A material (as defined in NFPA 220, Standard on Types of Building Construction) not complying with the definition of noncombustible material that, in the form in which it is used, has a potential heat value not exceeding 8141 kJ/kg (3500 Btu/lb) and complies with one of the following: (a) materials having a structural base of noncombustible material, with a surfacing not exceeding a thickness of 3. 2 mm (1⁄8 in. ) and having a flame-spread rating not greater than 50 or (b) materials, in the form and thickness used, other than as described in (a), having neither a flame-spread rating greater than 25 nor evidence of continued progressive combustion and of such composition that surfaces that would be exposed by cutting through the material on any plane would have neither a flame-spread rating greater than 25 nor evidence of continued progressive combustion.