Waterproofing & Construction
How shower waterproofing actually works
Last reviewed
Short answer
Tile and grout are not waterproof — water passes through them. What keeps a shower dry is a continuous barrier installed behind the tile, bonded to the substrate, wrapped through every corner and connected properly to the drain. If that barrier is interrupted anywhere, the shower will eventually leak there.
Key takeaways
- Grout is not a waterproofing layer; it is a filler between tiles.
- Cement board is water-durable, not waterproof; it still needs a membrane.
- Never mix components from two different manufacturers' systems.
- A flood test before tiling is the only way to prove the pan holds.
What the assembly looks like, layer by layer
- 1Framing, corrected flat and with blocking where anything will be fastened.
- 2Substrate: foam board, cement board or an equivalent rated for wet areas.
- 3Waterproofing membrane, bonded and continuous across every plane.
- 4Reinforcing band at every inside corner, seam and pipe penetration.
- 5A sloped pan falling to a drain compatible with that membrane system.
- 6Thin-set and tile, which are finish materials, not water barriers.
- 7Flexible sealant — not grout — at every change of plane.
Why the drain has to match the membrane
Different waterproofing systems manage water at different heights in the assembly, and the drain has to collect it where that system delivers it. Mismatching them is one of the most common causes of a shower that leaks despite looking correctly built.
| Drain | Belongs with | Collects water at |
|---|---|---|
| Bonding flange | Bonded sheet or liquid membrane | The membrane plane, just under the tile |
| Clamping / weep hole | Traditional mortar bed over sheet liner | The liner, below a thick mortar bed |
| Linear (bonded) | Bonded membrane systems | One edge of a single-plane sloped floor |
A bonded membrane over a clamping drain traps water in the mortar bed with nowhere to go. It is a common and expensive mistake.
The flood test
Plug the drain, fill the pan, mark the water line and leave it. If the level holds, the pan and the drain connection are sound. If it drops, you have found a problem while it still costs nothing to fix. Skipping this step saves an afternoon and risks the entire floor.
Frequently asked questions
Related services and areas
Planning a shower project?
Tell us about the shower you have and the one you want. We will come back with questions, then arrange to see the room before anything is quoted.