As applied to a building construction material, other than interior finish, means a material not complying with the definition of noncombustible material that, in the form in which it is used has a potential heat value not exceeding 3500 Btu/lb. (8. 14 X 10⁶ J/kg) and complies with one of the following paragraphs, (a) or (b). (a) Materials having a structural base of noncombustible material with a surface not exceeding a thickness of 1/8 in. (0. 3 cm) that has a flame spread rating not greater than 50. (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. Materials subject to increase combustibility or flame spread rating beyond the limits herein established through the effects of age, moisture, or other atmospheric condition shall be considered combustible. See: NFPA 259, Standard Test Method for Potential Heat of Building Materials, and NFPA 220, Standard on Types of Building Construction