A one-hour fire-resistive ceiling shall be constructed of not less than 15/8 inch joists, spaced not more than 16 inches center to center, protected on the under side by metal or wire lath and 3/ 4 inch of gypsum or Portland cement plaster or by 3/8 inch perforated gypsum lath and 1/2 inch of gypsum plaster, the joints of the lath to be reinforced with 3 inch wide strips of metal lath. Other materials and methods of construction having an equivalent fire-resistive rating may be used if approved by the department, as specified under s. Ind 50.12, 1956.
Wis. Admin. Code Department of Safety and Professional Services SPS 375.11