Roof Rot: Ignorance Is an Easy Way to Damage Low-slope Residential Roofs

Change often brings with it unintended consequences, and the issue of reflective roof surfaces in North America is no exception. In the late 1990s, U.S. cities in northern climates started to mandate the use of reflective roof—more for politics, feel-good, pseudo-environmental reasons than sustainable, resilient and durable reasons. In my estimation, cool roofs often did more to lower the quality of buildings than enhance them. Furthermore, code and standard changes were made with no understanding of the result and no education to the architects of America.

Figure 1: Reduced attic space resulted in a roof section comprised of the following components from the interior to the roof cover.

Figure 1: Reduced attic space resulted in a roof section comprised of the following components from the interior to the roof cover.

Although the resulting unintended consequences affected commercial and residential buildings, it was the often-catastrophic results on low-slope residential buildings that went untold and left homeowners with tens of thousands of dollars of corrective work on basically new residences.

Following is a summary of how these concerns evolved in wood-framed residential construction. I’ve included case studies of failures, potential solutions and lessons learned.

HISTORY

During the industrialization of America’s large cities throughout the 1800s, the need for labor caused populations to explode. To house the labor migration, row houses (3- to 4-story structures, often with a garden level and four or more narrow units) were constructed approximately 3-feet apart, block after block, creating medium-sized apartment blocks. Most of these row houses were wood-framed, masonry veneer with low-slope roof structures. The interior walls and ceilings were finished in cementitious plaster, which provided a durable, fire-resistive finish. The plaster also performed as an effective air and vapor barrier, preventing interior conditioned air from penetrating into the non-insulated walls and ceilings where it could condense within the walls and roof on cold days.

Photo 1: A contractor was called out to fix the “soft roof” and found this catastrophic situation.

Photo 1: A contractor was called out to fix the “soft roof” and found this catastrophic situation.

Heating costs were low, so little—if any—insulation was installed in the walls and roof. Roofs were composed of built-up asphalt and coal tar, both smooth and aggregate surfaced. Attic spaces often 4 to 6 feet in height were vented via static vents. Any conditioned air that passed to the attic was able to dissipate through these static vents. This method of construction performed without significant attic condensation, and the roof systems and roof structure served these buildings for decades.

In the mid 1990s, researchers (theoretical researchers with no architectural, engineering, roofing, construction or practical building technology experience or knowledge) at research institutes conducted studies into the effects of minimizing solar gain through the roof via a reflective surface. Based on the researchers’ algorithmic findings and recommendations (regardless of their validity), environmental groups used the concept to promote change. Large cities started introducing new energy codes with reflective roofing requirements and prescribed reflectance values. These new codes contained greater insulation requirements, which was a benefit. However, in this one code adoption, roof systems, such as coal-tar pitch, that had performed for centuries were no longer permitted. Consequently, roofing contractors went out of business and so did some roofing material manufacturers because of unproven and suspect research.

Photos and Details: Hutchinson Design Group Ltd.

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Air and Vapor Barrier Creates a Seal Between a Low-slope Roofing System and the Building Below

F1 Air & Vapor Barrier from Mule-Hide Products Co.

F5 Air & Vapor Barrier from Mule-Hide Products Co.

F5 Air & Vapor Barrier from Mule-Hide Products Co. allows contractors to quickly and easily create an air- and vapor-tight seal between a low-slope roofing system and the building below.

F5 Air & Vapor Barrier is compatible with a wide variety of roofing systems and can be used on primed substrates, including concrete, plywood, exterior gypsum, DensDeck Prime and SECUROCK. It also can serve as a temporary roof for up to 120 days while work on the finished roofing system is completed.

The membrane is a 40 mil-thick composite consisting of 35 mils of self-adhering rubberized asphalt laminated to a 5 mil-thick woven polypropylene film. Rolls are 39 inches wide and 75 feet long and cover approximately 244 square feet of substrate surface.

