Roofing systems must meet or exceed building codes and insurance requirements. Fire ratings may pertain to resistance to fire from above the roof system (the familiar Class A, B, or C ratings), or fire exposure from the building interior (the underside of the roof deck).
How Are Roofs Rated?
ASTM E108 defines fire test methods for roof coverings. These tests may be conducted at UL Inc., FM Global, or any other certified testing laboratory. E108 defines the following conditions:
Over non-combustible roof decks
such as steel, poured gypsum, or concrete, only the spread of flame on the top surface of the roof system is evaluated. The maximum flame spread is 6 feet for a Class A rated roof, 8 feet for Class B, and 13 feet for Class C. The slope of the test specimen is preselected, and since steeper slopes are more of a challenge due to melting material feeding the fire, the rating applies to the maximum
Gravel-surfaced built-up roofing and ballasted single-ply systems usually meet Class A, while mineral surfaced roofings may be Class B. Some unsurfaced systems, such as asphalt glazing, may be Class C or even unrated.
If the roof deck is combustible, such as wood, plywood, or OSB, two additional tests must be conducted. Both are burn-through tests, and the ultimate roof systems rating is the lowest of the three tests.
- The burning brand: The brands consist of an ignited wood lattice placed directly on top of the test specimen. The Class A brand weighs 2,000 grams, Class B 500 grams, and Class C 9-1/4 grams. Failure is defined as the point where the roof deck ignites.
- The intermittent flame test: A gas flame is cycled on and off. To meet Class A, the test specimen must resist 15 cycles, Class B eight cycles, and Class C three cycles.
Fire resistance ratings (time-temperature) include structural elements and everything above, including the roof membrane and its surfacing. A one-hour rating would mean the structural elements have not yet reached their yield point when exposed to a under-deck heat load defined in ASTM E119 for steel structural members as reaching 1,070 degrees F.
Interior fire hazard is evaluated by FM Global and/or UL using differing test procedures. Both relate to a major interior building fire in Livonia, MI, back in 1953. In that case, an insulated steel building that might be expected to be highly fire-resistant failed catastrophically. Heat from an under-deck fire melted and vaporized the asphalt used to adhere the thermal insulation to a steel deck, feeding the fire. Some 30 acres of building burned to the ground.