Is drywall a suitable “solid backing” for methods B421 and B422?
B421 and B422 are shower methods in the TCNA Handbook in which a waterproof membrane is applied over an unspecified solid backing. The membrane must cover the solid backing under the entire area to be tiled. We are often asked if drywall is a suitable backing for these shower methods.
While some membrane manufacturers promote use of their membrane in such installations with drywall as the solid backing, other membrane manufacturers do not. At the same time, not all drywall manufacturers recommend their product be used in a shower, even if a membrane will be applied to the surface. Thus, the Handbook leaves the determination of a suitable membrane/backer board combination up to the membrane and backer board manufacturers to declare and up to the design professional to select/specify. Where materials manufacturers do not agree on a particular recommendation, a manufacturer is offering what’s called a proprietary recommendation. The International Building Code (IBC) is somewhat ambiguous on the topic. Many in the construction industry interpret it to say drywall is strictly prohibited in wet areas, regardless of whether a waterproof membrane would be applied to the surface of the drywall. Others feel the code only prohibits direct bond of tile to drywall in wet areas. Check with a local code official for your area requirements.