F5 Air & Vapor Barrier’s factory-controlled thickness helps ensure that the membrane has uniform barrier properties, reducing moisture movement through the roofing system and helping keep conditioned air inside the building and unconditioned air outside. The woven polypropylene film makes the membrane highly resistant to tears and punctures. The non-skid surface helps keep contractors safe on the job site and is suitable for the bonding of subsequent layers of the roofing system.

A siliconized one-piece release liner prevents the material from bonding to itself in the roll and is easily removed during installation of the barrier.

With Today’s ‘New Age’ Roofs, Removing All System Components May Not Always Be Required or in the Clients’ Best Interest

Years ago, reroofing design involved removing all roof-system components down to the roof deck and rebuilding a new roof system up from there.

PHOTO 1: This EPDM roof’s service has been extended for nine years and counting, approaching 30 years in-situ performance. Here, the restoration of perimeter gravel- stop flashing and lap seams, as well as detailing of roof drains, penetrations and roof curbs, is nearing completion.

PHOTO 1: This EPDM roof’s service has been extended for nine years and counting, approaching 30 years in-situ performance. Here, the restoration of perimeter gravel- stop flashing and lap seams, as well as detailing of roof drains, penetrations and roof curbs, is nearing completion.

Although that is still a viable option and often performed, the coming of age of many single-membrane roofs has altered the method of installing a new reroof system. Options now include EPDM roof restoration; removal of the roof membrane and the addition of new insulation and roof membrane; using the existing roof membrane as a vapor retarder and adding new insulation and roof membrane; removal of the roof cover and installation of new, leaving all the existing insulation in place.

When I first moved into roof-system replacement design some 35 years ago, the dominant roof systems being removed were bituminous, specifically gravel-surfaced asphaltic and coal- tar-pitch built-up roofs. As they aged, their surfaces often started to blister, crack and undulate with ridges—surfaces often unsuitable for roof recover. The bitumen often was deteriorating because of ultraviolet-light exposure; when that occurred, the deterioration of the felts was not far behind. The insulation was mostly perlite or high-density wood fiber; the amount was minimal (low thermal value) and, more often than not, flat or with very minimal slope. Drains were erratically placed, tapered insulation was not often the case and roof edges were predominately gravel stops. In the Midwest, many roof decks were cementitious wood fiber. The roof covers were often patched again and again, even as water infiltrated the system.

PHOTO 2: The re-flashing of roof curbs is an integral part of the restoration of EPDM roof membranes.

PHOTO 2: The re-flashing of roof curbs is an integral part of the restoration of EPDM roof membranes.

When replacement was necessary, the roof-edge sheet metal was removed; the entire existing roof system was removed down to the roof deck; and a new roof system was designed, often incorporating vapor retarders/temporary roofs so the removal of multiple layers of roofing could be accomplished, roof curbs raised, and enhancements of roof drains, curbs and roof edge could occur prior to the installation of the new roof cover. Tapered insulation designs be- came common; this would often require realignment of the roof drains to simplify the tapered design and installation. To accommodate the new insulation thickness, the roof edge had to be raised as did roof curbs, RTU curbs, plumbing vents and roof drains via extensions. Roof membranes changed from bituminous to those classified as “single plies”: EPDM, PVC, CPE, CSPE.

These new roof-system replacement designs resulted in superior roofs—85 percent of all the reroofs I have designed are still in place, still performing, still saving the owner money. Life cycles have moved from eight to 12 years, up to 18 to 25 years and longer. They certainly were more expensive than the original installation and, if a roof designer didn’t have a handle on costs to provide the owner with estimated costs of construction, were often shocking. But these roof systems were good for the client, economy, environment and public.

PHOTO 3: When restoring EPDM roof membranes, the removal of roof penetration flashings and installation of new with target patches will provide another 20 years of watertight protection.

PHOTO 3: When restoring EPDM roof membranes, the removal of roof penetration flashings and installation of new with target patches will provide another 20 years of watertight protection.

