How are roof coverings classified in terms of fire ratings?

Fire ratings for roof coverings are either “Class A,” “Class B,” or “Class C,” based on passing a fire test standard. Class A is the highest rating (that is, the best performance) for classified roof coverings. Not all roof coverings have fire rating classifications. The tests within the standard address surface flame spread over the surface of the roof, fire penetrating through the covering and into the attic as result of exposure to exterior flames or burning brands or embers, and any tendency of the roof covering to generate embers during the test. The severity of the fire exposure (for example, the size of the burning brand) increases as the desired rating from a successful test increases from C to B to A. Examples of a Class A roof covering include fiber cement materials, fiberglass asphalt composition shingles, and metal roofs. An example of an unrated covering is an untreated wood shake roof not pressure impregnated with a fire retardant.