Construction Concerns: Combustible Metal Deck Roofs

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.

On top of the layers of insulation, a single layer of ethylene propylene diene monomer (EPDM, a type of synthetic rubber) roof membrane (or a similar rubber or plastic product), is laid down; joints are glued or solvent-welded. This membrane is sealed around pipes, vents, ducts, and other roof penetrations; and is ballasted with round gravel. With a fire below, this type of roof behaves like the older-style built-up roof and can develop a fire above the steel roof deck, which then will drip burning and molten plastic and rubber down into occupied space that is not yet affected by the original fire. This behavior could be made worse if the roofer substituted less expensive plain foam insulation board for the flame-retardant-treated foam board that is usually specified.
 
Roofs of these types are sometimes inaccurately advertised as “fire-rated.” They do not have a “fire rating” from testing under National Fire Protection Association (NFPA) 251, 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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Photo 3 shows a blueprint detail of the most recent development in metal deck roofs. A noncombustible thermal barrier (arrow) is laid down on top of the steel roof deck to separate it from the plastic foam insulation board. This thermal barrier provides enough separation between the steel deck and the foam insulation so that if there is a fire below the roof deck, it would be unlikely to start a second fire above the roof deck. The noncombustible thermal barrier is usually a moisture-resistant, fiberglass-faced, Type X gypsum board or a type of perlite board, ¼- to ¾-inch thick that meets standards like ASTM E-136 for noncombustibility and NFPA 255, 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.

 

Subjects: Building construction for firefighters

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