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.
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.
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.