§ 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.