Mechanical Code…
As applied to building construction material, means a material that in the form in which it is used is either one of the following:
(1) A material that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Materials that are reported as passing ASTME 136, Standard Test Method for Behavior of Materials in a vertical Tube Furnace at 750°, are considered noncombustible material. [NFPA 220:3.3.4]
(2) Material having a structural base of noncombustible material as defined in 1 above, with a surfacing material not over one-eighth (1/8) inch (3.2 mm) thick that has a flame-spread index not higher than fifty (50).
Noncombustible does not apply to surface finish materials. Material required to be noncombustible for reduced clearances to flues, heating appliances, or other sources of high temperature shall refer to material conforming to 1 above. No material shall be classed as noncombustible that is subject to increase in combustibility or flame-spread index beyond the limits herein established, through the effects of age, moisture, or other atmospheric condition.
(Uniform Mechanical Code® 216.0)