Building managers and owners need to be aware of the many hazards that must be addressed to keep their buildings operational. The building envelope deserves attention for its vulnerability, as it receives the full brunt of the weather.
In the case of roofing systems, building stresses become magnified at walls, transitions, and other closures. To add to these expected challenges, we have the human factor: abuse, misuse, vandalism, and neglect.
When conventional asphaltic built-up roofs were dominant, the flashings at walls and curbs were always beefed up. The system generally started with a cant strip to break the angle between the horizontal and vertical elements, softening the angle of change and to provide support for all the plies that would follow. Ply felts, adhered using hot asphalt, were mopped to the cant strip and to each other, followed by a “backer-ply” that ran up the wall or curb for extra kick-through resistance. Finally a fabric-reinforced asbestos-flashing sheet was bonded to the backer sheet. (See Fig. 1A and Fig. 1B.)
This was a heavy system; to resist gravity’s pull, the flashing sheet was top-nailed at close intervals. These nails were then covered with felt stripping to seal the wall against water penetration. Metal counter flashing was also incorporated into the design to redirect rainwater or snowdrift melt-water away from the top edge of the base
When reinforced asbestos base flashings were withdrawn from the marketplace in the 1980s, it was found that the substituted glass fiber mat systems did not provide the same conformability, durability, and toughness as their predecessors. Fortunately, polymer-modified bitumens (also referred to as mod bits or MBs) were being introduced first in Europe, and then here, which were robust enough to fill this gap in requirements.
Another trend brought to us from Europe during this period was substitution of the heat of a propane torch flame for the complicated process of mopping hot asphalt. The backing bitumen on the MB sheets was factory applied, and the roofing contractor needed to simply reheat this material to achieve a durable, waterproof assembly. (See Fig. 2.) The logistical necessities were thus reduced to a bottle of propane, a hose, and a torch head compared to the previously required cartons of asphalt on the ground (or a tanker), a kettle-man, pumps, hot luggers, mop carts, and the like. The needed heat was delivered instantly by pulling on the propane torch trigger rather than relying on the long-distance chain involved in getting hot material from the kettle to the point of application.
The simplicity of the torch-melted systems offers other benefits as well. Repairs could more be more feasibly made to weathered built-up roofing, as well as to MB systems. Rather than lug 5-gallon pails of mastic to sites of broken blisters, puncture damage, or slipped flashings, a handheld propane torch could soften damaged materials; evaporate snow, water, or ice; permit embedment of roofing granules at tie-ins and laps; and compensate for ambient conditions, such as freezing weather. Once the back of the MB patch was heated, and the patch pressed in, we were done.
The Fire Concern
With all the virtues of torched-applied MB systems, there came a new downside – the fire hazard. Without the proper precautions, a fire could occur during application, or even several hours after the roofing crew had left the jobsite. These hotspot fires could result from smoldering combustible underlayment materials, such as fiberboard cant strips, lint-coated ducts, unprotected wood
curbs, and the wood deck itself. European constructions tended to be monolithic concrete, limiting potential fire danger, whereas, in the United States, there are many air-permeable, wood, OSB, and steel decks overlaid with combustible wood fiber, glass fiber, or perlite thermal insulations.
FM Documentation of Fire Occurrences
FM Global published a Loss Prevention Data Technical Advisory Bulletin in October 1988 to address some of these concerns:
Select excerpts follow:
Fire Started by Roofer’s Torch
Repair activity to MB-recovered BUR consisted of 2- by 6-foot patch being applied with a propane torch. Fire watch was 10 feet away, and a 25mph wind was blowing. About 2 squares of glass fiber insulation and new MB roof covering were fire damaged, as well as five roof-mounted air-handling units.
This loss illustrates the need for an adequate fire watch, the use of charged hose lines, and the use of a base sheet over certain insulations.
Roofer’s Torch Ignites HVAC Unit Support Structure
HVAC units were mounted on the roof of an electronic assembly and machining area. The roof was constructed of plywood on wood glulam beams. The support structure for the HVAC units was also wood. Roofers were installing base flashing with a torch along the perimeter of an HVAC unit located over a clean room area. The open flame from the roofer’s torch ignited the wood support structure for the HVAC unit. Smoke was noted exiting the perimeter of the HVAC unit. A dry chemical extinguisher was used unsuccessfully. Three sprinklers and hose streams controlled the fire about 25 minutes later.
The HVAC unit was fire damaged, and water and soot damage occurred in the clean room area. This loss illustrates the need to exercise caution when working around roof penetrations.
Wood Fiber Cant Strip Ignites
This building is used for the storage of food products. A torch-applied roof cover was being applied at a flashing detail for a plastic heat and smoke vent. A wood fiber cant strip was provided around the perimeter of the vent frame. This was part of a re-roofing system over an existing built-up roof.
