What is behind your tile
Tile and grout do not keep water out. The board behind them, and what is painted or sheeted over it, is what does.
- Cement board
- A rigid panel of cement and fibreglass mesh. Water does not rot or soften it, which is why it belongs behind tile in a shower. It is not waterproof by itself: water passes through it, so it still needs a membrane over the top.
- Foam board
- A lighter panel with a waterproof core, such as Schluter KERDI-BOARD or Wedi. The board and its sealed seams are the waterproofing, so it does both jobs at once. It costs more per sheet and saves a step.
- Green board
- Moisture-resistant drywall, and still drywall. It belongs outside the shower at most. Behind wet-area tile it eventually goes soft, the tile loses its backing, and the repair is the whole wall. We do not use it in a wet area.
- Waterproofing membrane
- A sheet or liquid coat over the backer board that stops water reaching the structure behind it. It is invisible the moment the tile goes on, and it is the single layer that decides whether a shower lasts for decades or leaks within a year.
| Board | Waterproof on its own | Where it belongs |
|---|---|---|
| Cement board | No, needs a membrane over it | Wet areas, under a membrane |
| Foam board (KERDI-BOARD, Wedi) | Yes, with its seams sealed | Wet areas |
| Green board | No | Outside the shower, never behind wet-area tile |