The waterproofing membrane must run continuously from the wall, over the drain’s sealing flange, and around the drain body — the drain is not a separate element to be sealed at its edge. Any break in the membrane at the drain is a direct leak path.
The principle
A shower drain is a penetration through the waterproof layer. Water that reaches the membrane flows toward the drain and must be able to enter it. The membrane is therefore bonded to a flange on the drain body, creating a continuous waterproof surface that terminates inside the drain rather than at its edge.
Correct sequence
- Install and level the drain body (before waterproofing).
- Prime the substrate and the drain flange with the membrane manufacturer’s primer.
- Apply the membrane up the walls and across the floor.
- Bring the membrane over the flange and bond it fully — no gaps, no folds.
- Apply the reinforcement or sealing collar detail around the drain if the system requires it.
- Water test before screeding and tiling.
Common failures
| Failure | Result |
|---|---|
| Membrane stopped at the drain edge | Water tracks under the drain body |
| No primer on the flange | Poor bond; membrane lifts |
| Membrane applied after screeding | Cannot be bonded continuously |
| Drain moved after waterproofing | Membrane tears at the flange |
| Wrong membrane/adhesive compatibility | Delamination over time |
Practical notes
- Use a drain with an integrated flange — retrofit sealing rings are less reliable.
- Follow one manufacturer’s system for membrane, primer and sealing accessories rather than mixing brands.
- Water test before tiling; after tiling the leak can only be found by removing the floor.
- On timber subfloors, treat the drain penetration with particular care — see wooden subfloor solutions.