Over the years, codes and standards have changed, especially in the past decade, requiring increased insulation values and roof-edge sheet-metal compliance with greater attention to wind-uplift resistance. As the new millennium arrived, these “new age” roofs came of age and owners started to look at their replacement—often with increased costs stifling their budgets.

LEAN THINKING

A factor that increased the performance of many roof systems in the past 20 years was the emergence and growth of the professional roof consultant, often degreed in architecture or engineering, educated in roofing, tested and certified. These professionals brought a scientific approach to roof-system design. Raleigh, N.C.-based RCI Inc. (formerly Roof Consultants Institute) was the conduit for this increased level of knowledge, professionalism and the growth in quality roof-system design and installation.

PHOTO 4: On this roof, the existing loose-laid membrane was removed, open insulation joints filled with spray-foam insulation and new insulation added to meet current code requirements. A new 90-mil EPDM membrane was installed and existing ballast moved onto it to 10-pounds-per-square-foot coverage.

PHOTO 4: On this roof, the existing loose-laid membrane was removed, open insulation joints filled with spray-foam insulation and new insulation added to meet current code requirements. A new 90-mil EPDM membrane was installed and existing ballast moved onto it to 10-pounds-per-square-foot coverage.

As these professionals started to examine the older “new age” roofs, those whose first responsibility was doing what was best for the client saw greater opportunity than just a costly full-roof replacement. Although many roofs today still need to be fully removed, prudent professionals see other opportunities, such as the following:

ROOF RESTORATION
EPDM membrane ages with little change in physical characteristics as opposed to its built-up roofing predecessor; therefore, EPDM membranes often can be “restored” in lieu of removing and replacing the roof. (Studies to support the lack of change in EPDM’s physical characteristics while it ages include Gish, 1992; Trial, 2004; and ERA, 2010.)

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VaproShield Used on Housing Prototype Commissioned by New York’s Office of Emergency Management

VaproShield was honored to have WrapShield SA Self-Adhered Water Resistive Vapor Permeable Air Barrier Sheet selected for use on an experimental, post-disaster housing prototype located in Brooklyn, N.Y. In development since 2008, the prototype was commissioned by New York’s Office of Emergency Management (OEM) with funding from FEMA. The U.S. Army Corps of Engineers (USACE) was designated as project manager for the prototype’s construction. Designed by Garrison Architects, the “townhouse” style post-disaster housing consists of five modular units—fabricated by Mark Line Industries—which are stacked on top of each other.

“This is an exciting innovation,” comments Phil Johnson, VaproShield Managing Partner. “These [post-disaster housing] units have the potential to provide safe, reliable housing to the victims of natural disaster as they work to rebuild their communities. VaproShield is a proud contributor.”

While the post-disaster prototype will be on-site in Brooklyn for a year, the modular units are designed to be mobile. The modular units were constructed for easy installation, deconstruction and transport.

“The idea is that no matter where a disaster occurs, these modular units can be placed on a truck and taken there,” says Johnson. “The units need to perform well in every climate, as there is no telling where they may end up, and WrapShield SA Self-Adhered helps with that.”

WrapShield SA Self-Adhered Water Resistive Vapor Permeable Air Barrier Sheet helps to regulate air flow to keep units cool in the summer and warm in the winter, thus reducing energy costs. WrapShield SA Self-Adhered was selected for use on this initial prototype as it provides superior weather protection as well as the durability to withstand exposure in a wide variety of climates.

Vapor Retarders

The need for, use and design of a vapor retarder in the design of a roof system used to be a hotly debated topic. It appears now—when vapor retarders are needed more than ever—the design community seems to have lost interest, which is not good, considering how codes and standards (altered through concerns for energy savings) have changed how buildings are designed, constructed and operated. Most notably, positive building pressures are changing the game.

If not controlled, constructiongenerated moisture can have deleterious effects on new roof systems.

PHOTO 1: If not controlled, construction-generated
moisture can have
deleterious effects on new
roof systems.

