Article and photos by Gregory Havel
For decades, builders have been using metal roof decking supported by bar joists, covered with combustible insulation board and a roof membrane that is often topped with gravel. The earliest of these roofs had melted asphalt mopped onto the steel roof deck to hold down the insulation board with additional melted asphalt mopped between and on top of the layers of roofing felt. This was known as a “built-up” roof and was inexpensive when compared with other types of roofs.
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A potential problem with this type of roof was recognized more than 50 years ago: If a fire heated the underside of the roof deck, it could melt, vaporize, and ignite the asphalt on top of the deck, starting another fire in addition to the original one inside the building (see Brannigan’s Building Construction for the Fire Service 4th Edition, 213-214). This roof fire could spread far ahead of the original fire and ignite other fires when burning asphalt found its way through the steel deck. You can identify this type of roof by the asphalt “icicles” that often form along walls (photo 1) and at seams in the steel deck as a result of too much or too hot asphalt being used to assemble the roof. These “icicles” also form during and after a roof deck fire.
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Since the 1980s, building codes have required more insulation on roofs, and reliable elastomeric (rubber or plastic) roof membranes have been developed. Photo 2 shows a typical modern metal deck roof. Multiple layers of plastic foam insulation board are laid on the steel roof deck, with offset joints. This foam board is supposed to be flame-retardant-treated extruded polystyrene or polyisocyanurate foam board. These materials will burn if heated or exposed to flame but will self-extinguish when the heat or flame is removed. The foam insulation is sometimes faced with roofing felt.
Standard Method of Tests of Fire Endurance of Building Construction and Materials (ASTM E119) like a wall or a floor-ceiling assembly. Rather, they have been tested under NFPA 256,
Standard Method of Fire Tests of Roof Coverings (UL 790; ASTM E-108) for exposure to fires originating outside the building. They are rated Class A (severe), B (moderate), or C (light), based on the severity of fire exposure they can withstand. Tests for the rating include ignition from flaming brands, intermittent flame exposure, rain, weathering, and flame spread.
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Standard Method of Test of Surface Burning Characteristics of Building Materials 2006 edition (ASTM E-84 or UL 723) for low flame spread and smoke development. Search the Internet for “thermal barrier roof underlayment” and similar terms for more information from roofers’ associations and manufacturers.
Installing a thermal barrier between the steel deck and the plastic foam insulation on a modern roof is usually about five percent of the cost of the completed roof. However, building owners usually have tight budgets and often give up this fire-resistance feature so that their money can be spent where it will show. After all, the thermal barrier is of no value to the owner or to the business unless there is a fire, and most building owners don’t believe that this can happen to their building.
Gregory Havel is a member of the Burlington (WI) Fire Department; a retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II and fire officer II, an adjunct instructor in fire service programs at Gateway Technical College, and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.
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Subjects: Building construction for firefighters
