A drain leak almost always originates at the waterproofing connection — where the membrane meets the drain flange — or at the outlet joint below. Water appearing on a ceiling below does not mean the drain body itself has failed.
Likely causes, in order of probability
| Cause | Signs | Fix |
|---|---|---|
| Membrane not bonded over the flange | Slow persistent damp; staining on the ceiling below | Expose the drain perimeter and reinstate the membrane |
| Membrane terminated at the drain edge | Water tracks sideways from the drain | Rebuild the joint with a continuous membrane over the flange |
| Outlet joint failure | Leak appears only when water flows, stops when it drains | Reseal or replace the outlet connection |
| Movement of the drain body | Leak started after floor loading or movement | Re-support the body; rebuild the connection |
| Cracked body or damaged flange | Visible damage | Replace the drain body |
| Leak from elsewhere | Water appears but the shower is not used | Check supply pipework and adjacent areas |
How to narrow it down
- Stop using the shower for a period and see whether damp persists. If it does, the source may not be the shower drain at all.
- Flood test the shower floor without running the shower — this tests the membrane connection.
- Flow test — run the shower and watch. A leak that only appears during flow points to the outlet joint or the pipe run.
- Check the walls — water can travel from a wall connection and appear at the drain.
- Inspect from below if the ceiling allows access.
Prevention at installation
- Bond the membrane continuously over the flange — the most common single failure
- Support the drain body rigidly, especially on timber subfloors
- Water test at membrane stage, before screed and tile
- Use one manufacturer’s waterproofing system for membrane, primer and sealing accessories
Related: waterproofing around a drain · testing an installation