Approximately 3.5 hours after work was completed, a sprinkler water-flow alarm was received. One sprinkler and small hose streams were utilized to extinguish a storage fire inside the building. Damage included fire, smoke, and water damage to contents; melting of the heat and smoke vent; and fire damage.
Overheating of a wood fiber cant strip that smoldered and later burst into flames caused the fire. This loss illustrates the need for an adequate fire watch and a base sheet over wood fiber cant strips.
Fire in Roofing System Spread to Contents in Building
An existing Class II insulated steel roof deck was being re-roofed using a torch-applied roof system. The existing assembly consisted of 2 inches of glass fiber insulation and original 5-ply built-up roof, and an additional 5-ply built-up roof installed some time after the original construction to repair roof leaks. A layer of 3/8-inch wood fiber insulation with a pre-applied base sheet on its top side was secured to the existing assembly. A modified-bitumen roof covering was then installed using a propane torch that was 2-inches wide, and an open expansion joint was provided between steel decking and an adjacent concrete block wall in an area where work had recently been completed. After work was completed, the roofer reportedly checked the top surface of the roof for hotspots. Approximately 2 hours after work was completed in this area, a fire was discovered in an area used for the storage of finished garments located below the expansion joint. The fire department responded and found that one sprinkler operated and controlled the fire in the storage; however, they noted flames in the underside and above the roof. Hose streams were used to extinguish roof flaming. The propane torch at the expansion joint excessively heated the roofing system, causing smoldering and subsequent flaming. Burning asphalt dripped through the expansion joint and ignited storage below. This loss illustrates the need for exercising caution near roof penetrations and conducting a satisfactory fire watch, including inspection inside the building.
As fire loss experience grew, the roofing industry reacted aggressively. The Midwest Roofing Contractors Association (MRCA), with the assistance of the Roofing Industry Educational Institute (RIEI), developed a two-level educational program and initiated a Certified Roofing Torch Applicator training program (CERTA). (See Fig 3). The first program was to train trainers and establish criteria for certification of applicators. The second component was an 8-hour program that covered everything from propane safety to actual hands-on techniques for safely applying torched-on roof systems.
In 2003, the National Roofing Contractors Association (NRCA) joined in, making the CERTA programs available on a nationwide basis. The goal was to reduce the incidence of torch-related fires through a comprehensive training program delivered by trained trainers.
The January 2009 issue of Professional Roofing magazine featured an updated safety article by Mark S. Graham, NRCA’s associate executive director of technical services, entitled Fahrenheit 570. (You may recall Ray Bradbury’s novel Fahrenheit 451, where 451 degrees F. was the theoretical temperature at which paper [books] would ignite.) The Fahrenheit 570 article was based on the idea that understanding the exact temperature that combustible substrates materials would reach their ignition point, temperature during the application process could be controlled and kept below those points, and the likelihood of fire would be greatly diminished.
To explore the concept as it applies to torch-applied polymer-modified-bitumen roofing products, the NRCA and Hughes Associates Inc. devised a program to test whether, and at what temperature, commonly encountered combustible substrates will likely ignite during torching. Tests revealed that the minimum ignition temperature of such substrates is 570 degrees F.
The NRCA/MRCA curriculum from 2004 through April 2008 instructed installers to understand that direct torching of base flashings was not the best safety practice. The installation method taught in CERTA – torch and flop – was based on best safety practices. (See Fig. 3.) (Note that many torch-grade MB systems have a backer film of polyethylene that must be completely melted to ensure good adhesion.)
Although torching the back surface of a polymer-modified-bitumen flashing sheet and flopping it into place may work in ideal conditions with some materials, it’s certainly a very risky technique for installing what are arguably the most important sections of a roof membrane: the flashings.
The polymer-modified-bitumen membrane type, the installer’s familiarity with the product, the installation technique, and the ambient conditions all play important roles in forming a watertight bond in torch-applied flashing applications. To address these considerations while still providing a best practice safety guide, direct torching with detail torches under certain conditions, to polymer-modified bitumen base flashing, has since become incorporated into the part CERTA curriculum and training program. Flashing applications will be because of it. (See Fig. 4.)
The NRCA will recommend the use of direct torching of polymer-modified-bitumen sheet product membrane flashings over various substrates, including plywood, oriented strand board, wood board or wood plank, and wood fiber board flashing substrates using a single-burner low-output (105,000 Btu or less). This recommendation also applies where the substrate has been covered with an air impermeable flashing backer, including a layer of fiber glass ply sheet, fiber glass base sheet, or polymer-modified-bitumen base sheet mechanically fastened to the substrate and an additional layer of a minimum of one layer of glass ply sheet or polymer-modified-bituminous base sheet adhered to the underlying layer in a solid mopping of hot asphalt. For more information about these changes, go to www.nrca.net/rp/education/nrca/0508_certa.pdf.
A very helpful video on handling MB applications can be found at www.gaf.com/training/ROOFING-VIDEOS/tri-ply-torch-application.html.
FM Global Data sheets are available at www.RoofNav.com.