll look at four key conditions on the roof that I see as the most erroneously conceived and installed:

  1. The roof system’s transition to the gutters
  2. Two-way structurally sloped roof decks with roof drains above the low point
  3. Four-way structurally sloped roof decks with drains above the low point
  4. Roof drains on level roof decks with tapered insulation

Accumulated Debris at Gutters

As perhaps you know and will see within this article, there are many things that irk me; one is walking on a new roof and seeing a 3- to 4-foot wide swath of black accumulated dirt and airborne components in front of the gutter. This situation

Photo 2. Owners do not like seeing ponding in front of their gutters, especially when it’s egregious. Proper design and installation would have prevented this problem. Images: Hutchinson Design Group Ltd.

results from restricted water drainage, and it is especially noticeable on reflective roof covers. (See Photo 2.) This restriction of water drainage can be due to several possible factors, including roof edge wood blocking that is too high, insulation that is too low, and the accumulation of roofing material above the slope plane. The roof deck itself can also be set too low.

When designing roof edge gutters, there are key design elements to consider:

  • Wood blocking:In addition to being of appropriate width and anchorage, wood blocking should be sloped to drain, even with sloped roof decks with an elevation 1/4 inch to 3/8 inch below the anticipated roof insulation height. The greatest error I see with most architects is that they do not draw the detail to scale. Insulation is not of the correct thickness, the wood is too big or too small, or it is depicted as one giant block floating atop the wall with no mention of anchorage.
  • Insulation:Please read the ASTM standard for polyisocyanurate and you will learn that the ISO has an allowable dimensional change. Thus, if you specified two layers of 2.25-inch ISO to match three layers of two-by wood blocking, you might be in for a surprise. You might get to the field and see that your two layers of insulation are 3/8 of an inch below the top of the wood, and the manufacturer whom you’ve complained to will pull out the ASTM standard and say, “We are within tolerances.”
  • Material layering:When the roof membrane is taken over the wood (yes you should do this) and sealed to the wall substrate, and the gutter is set in mastic and then stripped in, the accumulated material thickness can exceed 3/8 of an inch. Not much, you say, but on a roof with a 1/4-inch-per-linear-foot slope, that can result in 18 inches of ponding right in front of the gutter. Ouch.

Design recommendations for achieving complete drainage at the roof edge with gutter include:

  • Communicate with the structural engineer.Coordinate with the structural engineer to determine the elevation of the wall (less wood blocking) with the structure and roof deck. If perimeter steel angles attached to the wall rise above the roof deck, discuss with the structural engineer turning the angle downward or changing the angle to one with a vertical leg that doesn’t rise above the roof deck. Angles that rise above the roof deck create a void when

    Photo 3. Even when using tapered insulation and on four-way sloped roof decks, it is advantageous to accentuate the slope into the drain. Here a 1/2-inch-per-foot tapered insulation sump matches up to the tapered insulation with the help if a 1/2-inch tapered edge strip. Images: Hutchinson Design Group Ltd.

    the first layer of insulation is set that is most often not sealed, resulting in a thermal short and a place where dew points can be reached and condensation can occur. If reinforcing paper facers are on the insulation, mold growth can result.

  • Properly detail the wood blocking. I prefer and recommend the use of two layers of wood blocking. First off, do not use treated wood; use untreated Douglas fir. The wood should be at a minimum 8 inches wide (preferably wider) so that the gutter flange can have nail locations back far enough to allow for 3-inch minimum overlap on the stripping-in ply.

Often it is best if the top of the wall is sealed prior to the installation of the wood to prevent air/moisture transport to the wood, and on precast, to prevent the migration of “damp” into the wood. The first layer of wood should be anchored to the structure (wall or framing). While not always required, I prefer to set anchors at 2 feet on center, staggered. This spacing prevents the warping of the wood. The second layer of wood should match the first in width. I suggest that this second layer of blocking be sloped, and placing a continuous shim along the roof side on the first layer will provide the proper slope. The shim width and thickness are dependent on the wood size, but for two-by-ten wood blocking, a shim of 1/2 inch by 1.5 inches will work well. The second layer of wood blocking should be set with joints offset from the lower layer and then screw fastened at 12 inches on center, staggered. Joints on both layers should be scarfed at 45degrees and screwed tight. On your detail, the height of the wood blocking at the interior side above the roof deck should be dimensioned. This will allow contractors to identify height concerns well before the installation of the insulation so adjustments can be made if necessary. I suggest that this distance be 1/4 inch to 3/8 inch below the top surface of the roof insulation or cover board atop the insulation. (See Figure 1.)

