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.