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MSHA - Title 30 CFR
30 CFR § 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
(b)(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.