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Lessons Learned in the Field: “The Washboard Effect” of Torched Applied Roofing Membranes

03/01/2017, by kevink, in News & Media, 0 comments

By: Zachary Johnson, Assistant Project Manager


Throughout the career of an experienced roof observer, you will inevitably run across roof deficiencies and installation errors. These ultimately lead to premature aging or premature roof failure. The roof is a costly component of the building envelope with its integrity being a large factor in the efficiency of the facility. Unlike many other building envelope component, roofs are susceptible to severe weathering, expansion, contraction, and abuse, which can result in additional roof system degradation. The origin of a roof ’s failure offers vital insight into refining installation practices that can prolong the life and performance of roof systems. With the latest technological advances in the roofing industry, we have come a long way from the 90 pound granulated roll roofing system utilized in historic building construction. Today manufacturers offer warranties upwards of 30 years and with the correct installation and proper roof asset maintenance program, this can easily be achieved. The procedures implemented during the roof system installation such as manufacturer’s training and published guidelines are imperative tools designed to aid installers. During install, companies should ensure specifications, building codes, regulations and ordinances are adhered to and followed by hiring a roof observer or third-party consultant.

Recently, consultants and roof observers alike have observed an anomaly with torch applied modified asphalt membranes and hot mop applications that is being termed “the washboard effect” (Figure 1). The term originated through observing ridge patterns seen at certain hours of the day. The ridge patterns are more visually prominent when the angle of the sun casts a shadow observable between each protrusion. The ridge patterns can be in localized areas, or in severe cases, throughout the entire torch-applied builtup membrane installation.

Figure 2: Modified Asphalt pool at Roll Base from Excess Heat during Application.

Built-up roof systems are composed of: layer(s) of insulation adhered with adhesive or mechanically fastened, a cover board adhered or mechanically fastened, and 2 to 3 layers of asphalt membrane bonded by utilizing a hot mop or propane torch. The washboard appearance is caused by two factors: an uneven distribution of heat at the torch applied membrane and inconsistent pulling or kicking of the membrane roll during installation.

The torch flame is directed to a localized area at the face of the membrane roll while stationary, the face of the roll begins to soften, and with excess heat will liquefy, creating a pool of modified asphalt in front of the roll (Figure 2). The membrane roll is then pulled or rolled forward over the pool of modified asphalt that is cooling quickly upon contact with the roof substrate thus creating a small hump or ridge beneath the membrane. When this application method is repeated, you begin to observe a ridge pattern anomaly in the roof membrane.

Figure 3: Uneven Distribution of Modified Asphalt beneath Membrane.

Examination of a roof sample containing the ridge pattern anomaly revealed small humps in the membrane which are apparent with a straight object positioned across the ridges. The modified asphalt bond is unevenly distributed beneath the membrane due to improper torch application and excess heat (Figure 3). The point of application temperature the membrane is exposed to during installation is critical.

An experienced torch applicator applies a consistent “L” shaped distribution of heat to the face of the roll and partially up the seam. This is done in conjunction with smoothly pulling or pushing the roll as typically specified per the manufacturer’s instruction. During a forensic evaluation of the anomaly, evidence of uneven distribution of heat at the point of application is apparent from the burn pattern on the gypsum cover board layer after membrane removal (Figure 4).

In addition to the aesthetics of the roof system, building owners should be concerned the effect it has on the longevity of the roof system. Overtime, the prominent humps of the washboarding generate an increase in granule loss at their ridges. These increase pattern ridges of the cap sheet layer will lead to a reduction in the roof membranes ultraviolet (UV) resistance. When the unprotected, modified asphalt is exposed to UV rays it will begin to rapidly degrade and alligator, ultimately leading to premature roof membrane failure.

Figure 4: Uneven Distribution of Heat apparent during Forensic Analysis

The occurrence of the washboard effect or ridge pattern anomaly is dependent on the quality of the roof membrane installer. The roofing industry needs to recognize the damaging effects to the longevity of a roof system created by an inexperienced installer. Educating the torch applicators thoroughly and providing adequate hands-on training is needed to ensure installers are informed on the proper roof application techniques and procedures.

As the industry continues to evolve there will be ever-changing variables and challenges with roofing materials that the installer must adequately adjust to during the application process. We are a
technology-driven society fueled by the latest products and trends. Installers must thoroughly understand the newest roofing products, materials and procedures implemented by the manufacturers to stay ahead in the industry. It is the responsibility of the employers and manufacturers alike to provide continuing education workshops to keep installers on the cutting-edge of the roofing industry and ensure the future success of their roof systems. Much like the roofing industry, the construction science field will need to continue to evolve, exploring new methods of proper building envelope system applications and techniques so the industry can optimize performance of the entire building envelope.

During proper heat application of torch applied membrane, the roll can be rolled back and “stringers” of modified asphalt can be observed. This is an indicator of adequate heat at the point of application.