A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system, where it can condense. Everyone knows that water in roof systems is never a positive. Typically, a vapor retarder has to have a perm rating of 1.0 or less to be successful. Through my recent observations, the lack of or poorly constructed vapor retarders contribute to ice under the membrane, soaked insulation facers, destabilized insulation, rusting roof decks, dripping water down screw-fastener threads, compromised fiber board and perlite integrity, mold on organic facers and loss of adhesion on adhered systems, just to name a few. Oh, and did I fail to mention the litigation that follows?

The codes’ “air-barrier requirements” have confused roof system designers. Codes and standards are being driven by the need for energy savings and, as a consequence, buildings are becoming tighter and tighter, as well as more sophisticated. This article will discuss preventing air and vapor transport of interior conditioned air into the roof system and the need for a vapor retarder. The responsibility of incorporating a vapor retarder or air retarder into a roof system is that of the licensed design professional and not that of the contractor or roof system material supplier.

It should be noted that all vapor retarders are air barriers but not all air barriers are vapor retarders. In so much that the roof membrane can often serve as an air barrier, it does nothing to prevent this interior air transport.

WHEN TO USE A VAPOR RETARDER

So the question arises: “When is it prudent to use a vapor retarder?” This is not a simple question and has been complicated by codes, standards, costs and building construction, changing roof membranes and confusion about air barriers. Then, there is the difference in new-construction design and roof removal and replacement design. Historically, it was said that a vapor retarder should be used if the interior use of the building was “wet”, such as a pool room, kitchen, locker shower rooms, etc.; outside temperature in the winter was 40 F or below; or when in doubt, leave it out. In my experience, changes in the building and construction industry have now made the determination criteria more complex.

I find there are typically three primary scenarios that suggest a vapor barrier is prudent. The first is the interior use of the building. The second is consideration for the control of construction-generated moisture, so that the roof can make it to the building’s intended use (see photo 1). The third consideration is the sequence of construction. In all three situations I like to specify a robust vapor retarder that “dries in” the building so that interior work and construction work above the vapor retarder can take place without compromising the finished roof. Consider the following:

BUILDING USE

This characteristic is often the most determinant. If the interior use of the building requires conditioned air and has relative-humidity percentages great enough to condense if the exterior temperatures get cold enough, a vapor retarder is needed to prevent the movement of this conditioned air into the roof system where it can condense and become problematic.

Most designers consider building use only in their design thinking, and it is often in error as the roof system can be compromised during construction and commissioning (through interior building flushing, which can drive moist air into the roof system) before occupancy.

To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap sheet, eliminating the glaze coat.

PHOTO 2: To seal two-ply asphaltic felts set in hot asphalt on a concrete roof deck, an asphaltic glaze coat was applied at the end of the day. Because of the
inherent tackiness of the asphalt until it oxidizes, Hutch has been specifying a smooth-surfaced modified bitumen cap
sheet, eliminating the glaze coat.

CONTROL OF CONSTRUCTION-GENERATED MOISTURE

I have seen roof systems on office buildings severely compromised by construction- generated moisture caused by concrete pours, combustion heaters, block laying, fireproofing, drywall taping and painting. Thus, a simple vapor retarder should be considered in these situations to control rising moisture vapor during construction, which includes the flushing of the building if required for commissioning.

CONSTRUCTION SEQUENCING AND MATERIALS

Building construction takes place year round. It is unfortunate decision makers in the roofing industry who are pushing low-VOC and/or water-based adhesives do not understand this; problems with their decisions are for another article. If the roof is to be installed in late fall (in the Midwest) and interior concrete work and/or large amounts of moisture-producing construction, such as concrete-block laying, plastering, drywall taping or painting, are to take place, a vapor retarder should be considered.

How will the building, especially the façades, be constructed? Will they be installed after the finished roof? This creates a scenario for a damaged “completed” roof system.

PHOTOS: Hutchinson Design Group Ltd.

