§57.22221. Overcast and undercast construction (I-A, II-A, III, and V-A mines).

Constructed of noncombustible materials; or

Where constructed of combustible materials, the outside surfaces shall be coated with at least one inch of construction plaster containing perlite and gypsum; at least one inch of expanded vermiculite, Portland cement and limestone; or other coatings with equivalent fire resistance;