Roof assemblies play important roles in buildings' overall abilities to resist fires. For this reason, building codes generally require buildings' roof assemblies to meet specific fire-resistance characteristics. Roof assemblies typically are required to resist the spread of fires originating from buildings' exteriors and interiors. The specific degree of fire resistance required primarily is based on a building's construction, occupancy and location.
Roof assemblies' abilities to resist fires originating on buildings' exteriors are determined by subjecting representative small-scale specimens of roof assemblies to laboratory testing using one of several recognized standard test methods: ASTM E108, "Standard Test Methods for Fire Tests of Roof Coverings"; UL 790, "Standard for Tests for Fire Resistance of Roof Covering Materials"; or FM 4470, "Approval for Class I Roof Covers." These test methods provide the basis for Class A, Class B and Class C fire ratings.
Class A-rated roof assemblies are effective against severe fire test exposure. A Class A fire rating is the most stringent rating available for exterior fire resistance. Class B-rated roof assemblies are effective against moderate fire test exposure. And Class C-rated roof assemblies are effective against light fire test exposure. A Class C fire rating is the least stringent rating available for exterior fire resistance. If a roof assembly cannot pass the Class C criteria, it typically is considered to be nonclassified.