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Air, Vapor and Water Barrier Products for All Six Sides of the Building Envelope

Garland Aero-Block product line offers solutions for all six sides of the building enclosure

The Garland Aero-Block product line offers solutions for all six sides of the building enclosure.

As the push for more energy-efficient buildings continues, numerous building codes are now including air barrier requirements. To address these changing standards, Garland has engineered a new product line of high-performance solutions that prevent unwanted air, vapor and water from penetrating the building envelope. The Garland Aero-Block product line offers solutions for all six sides of the building enclosure, as well as for gaps in walls or between sections of walls.

This new polymer-modified-asphalt technology is available in three formats. The fluid-applied solvent-based polymer and fluid-applied water-based polymer versions can be applied by brush, spray or roller. There is also a pre-fabricated, self-adhering multi-layer membrane. The entire Aero-Block family create vapor-closed protection, meeting the requirements for a Class I air/vapor barrier.

A companion vapor-open product line, Aero-Perm, will become available later this year. Aero-Perm will offer the same capabilities as Aero-Block systems, while providing permeability to water vapor.

Attention Roof System Designers: Numerous Roof Components Work Together to Affect a Building

There has been a great deal of opinion expressed in the past 15 years related to the roof cover(s), or the top surface of a roof system, such as “it can save you energy” and “it will reduce urban heat islands”. These opinions consequently have resulted in standards and code revisions that have had an extraordinary effect on the roofing industry.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

The building type should influence the type of roof system designed. Some spaces, like this steel plant, are unconditioned, so insulation in the roof system is not desired.

Let’s say it loud and clear, “A single component, does not a roof make!”. Roofs are systems, composed of numerous components that work and interact together to affect the building in question. Regardless of your concern or goal—energy performance, urban heat-island minimization, long-term service life (in my opinion, the essence of sustainability) or protection from the elements—the performance is the result of an assembled set of roof system components.

Roof System Components

Energy conservation is an often-discussed potential of roofs, but many seem to think it is the result of only the roof-cover color. I think not. Energy performance is the result of many factors, including but not limited to:

Building use: Is the building an office, school, hospital, warehouse, fabrication facility, etc.? Each type of building use places different requirements on the roof system.

Spatial use and function be low the roof deck: It is not uncommon in urban areas to have mechanical rooms or interstitial spaces below the roof—spaces that require little to no heating or cooling. These spaces are typically unconditioned and unoccupied and receive no material benefit from the roof system in regard to energy savings.

Roof-deck type: The type of roof deck—whether steel; cast-in-place, precast and post-tensioned concrete; gypsum; cementitious wood fiber; or (don’t kill the messenger) plywood, which is a West Coast anomaly—affects air and moisture transport toward the exterior, as well as the type of roof system.

Roof-to-wall transition(s): The transition of the roofing to walls often results in unresolved design issues, as well as cavities that allow moisture and vapor transport.

Meanwhile others, like this indoor pool, require extreme care in design and should include a vapor retarder and insulation.

Meanwhile others, like this indoor pool, require extreme care in design and
should include a vapor retarder and insulation.

Roof air and/or vapor barrier: Its integration into the wall air barrier is very important. Failure to tie the two together creates a breach in the barrier.

Substrate board: Steel roof decks often require a substrate board to support the air and vapor barrier membranes. The substrate board also can be the first layer of the roof system to provide wind-uplift resistance.

Insulation type: Each insulation type—whether polyisocyanurate, expanded polystyrene, extruded polystyrene, wood fiber, foam glass or mineral wool—has differing R-values, some of which drop with time. Many insulation types have differing facer options and densities.

The number of insulation layers: This is very important! A single layer of insulation results in a high level of energy loss; 7 percent is the industry standard. When installing multiple layers of insulation, the joints should be offset from layer to layer to avoid vapor movement and thermal shorts.

Sealing: Voids between rooftop penetrations, adjacent board and the roof-edge perimeters can create large avenues for heat loss.

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