In a conventional bathroom, the shower tray catches the water. In a wet room, the whole floor does that job. That makes a wet room less forgiving: the tiles are only the surface, and what actually keeps water out of the floor and the ceiling below is a waterproof layer under them, a drain sealed into that layer, and a floor laid so the water runs towards the drain. When any one of those fails, water goes somewhere it should not.
This guide explains where wet rooms usually leak, the signs that point to each cause, and how a flood test and moisture mapping narrow things down before anyone starts lifting tiles.
Why do wet rooms leak?
A wet room works as a system of three parts.
- The tanking. A waterproof membrane, liquid coating or waterproof board system under the tiles, lapped up the walls. Grout and tiles are not waterproof on their own, so this layer is the real barrier.
- The drain. A linear channel or a round gully, sealed into the tanking so that water reaching the membrane is carried away rather than getting round the edge.
- The falls. The floor is laid, often on a sloped former or screed, so that water runs to the drain instead of sitting on the surface or flowing to the doorway.
Add the supply pipes and the shower valve, which are in the walls, and you have the full list of places a wet room can leak.
Where does a wet room usually leak?
| Source | Common failure | Typical pattern |
|---|---|---|
| Tanking | Gaps at corners, board joints, wall to floor junctions and pipe penetrations; membrane not taken high enough up the walls | Damp after use, often at the edge of the room or below a particular wall |
| Drain | Membrane not properly clamped or bonded to the drain body; a cracked drain body or loose clamping ring | Damp directly below the drain, appearing soon after the shower runs |
| Waste pipe and trap | Loose or cracked joint below the former; a blocked trap backing up | Leak when a lot of water goes down at once, sometimes some distance along the pipe route |
| Falls | Floor too flat, or sloping the wrong way in places | Puddles that stay after showering, water reaching the door threshold, damp in the next room |
| Supply pipes and valve | Weeping joint or valve connection in the wall | Damp that continues when the room has not been used |
What are the signs that a wet room is leaking?
- a stain or drip on the ceiling below, particularly under the drain or along one wall;
- grout that stays dark long after the room has dried, or tiles that sound hollow or move;
- damp or swelling at the bottom of the door frame or on the other side of the doorway;
- skirting or plaster in the next room that is damp at low level;
- a musty smell that does not clear with ventilation;
- standing water that never quite drains away.
As with any bathroom leak, when the ceiling gets wet tells you a lot. If you have a conventional shower rather than a wet room, our guide to a bathroom ceiling leaking below the shower covers trays and enclosures.
How is a wet room flood test done?
A flood test checks the tanking and the drain seal by holding water on the floor and watching what happens below. It is done in stages so that each stage tests one thing.
- Check the room below first. If the ceiling is already saturated, sagging or near a light fitting, deal with that before adding more water. Isolate the lighting circuit if in doubt.
- Test the supply side separately. Leave the room unused and watch the damp. If it keeps spreading, the pipes or valve are the stronger suspect and a pressure test comes first.
- Test the drain and waste. Run water from a hand-held head or a hose straight into the drain, keeping the rest of the floor and the walls dry. A leak now points to the drain body, the trap or the waste pipe.
- Test the floor tanking. Block the drain with a suitable temporary seal, build a small dam at the doorway if needed, and put a shallow depth of water on the floor. Mark the water level and watch it, and the ceiling below, over several hours.
- Test the walls. Wet each wall in turn with the hand-held head, pausing between walls, to see whether one side lets water through.
- Release and observe. Remove the seal and let the water drain in one go, which puts the waste run under a full flow.
A falling water level with no leak below can simply be evaporation or water soaking into grout, so the level is read alongside what is happening underneath. The test has a cost: if the tanking has failed, it puts more water into the floor. That is why it is done with a plan, and with someone watching the ceiling below.
How does moisture mapping below the wet room help?
Moisture mapping is the quieter half of wet room leak detection. Readings are taken in a grid across the ceiling below, the adjoining walls and the floor at the doorway, using moisture meters and thermal imaging. The pattern shows where the water is concentrated.
- Wettest directly under the drain points to the drain seal or the waste.
- Wettest along one edge points to the tanking at that wall to floor junction.
- Wettest at the doorway points to the falls, a missing upstand, or tanking that stops short of the threshold.
- Wet along a line away from the room points to the waste pipe route or a supply pipe.
Repeating the readings before and after each stage of the flood test turns a vague damp patch into clear evidence of which part failed. Our damp and moisture detection service covers this type of mapping.
Can poor falls cause a wet room to leak?
Yes, indirectly. Poor falls do not break the tanking, but they leave water sitting on the floor for longer, often against the edges where the tanking is most likely to have a weak point. They can also send water towards the doorway, where the membrane may not extend. Signs of a falls problem are puddles that remain after showering, water tracking out of the showering area, and damp at the threshold rather than under the drain.
A simple check is to pour a small amount of water at different points across the floor and watch where it goes. Water that runs away from the drain, or sits still, tells you the falls need attention as part of any repair.
Can a leaking wet room be repaired without ripping it out?
Sometimes, and the answer depends on what the tests show.
- Drain seal or trap: some drains allow access to the trap and clamping ring through the grating, so a loose ring or blocked trap can be dealt with from above.
- Waste pipe joint: may be reachable from the ceiling below or a floor void, through a small opening.
- Supply pipe or valve: often reachable through the valve faceplate, an access panel or a small opening on the other side of the wall.
- Localised tanking failure: usually means lifting the tiles over that area and making good the membrane before re-tiling.
- Widespread tanking failure or poor falls: generally means stripping the floor and re-tanking it properly.
Regrouting or resealing over a failed membrane rarely lasts, because grout is not the waterproof layer. It can make the room look fine while water carries on getting through. Before the floor is lifted, the wet screed or boards underneath also need time to dry, and our guide to finding a leak under a tiled floor covers what happens when tiles have to come up.
When should you call a leak detection specialist?
Call a specialist when:
- the ceiling or the room next door is damp and you cannot tell whether it is the drain, the tanking or a pipe;
- regrouting or resealing has not stopped the leak;
- the wet room is on an upper floor of a flat and the neighbour below is affected;
- you need evidence of the cause for an insurer, a builder or a managing agent before work starts.
Our combined detection package, which covers pressure testing, thermal imaging and moisture readings, is £250 to £500. A damp and moisture survey on its own is £250 to £450. Call 020 3912 0200 to discuss your wet room.
Frequently asked questions
Why is my wet room leaking through the ceiling?
The usual causes are a gap or split in the tanking, a drain that is not properly sealed to the tanking, a leaking trap or waste pipe, poor falls that leave water at the edges, or a leaking supply pipe or valve in the wall. Testing each part separately shows which one it is.
How do you test a wet room for leaks?
By testing one part at a time: leaving the room unused to check the supply pipes, running water only into the drain, holding a shallow depth of water on the floor with the drain sealed, and wetting each wall in turn, while watching and measuring the damp below.
Can I just regrout my wet room to stop the leak?
Only if the tanking underneath is sound and the grout has simply failed. Grout is not waterproof, so if water is reaching the ceiling below, the tanking or drain seal is usually involved and regrouting alone will not fix it.
How long should a wet room flood test run?
Long enough for a slow leak to show below, which is usually several hours. Water levels and moisture readings are compared at the start and end rather than relying on a single look.
Does the whole wet room floor have to come up?
Not always. Drain, trap, waste and pipe leaks can often be reached through small openings. A tanking failure in one area usually means lifting the tiles there, and only widespread failure or poor falls tend to need the whole floor redone.
