Shower methods B421 and B422 leave the backing choice to the makers
B421 and B422 are shower methods in the Tile Council handbook. A waterproof membrane covers a solid backing under the entire area to be tiled. The FAQ's question is whether drywall counts as that backing.
Makers do not agree
Some membrane manufacturers promote drywall under their membrane. Others do not. Not every drywall maker recommends the board in a shower even when a membrane will cover it. The handbook leaves the combination to the membrane maker, the backer maker, and the design professional. Where the makers disagree, the recommendation is proprietary.
The building code is read two ways
The FAQ says many people read the International Building Code as barring drywall in wet areas even with a membrane on its face. Others read it as barring only a direct bond of tile to drywall in a wet area. The page tells the reader to ask the local code official. This note does not decide which reading applies.
Source
Fetched 2026-10-11 from Tile Council showers FAQ. The sentences above follow that page. They are not a contractor promise.
Related notes
- Tile notes index
- ANSI tile standards cited by the Tile Council exclude OSB
- Steam-room grout passes vapor even when the glaze does not
- Floor photos
Send photos if you want another person to look at the area. This note does not set a response time.