Leak Under the Bath? Here's How to Find and Fix It

A bath leaking underneath is almost always down to one of three things: the sealant around the edge has failed, the waste trap under the plughole is dripping, or the overflow connection is letting water past when the bath is filled close to the top. Each one leaves a different clue, which is why testing them separately is faster than guessing and pulling the whole panel off.

Failed sealant is the most common cause by a wide margin, especially in older bathrooms where the silicone has never been replaced — it shrinks, cracks and pulls away from the tile line, letting bathwater run straight down behind the panel every time someone bathes rather than showers. A dripping waste trap or a leaking overflow connector are the next most likely faults if the sealant line looks intact.

A bath leak that's come through to the ceiling below means water has been running for some time, tracking along the joists under the bathroom floor before finding a gap in the plasterboard — it's rarely a one-off splash. In London flats and converted terraces where a bathroom sits directly above a bedroom or living room, this is one of the most common leak callouts we see, particularly in Tower Hamlets, Hackney and Islington's older housing stock.

A brown ring, a soft patch, or bulging paint on the ceiling below is the giveaway sign — if you spot any of these, treat it as more urgent than a simple drip, since the leak has already been running long enough to soak through plasterboard.

You can isolate the source in about fifteen minutes by testing each one under the specific condition that makes it leak. Sealant and overflow faults only show when the bath is full and standing; a waste trap fault only shows while water is actively draining through it.

• Sealant: run the bath, put the plug in, and check under the panel while it sits full and still — a drip with the water static and not draining points to the sealant line, not the waste.
• Waste trap: pull the plug and watch the trap and its joints while the water runs out — a leak that only appears during draining is the waste, not the sealant or overflow.
• Overflow: fill the bath close to the overflow slot specifically and check the overflow pipe and its connector — this only leaks under that condition, so a normal half-full bath won't reveal it.

Most baths have an access panel built into the end nearest the taps, and that section usually unclips or comes away with a couple of screws without disturbing the rest of the panel run. That's where the waste trap and overflow connection live, so it's normally all you need to open.

If your bathroom has no access panel at all — common in older London bathrooms that were tiled or panelled before an inspection point was fitted — cutting a discreet hatch into the panel with a jigsaw is far quicker and less disruptive than lifting the whole panel or tiling off. A plumber can fit a proper access panel while they're there, so you're not left with a rough cut edge.

Tracing a bath leak takes about fifteen minutes if you work through the sealant, waste and overflow in order, and you don't need to disturb the bath itself to do it.

1. Check the sealant line along the bath edge for cracking, gaps, or peeling silicone.
2. Open the access panel nearest the taps to reach the waste trap and overflow.
3. Fill the bath and check under the panel while it's full and standing — a drip here points to sealant or overflow.
4. Drain the bath and watch the waste trap and its joints while the water runs out.
5. Fill close to the overflow slot separately and check the overflow pipe and connector.
6. Re-seal the bath edge, or tighten/replace the trap, washer or overflow connector, depending on what you've found.
7. Check the ceiling in the room below for staining or a soft patch, confirming how much water has already got through.

Re-sealing is a job most people can do themselves with a sealant gun and a bit of patience — strip the old silicone out completely first, since applying fresh sealant over old cracked silicone just fails again within weeks.

Call a plumber if re-sealing doesn't stop the leak, if the waste trap or overflow connector needs replacing rather than tightening, or if the ceiling below is already stained and you need the source confirmed before repairing it. RenoPlumb carries traps, washers, overflow connectors and sealant on every van and covers Tower Hamlets, Hackney, Islington, Newham, Southwark and the rest of East and Central London.

For a full diagnosis, including leaks that seem to disappear as soon as you start looking for them, see our leak detection & repair service or call 07460 824073 for a free quote.

The cost depends on whether the fix is a re-seal, a trap or overflow replacement, or extends to repairing a ceiling below, so we always confirm the price with a free quote before any work starts. For a general sense of how plumbing jobs in London are priced, see our pricing page.

Need this fixed by someone local? See our page for a plumber in Newham for borough-specific coverage and response times.