Waterproof the Shower Valve With an Embedded Gasket Seal

The Problem

Shower walls get waterproofed and tiled, and then a hole gets cut through all of it for the mixing valve — and at that opening, the backer board's raw edge is exposed. Kids splashing in the tub or a sloppy tile cut around the trim plate lets water reach that edge, and it wicks into the wall. The result shows up months later as mold behind the tile. Purpose-made mixing-valve seals close this off for about ten dollars and five minutes of work.

The Technique

1. With the wall waterproofed and the valve opening cut, mix a small batch of thinset.
2. Apply thinset around the valve opening — a 3-inch joint knife works for spreading, then comb it with the notched trowel.
3. Embed the mixing-valve seal into the thinset. The critical part: the seal's rubber gasket collar goes into the wall opening, so it covers the exposed cut edge of the backer board. That edge is the vulnerable spot on any backer — foam board, cement board, membrane-faced board, all of them.
4. Smooth out the thinset around the seal so it won't telegraph through or interfere with the tile that goes over it.
5. Tile the wall as normal; the valve trim plate installs over the sealed opening.