The stair and elevator shafts of all buildings except private dwellings, hereafter erected more than two (2) stories high, shall be enclosed continuously by incombustible material, consisting of reinforced gunite not less than two and one-half inches (21/2") thick, or of solid Portland cement plaster not less than two and one-half inches (21/2") thick on metal lath and metal frame, or of reinforced concrete not less than three inches (3") thick, or of any fireproof material or construction that will pass the standard fire test of the Underwriters' Laboratories for a period of at least two (2) hours. The thickness in all cases must be sufficient to give rigidity. (1964 Code § 299)