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
(c) Kept clear of obstructions.