30 CFR 57.4533 - Mine opening vicinity.
Surface buildings or other similar structures within 100 feet of mine openings used for intake air or within 100 feet of mine openings that are designated escapeways in exhaust air shall be -
(a) Constructed of noncombustible materials; or
(b) Constructed to meet a fire resistance rating of no less than one hour; or
(c) Provided with an automatic fire suppression system; or