Class A roof coverings are noncombustible like tile, metal, and slate. An example of a Class B roof covering is fire retardant-treated wood shingles. An example of an "unrated" roof covering is plain wood shingles. However, a Class B roof covering may receive a Class A rating if it is installed as part of a system that includes additional fire-resistive elements in the assembly. These four classes of roof coverings are incorporated into most building and some fire codes, and are usually noted on the manufacturersa?™ literature and materials labels.