  • Make sure the insulation is higher than the wood blocking.We will not discuss insulation types, substrate boards (vapor barriers) and cover boards in this article; please see earlier articles on the topic. In designing the roof edge and discussing/coordinating with the structural engineer, the goal is to have the insulation system: substrate board, vapor retarder, cover board. The thickness should be 3/8 of an inch greater than the interior top corner of the wood blocking. One key item to remember is that spray-and-bead polyurethane adhesive adds 3/8 of an inch thickness per layer. Designing the insulation to be higher than the wood blocking is important, as it compensates for that allowable dimensional change mentioned above, as well as the thickness created by the layers of gutter flange and roofing. The goal is to create a condition in which water will flow over and into the gutter.

Two-Way Structurally Sloped Roof Decks

Often long, narrow roof areas are designed with a two-way structurally sloped roof deck designed to move water from the outer roof edge to a central point. Prudent designers would like the roof drains to be located at the low point of the structurally sloped roof deck. Typically, though, there is a steel beam at the low point, which prevents the installation of the roof drain at the low point. Consequently, the roof drains must be located on the plumbing drawings up slope from the low point. I have tried for years to explain to plumbing engineers that water doesn’t typically flow uphill, but to no avail, so we as the roof system designer have to fix it. How? By moving the low point.

How is this design goal accomplished?

Let’s start with our roof system design for the following example: a new construction project in Chicago (R-30 minimum) with a steel roof deck, two-way structural slope and the low point over a steel beam. The plans call for the drains to be installed 2 feet up slope, and thus they will be more than 1/2 inch above the low point.

The goal will be to move the structural low point to the drain line. With a structural slope, to meet the thermal value we are looking at two layers of 2.6-inch insulation. Run the first layer of 2.6-inch insulation throughout the roof. Then the fun begins: Draw a line down the center of the roof drains. From this centerline, come out 4 feet on each side with a 1/2-inch-per-foot tapered edge board (Q panel, for those who know). The next layer of 2.6-inch insulation abuts the taper. The tapered insulation at the drain line effectively moves the low point to the drains. (See Figure 2.)

Now that the water is being moved to a new low point, it then needs to be moved to the drains. This is accomplished by saddles. (See Figure 3.) Sounds simple enough, but 95 percent of the saddles I see are incorrect, and water ponds on them, over them and along them. This situation leaves, once again, a bad taste in the mouth of the owner, general contractor, construction manager, and architect — even though it’s the designer’s problem. So, I will now, for the first time, reveal my secret developed years ago: The taper of the saddles mustbe twicethe roof deck slope. If the deck slopes 1/4 inch per foot, the saddles must slope at 1/2 inch per foot. If the deck slopes at 3/8 inch per foot, as it often does, the saddle needs to be at 3/4 inch per foot. And, architects and designers, the slope of the saddle is to the valley line, not the drain. The width of the saddle is the key and determining the width of the saddle is my secret.

It’s a simple formula:

(Distance Between Drain)x 33% = X

2

Increase X to the next number divisible by 4

Example: If the drains are 60 feet apart, divide 60 by 2 to get 30 feet; multiply 30 feet by 33% = 9.9 feet. Increase 9.9 to the next number divisible by 4 to get the answer: 12 feet.

Thus, the saddles at the mid-point apex should extend out three full tapered insulation boards. It’s best if you dimension this width on the detail.

On large buildings, the saddle width and thickness can be quite high, so be sure to double-check the insulation height with the height of the roof edge. I could tell you about a roof where the insulation rose several inches above the perimeter height because someone didn’t draw the detail to scale, but that is a story for another time.

