Section 57.22221 - Overcast and undercast construction (I-A, II-A, III, and V-A mines)Overcasts and undercasts shall be-
(a) Of substantial construction;(b)(1) Constructed of noncombustible materials; or(2) 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;(c) Kept clear of obstructions.