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
(c) Provided with an automatic fire suppression system; or
(d) Covered on all combustible interior and exterior structural
surfaces with noncombustible material or limited combustible material, such
as five-eighth inch, type "X" gypsum wallboard.
For further information: See MSHA's Program Policy Manual