(1) Except as provided in Sentences (2) and (3), combustible interior wall and ceiling finishes described in Clause 3.1.13.1. (1) (b) that are not more than 1 mm thick are permitted in a building required to be of noncombustible construction.
(2) Combustible interior wall finishes, other than foamed plastics, that are not more than 25 mm thick are permitted in a building required to be of noncombustible construction, provided they have a flame-spread rating not more than 150 on any exposed surface or any surface that would be exposed by cutting through the material in any direction.
(3) Except as provided in Sentence (4), combustible interior ceiling finishes, other than foamed plastics, that are not more than 25 mm thick are permitted in a building required to be of noncombustible construction, provided that,
(a) they have a flame-spread rating not more than 25 on any exposed surface or on any surface that would be exposed by cutting through the material in any direction, and
(b) not more than 10% of the ceiling area within each fire compartment where such finishes are installed has a flame-spread rating not more than 150.
(4) Combustible interior ceiling finishes made of fire-retardant treated wood are permitted in a building required to be of noncombustible construction, provided they are not more than 25 mm thick or are exposed fire-retardant treated wood battens.