Overcasts and undercasts shall be—

1. (a) Of substantial construction;

2. (b)

   1. (1) Constructed of noncombustible materials; or
   2. (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;
3. (c) Kept clear of obstructions.