If your bath is leaking, time when the water appears. Water only during a shower or bath, tracking along the edge, usually means failed sealant around the rim. Water that keeps appearing after the bath is empty, or drips as it drains, points to the waste pipe or overflow. A ceiling stain below the bathroom means it's been going on longer than you think and needs checking properly.
In This Guide
Why timing tells you almost everything
A bath leaking rarely needs expensive detective work if you pay attention to when the water shows up. Sealant problems and pipe problems behave differently, and a few minutes of watching, rather than guessing, usually points you straight at the cause.
Run a bath, get in, and watch the skirting board or the ceiling below (if you can get access) during and just after. Then watch again once the bath has been empty for twenty minutes. The pattern you see across those two windows does most of the diagnosis for you.
Sealant failure: what it looks like
Sealant, sometimes called caulk, runs along the joint between the bath and the wall tiles, and around the base where the bath meets the floor. It's there to stop splashed water getting behind the bath panel or into the floor, not to hold back any real volume of water.
When sealant fails, water only appears while there's active splashing, meaning during a shower over the bath or while someone's getting in and out. It typically shows as a damp patch right at the join, sometimes with visible cracking, peeling or a gap you can see daylight through. Once the water stops moving, the leak stops too.
Quick sealant check
Run a fingernail gently along the sealant line. Cracked, brittle or gapped sealant will usually show itself this way, and a section that's gone grey, mouldy or has pulled away from either surface is the likely entry point.
Waste pipe and overflow leaks
The waste pipe carries water away when you pull the plug, and the overflow is the safety outlet near the taps that stops the bath flooding if it's filled too high. Both have joints, seals and washers that can fail over time.
A waste pipe leak usually shows while the bath is draining, sometimes for a few minutes after, as the last of the water works its way through a loose joint. An overflow leak often only appears when the bath is filled close to the top, since that's the only time water reaches the overflow outlet at all.
Watching the waste while it drains
If you can see the pipework under the bath, watching it during draining is one of the most useful five minutes you'll spend. A drip at a joint while water is flowing, that stops once the bath is empty, is a strong pointer to a loose or worn waste connection.
The access panel test
Most baths have a removable panel on the front, sometimes held with clips, sometimes screwed in. Taking it off gives you a direct look at the waste trap, the overflow pipe and the underside of the bath itself, rather than guessing from stains outside the panel.
Run the bath, pull the plug, and watch the panel opening during draining with a torch. Note exactly which joint or fitting is wet. This single check often does more to narrow things down than anything else on this list, because you're looking directly at the source rather than at where water has travelled to.
If you can smell damp or see mould inside the panel area that you didn't expect, the leak has probably been running for a while before you noticed it on the surface. Don't just reseal and close the panel back up without checking further.
When it's neither sealant nor waste pipe
Sometimes neither test points anywhere obvious, and that's usually when a hidden supply pipe is involved instead. A leaking hot or cold feed to the taps, or a mixer valve fault inside the wall, can produce dampness that has nothing to do with the bath's own fittings, and it won't show up during a simple fill-and-drain test because it's leaking constantly, tap on or off.
This is where a proper plumbing leak detection survey earns its keep. Acoustic listening equipment can pick up the sound of water escaping a pressurised supply pipe behind tiles, without lifting a single one to look.
Tested it yourself and still not sure
We use acoustic and thermal methods to trace a bathroom leak back to its source, whether it's a supply pipe, waste pipe or something behind the tiles.
What a stained ceiling below tells you
If you have a bathroom above a room downstairs and a stain has appeared on that ceiling, the leak has been running long enough to soak through the floor structure. That takes more than a single splash of water from a cracked sealant line; it usually means either a repeated pattern of splashing over weeks, or a constant drip from a pipe joint.
A ceiling stain is also worth checking against the position of the bath above. A stain roughly under the bath's edge points toward sealant or the bath's own fittings; a stain further along, tracking toward where the supply pipes run, points more toward a hidden pipe leak.
| Sign | Sealant failure | Waste pipe / overflow | Hidden supply pipe |
|---|---|---|---|
| When water appears | During use only | During or just after draining | Constant, tap on or off |
| Location | Bath-to-wall or bath-to-floor join | Under the bath, near the trap | Behind tiles, in the wall |
| Visible clue | Cracked, gapped or mouldy sealant | Damp or drip at a pipe joint | Often none, until ceiling below stains |
| Fix | Reseal the joint | Tighten or replace the fitting | Needs pipe location and repair |
Fixing sealant vs fixing a pipe
Resealing is a straightforward DIY job for most people: strip out the old sealant, clean and dry the joint fully, then apply a new bead of bathroom sealant and let it cure before using the bath again. A loose waste pipe joint is often just as simple, sometimes needing nothing more than tightening a nut or replacing a worn washer.
A hidden supply pipe leak is a different job entirely. It usually means opening tiles or plaster to reach the pipe, which is exactly why finding the precise spot first matters. We see this most weeks: someone reseals the bath, the damp patch dries up briefly from the redecorating, and then returns a few weeks later because the sealant was never the actual problem.
- Time it. Watch for water during use, during draining, and twenty minutes after the bath is empty.
- Check the sealant line. Cracking, gaps or mould are visible clues you can spot yourself.
- Open the access panel. A direct look at the waste trap beats guessing from outside stains.
- Check the ceiling below. A stain there means the leak has been running longer than it looks.
- Call in a survey if it's constant. A leak with no clear on/off pattern is usually a supply pipe.
Frequently Asked Questions
Watch when the water appears. Sealant leaks only show up during active use, like a shower or someone getting in and out. Pipe leaks, especially from the waste, tend to appear during or after draining, while a supply pipe leak can be constant regardless of use.
Yes, in most cases. Strip out the old, cracked sealant, clean and fully dry the joint, then apply a fresh bead of bathroom sealant and leave it to cure before using the bath. It's one of the more straightforward bathroom repairs for a competent DIYer.
Water appearing without any recent use points away from sealant, since sealant only leaks during active splashing. It's more likely a constant drip from a supply pipe, a slow leak at the waste trap joint, or condensation from pipework, and it's worth checking the access panel to see exactly where it's coming from.
It means the leak has been running long enough to soak through the floor structure, which usually rules out a single minor splash. It could still be a repeated sealant issue over several weeks, or a constant pipe drip, so it's worth getting checked rather than just repainting the ceiling.
There's no fixed schedule, since it depends on use and how well it was applied originally. Check it whenever you notice cracking, discolouration, mould or gaps, and replace it as soon as any of those appear rather than waiting for a leak to develop.
Generally no, since the overflow only carries water when the bath is filled close to its top edge. If you consistently fill the bath fairly high, though, it's worth checking the overflow pipe and its seal the same way you'd check the waste trap.
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Stop the leak before it reaches the ceiling
Don't let a small problem become a big one. If you've ruled out sealant and the waste pipe, we can trace a hidden bathroom leak with non-invasive equipment and give you an insurance-approved report.