Roof Drains in Four-Way Slope Roof Decks

Structurally sloped roof decks can be beneficial in that they can create positive drainage flow. But with four-way structurally sloped roof decks, the drain is not necessarily at the low point of the roof. How far off the low point is dependent on the plumbing contractor. I have seen drains installed several feet upslope. The plumbing drawings should have a note to the fact that the roof drain sump pan should be installed as close to the low point as possible.

Even when the drain is installed very close to the low point, it is still high and will result in water ponding in front of the drain. Thus, the low point needs to be artificially moved to the drain.

This is accomplished with a drain sump. Best practices suggest that the roof insulation be installed in two layers. This will allow for the installation of the sump.

Using Chicago as an example, which calls for R-30 or 5.2-inches of insulation, the first layer of insulation 2.6 inches thick is installed across the roof deck, to the roof drain. It should be cut to the roof drain extension ring. Fill the void between the roof drain and the insulation with spray foam; trim to the insulation. Next the tapered insulation sump is installed. To match the next layer of insulation, we use 1/2-inch-per-foot tapered insulation. It starts at 1/2 inch and, with a 4-foot panel, rises to a thickness of 2.5 inches. Placed around the drain, the sump created is 8 feet by 8 feet. The next layer of insulation is 2.5 inches and abuts the backside of the tapered insulation.

The 1/2-inch-per-foot slope is used as it doubles the slope of the structurally sloped roof deck, which in this case has a slope of 1/4 inch per foot.

Level Roof Decks With Tapered Insulation

Whether re-roofing or new construction, getting the drainage correct on level roof decks is still a challenge for most designers. Perhaps they don’t realize decks are not level; they have camber, they deflect, they undulate, and the drains are often near columns so the drain pipe can run along it. When the drain is near a column where no deflection takes place, it can often be high.

I like to first ensure the proper drain assembly has been selected and designed by the plumbing engineer: the roof drain, reversible collar, threaded extension ring, clamping ring, cast iron dome. (For more detail, see “Roof Drain Installation Tips” on page XX of this issue.) The sump pan should be selected and designed by the plumbing engineer and provided by the roof drain manufacturer — not by the metal deck supplier. (That the industry cannot get this correct is one on my pet peeves.) Do not raise drains off the deck with threaded rods. (See my article “Concise Details and Coordination Between Trades Will Lead to a Quality Long-Term Solution for Roof Drains,” RoofingMay/June 2016). If designing in a vapor retarder, it needs to extend to the roof drain flange and be clamped by the reversible collar. The first layer of insulation should be cut to fit and extend under and to the extension ring. Any voids should be sealed with spray foam.

To compensate for all the potential deck irregularities, I like to accentuate the slope into the roof drain by increasing the taper. More often than not, this means designing a 1/2-inch-per-foot slope sump into the drain. With a 4-foot board, this results in an 8-foot-by-8-foot sump. (See Figure 4 and Photo 3.) After detailing this sump, the main roof four-way tapered insulation can be designed and the heights at the perimeter calculated and noted on the plans. Just a reminder that the code-required thermal value needs to be attained four feet from the drain. So, for Chicago we detail to achieve R-30 at the backside of the tapered sump.

Final Thoughts

A new roof installation that results in ponding water at the drainage point is an unfortunate occurrence. Owners can be upset: “What is that?” “I didn’t pay to have water retained at the drains!” “Who is coming up and cleaning all this stuff off my roof?” Ponding water can be a standard of care issue for designers and result in damages. Learning to properly design rooftop drainage is not difficult, but it requires some thinking and some rooftop experience. Getting up on the roof during installations will help you visualize the needs to achieve proper drainage.

Making sure the roof system drains properly requires discussions with the structural engineers for new construction. I also find it helpful to have the plumbing contractor at pre-con meetings to review the interrelationship of the roofing and drains.

Getting water off the roof as quickly as possible has been a key priority for centuries — no matter the roof cover material. If the builders using stone can achieve complete and full drainage, then I challenge you to achieve it with the materials we use today.