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.

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.

Adapt Existing Roof Curb to New Rooftop Unit

A Thybar Multi-Zone Retro-Mate is custom made to adapt your existing roof curb to a new rooftop unit.

A Thybar Multi-Zone Retro-Mate is custom made to adapt your existing roof curb to a new rooftop unit.

A Thybar Multi-Zone Retro-Mate is custom made to adapt your existing roof curb to a new rooftop unit. It saves time and costly roof reconstruction, preserves roofing integrity, reduces system downtime, and takes advantage of existing multi-zone ductwork. Convert a single-zone constant or variable-volume rooftop unit into a variable-volume multi-zone. Our comprehensive library of new and old rooftop specifications lets us design a matching Retro-Mate without extensive field measurements. Reduced engineering and construction time lets you bid more competitively. Licensed P.E. on staff.

Adapt Existing Roof Curb to a New Rooftop Unit

A Thybar Retro-Mate is custom made to adapt the existing roof curb to a new rooftop unit.

A Thybar Retro-Mate is custom made to adapt the existing roof curb to a new rooftop unit.

A Thybar Retro-Mate is custom made to adapt the existing roof curb to a new rooftop unit. It saves time and costly roof reconstruction, preserves roofing integrity, reduces system downtime, and takes advantage of existing ductwork. The comprehensive library of new and old rooftop specifications allows for the design of a matching Retro-Mate without extensive field measurements.