I’m looking for the spot in the Handbook that states that you are not allowed to use pressure-treated lumber to build a bench or curbs.
In the shower methods of the TCNA Handbook for Ceramic, Glass and Stone Tile Installation, the requirements for wood studs are that they must be dry and well braced. The general requirements for wall bracing are found in ANSI A108.11 in section 4.1 Wood Framing. This section states all framing lumber should have a moisture content not in excess of 19%. Most pressure-treated lumber has moisture content ranges of 30% to 70%. As pressure-treated wood starts to dry out, it has a tendency to twist and contort. The rigidity of the tile assembly cannot generally handle that type of movement, and failures can result from it.