International Building Code 2509.1
Wet areas. Showers and public toilet walls shall conform
to Sections 1210.2 and 1210.3.
International Building Code 2509.2
Base for tile. Cement, fiber-cement or glass mat gypsum
backers in compliance with ASTM C 1178, C 1288 or C
1325 and installed in accordance with manufacturer recommendations
shall be used as a base for wall tile in tub and
shower areas and wall and ceiling panels in shower areas.
Water-resistant gypsum backing board shall be used as a base
for tile in water closet compartment walls when installed in
accordance with GA-216 or ASTM C 840 and manufacturer
recommendations. Regular gypsum wallboard is permitted
under tile or wall panels in other wall and ceiling areas when
installed in accordance with GA-216 or ASTM C 840.
2006 INTERNATIONAL BUILDING CODE. 503
GYPSUM BOARD AND PLASTER
International Building Code 2509.3
Limitations. Water-resistant gypsum backing board
shall not be used in the following locations:
1. Over a vapor retarder in shower or bathtub compartments.
2. Where there will be direct exposure to water or in areas
subject to continuous high humidity.
3. On ceilings where frame spacing exceeds 12 inches (305
mm) o.c. for 1/2-inch-thick (12.7 mm) water-resistant
gypsum backing board and more than 16 inches (406
mm) o.c. for 5/8-inch-thick (15.9 mm) water-resistant
gypsum backing board.