A bath hides a surprising amount of plumbing behind its panel: a waste, an overflow, a trap, two supply pipes, the tap connections and, often, the pipework for a shower over the bath. Any of them can leak, and because the space under a bath is enclosed and rarely looked at, the first sign is often a stain on the ceiling below, a soft floor or a smell.
This guide covers the signs of a leak under a bath, the parts that most often fail, how to inspect behind the panel, and a flood test that separates the causes. If the leak is below a shower tray rather than a bath, our guide to a bathroom ceiling leaking below the shower covers trays and enclosures.
What are the signs of a leak under the bath?
- a stain, bubbling paint or a drip on the ceiling of the room below, often near the tap end or the plughole end;
- a bath panel that is swollen, stained or warped at the bottom edge;
- flooring that lifts, feels soft or is discoloured next to the bath;
- a musty smell in the bathroom that does not clear;
- silicone along the bath edge that is black, lifting or has gaps;
- water on the floor after a bath or shower with no obvious splash;
- a bath that moves or creaks when you step in.
If water is already coming through the ceiling below, deal with that first. Our guide to water coming through a ceiling covers the immediate steps, including making the electrics safe.
What usually leaks under a bath?
| Part | Common failure | When it leaks |
|---|---|---|
| Waste fitting at the plughole | Perished or poorly seated seal, loose backnut | When the bath holds water, and while it drains |
| Overflow | Loose overflow fitting, perished washer, split overflow hose | Only when the water level reaches the overflow, or during a deep bath |
| Trap and waste pipe | Loose screw joint, cracked trap, blocked trap backing up, pipe pulled out of its fitting | While the bath drains |
| Taps and tap tails | Loose tap connector, weeping flexible tail, worn tap backnut | All the time, or while the taps run |
| Supply pipes | Weeping joint or isolation valve | All the time |
| Silicone and tiles | Gaps where the bath meets the wall, often made worse by a bath that flexes | During showers over the bath, or when water splashes on the edge |
| The bath itself | Cracked acrylic, often near the waste or where it is poorly supported | When the bath holds water |
How do you look behind the bath panel?
Most bath panels are held by clips, screws, magnets or a plinth strip along the floor. Some are sealed with silicone along the bath edge, which needs cutting carefully with a blade before the panel is eased away. Tiled or boxed-in baths may have a small access hatch, or none at all.
- Turn off the lights if water is near them and keep the floor dry.
- Remove the panel gently. Start at a corner and work along. Plastic panels crack if forced.
- Use a torch and a mirror. The waste and overflow are usually at one end, under the plughole. The tap connections are at the tap end, often high up and tight against the wall.
- Look for evidence. Water marks on the floor, white scale or green staining on fittings, rust on the bath cradle, and wet or dark timber.
- Dry everything you can reach. Then place dry kitchen towel under the trap, under the overflow and below each tap connection. This makes the test that follows much easier to read.
- Photograph what you find before changing anything.
If the bath has no panel access at all, the tests below still work, but you will be reading the results from the ceiling below rather than from the pipes.
How do you flood test a bath for leaks?
The idea is to test one part at a time, starting with the pipes that are always under pressure and ending with the parts that only leak when the bath is full. Allow time for each step to show, and keep the walls, edges and silicone dry until the last step.
- Leave the bath unused. Check the towel under the taps and supply pipes after a few hours. If it is wet with nobody using the bath, the taps, tails or supply pipes are leaking.
- Run the taps with the plug out. Let water run straight down the plughole for a few minutes, without splashing the edges. Wet towel under the trap now points to the waste fitting, trap or waste pipe. Wet towel at the tap end points to the tap connections under flow.
- Fill the bath below the overflow. Put the plug in and fill to a level well below the overflow. Mark the water level and leave it. A falling level or wet towel under the plughole points to the waste seal or a crack in the bath.
- Raise the level to the overflow. Add water until it just starts to reach the overflow opening. Wet towel behind the overflow now points to the overflow fitting or its hose.
- Release the water in one go. Pull the plug. The full flow tests the trap and waste pipe under the heaviest load.
- Test the edges last. Pour or spray water along the silicone and tiles, one side at a time, and watch the ceiling below.
If the ceiling below is already wet, check it after each step rather than only at the end. And if the bath has a shower over it, test the shower valve and its pipework separately, as they can leak whether or not the bath is used.
What can you fix yourself under a bath?
Some faults are simple to put right once found:
- A loose trap joint can often be hand-tightened. Check the rubber seal inside is present and seated first.
- A blocked trap can be removed and cleaned over a bucket.
- Failed silicone can be cut out, the surface cleaned and dried, and new sealant applied. Filling the bath with water before sealing, and leaving it full while the sealant cures, helps stop the joint tearing when the bath is next used.
Waste and overflow seals, tap connections and supply pipe joints are usually jobs for a plumber, particularly where access is tight or the fittings are old. A cracked bath, or one that flexes because its feet or cradle are not properly supported, generally needs attention from a bathroom fitter.
Why does the leak only show on the ceiling below?
Water under a bath lands on the floor first. On a timber floor, it soaks into the boards and runs along joists before reaching the ceiling, so the stain below may be some distance from the bath. On a solid floor, it can spread under the floor covering and into the next room. That is why the stain position is only a rough guide, and why moisture readings across the floor and ceiling help confirm the source.
Before any ceiling is cut or floor lifted, photograph the damage. If you are thinking about an insurance claim, our escape of water evidence checklist lists what to record.
When should you call a leak detection specialist?
- the bath is tiled in with no access and the ceiling below is wet;
- the flood test steps do not give a clear answer;
- the damp continues when the bath is not used, pointing to hidden supply pipework;
- a shower over the bath has a concealed valve;
- you need readings and a report for an insurer, landlord or neighbour.
Our water leak detection service uses pressure testing, thermal imaging and moisture mapping. A combined detection package is £250 to £500, and a thermal imaging survey is £150 to £250. Call 020 3912 0200 to discuss it.
Frequently asked questions
Why is water leaking from under my bath?
The usual causes are a waste or overflow seal, a loose or blocked trap, a leaking tap connection or supply pipe, failed silicone along the wall, or a crack in the bath. Testing each in turn shows which one it is.
How do I know if my bath overflow is leaking?
Fill the bath to just below the overflow and check under it, then raise the level until water reaches the overflow. If the area behind the overflow only gets wet at the higher level, the overflow fitting or hose is leaking.
Can a bath leak through the ceiling below?
Yes. Water under a bath runs across the floor and through to the ceiling below, often appearing some way from the bath itself.
Should I stop using the bath if it is leaking?
Yes, until the cause is found. If the supply side is leaking, close the isolation valves to the bath taps or the main stopcock as well.
Do I need to remove the bath to fix a leak?
Usually not. Most waste, trap and tap leaks can be reached with the panel off. Removing the bath tends to be needed only for a cracked bath or where there is no access at all.
