Tech Today

Understanding fire ratings

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.

Exterior exposure

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.

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