A leak under the bath rarely announces itself. Most of the time the first sign is somewhere else entirely: a soft patch of ceiling in the room below, a tide mark on a skirting, a floorboard that flexes underfoot, or a neighbour knocking on your door because their hallway light fitting has water in it. By then the leak has usually been running for weeks.
This page is written for the moment you have found it. First, how to stop the water and limit the damage. Then how to work out which of the six or seven things under a bath is actually leaking, because they all look identical from a wet floor. Then what the repair really involves in a Central London flat, where the bath is often boxed in, the pipework is undocumented, and the room below belongs to someone else.
The first ten minutes
Stop using the bath. That sounds obvious, but if the leak only appears when the bath is filled or drained, the single most useful thing you can do is stop adding water while you work out what is happening. A leak that is not being fed will slow down, and a slower leak gives you time to think.
Next, isolate the supply if you can. Under most modern baths there are two small isolation valves on the hot and cold tails feeding the taps. They are usually chrome, roughly the size of a large coin, with a slot for a flat-blade screwdriver. Turn the slot so it sits across the pipe rather than along it and the water to that tap is off. That is a safe job for a homeowner and it costs nothing to try.
If there are no isolation valves, or you cannot reach them, find your stopcock. In a purpose-built flat it is commonly under the kitchen sink, in an airing cupboard, or in a service cupboard on the landing. In a Victorian conversion it can be almost anywhere: behind a bath panel, under a floorboard in a hallway, at the back of a base unit, or in a communal cupboard shared with the flat next door. If you have never found yours, find it now while the room is dry and the light is on. Turn it a couple of turns each way once or twice a year so it does not seize.
Then deal with the water that has already escaped. Get towels under the bath, lift anything absorbent off the floor, and if the ceiling below is bulging, do not ignore it. A bulge means water is sitting in a pocket of paint or plaster and the weight is building. It is far better for a filled bulge to be released deliberately into a bucket than for it to fail on its own at three in the morning.
If the leak is running fast enough to be crossing into another flat, tell the flat below now, before you have a diagnosis. It is a much easier conversation early than late.
Working out what is actually leaking
Under a bath there are several separate systems, and the repair for each is completely different. Diagnosis matters, because guessing usually means opening the wrong thing.
The waste and trap. The large plastic assembly under the plughole. This only leaks when the bath drains. If your floor is dry all day and wet ten minutes after someone empties the bath, this is the first suspect. Traps loosen, the rubber washer under the plughole perishes, and the flexible waste pipe behind the bath gets knocked out of its seal when someone reaches in to retrieve something.
The overflow. The chrome cover near the tap end connects to the waste with a short pipe. If the bath is regularly filled high, or the seal behind that chrome cover has failed, water runs down the outside of the overflow pipe rather than through it. Classic symptom: wet only when the bath is filled to the brim.
The tap tails and connectors. The flexible hoses connecting the taps to the pipework. These weep slowly and steadily whether the bath is in use or not, and because they sit at the far end of the bath the water often travels along a joist before it appears somewhere else entirely. Constant damp with no relation to bath use points here.
The supply pipework itself. A compression joint that has been nudged, a soldered joint that has been stressed, or in older stock a length of pipe that is simply at the end of its life. In many Central London conversions the pipework under a bathroom floor is a museum of every plumber who has been there since the 1950s.
The seal around the bath. Not a plumbing leak at all, and probably the most common cause of all. Failed silicone or grout at the bath-to-wall junction lets shower water run behind the bath and straight onto the floor. Symptom: only wet when someone showers, never when the bath is filled and drained normally. If that is your pattern, the plumbing is fine and the problem is sealing and tiling.
The floor itself. In a wet room or a heavily tiled bathroom, water can pass through failed grout, run across a floor that was never properly tanked, and emerge metres away. If you are dealing with that, the honest answer is that the room is failing, not a fitting, and our guide to wet room waterproofing and tanking covers why.
The test that saves you money
Before anyone opens up your bathroom, run a simple sequence and write down what happens. Dry the floor completely. Fill the bath a few inches, leave it standing for twenty minutes, and check. Then fill it to just below the overflow and check again. Then pull the plug and watch. Then run the shower over the seal for two minutes with the bath empty and check once more.
Whichever step produces water tells you the system. Standing water only means waste or trap. Wet only at high level means overflow. Wet only when draining means waste. Wet at all times regardless means supply. Wet only when showering means seals or tiling. Ten minutes of this is worth more than an hour of somebody guessing, and it means the person who attends arrives with the right parts.
Why the flat below changes everything
In a house, a bath leak is your problem and your ceiling. In a Central London flat it is very often someone else's ceiling, and that turns a repair into a claim.
Escape of water travelling between flats typically engages the building's insurance rather than your contents policy, and in most leases the buildings policy is arranged by the freeholder or managing agent. What that means in practice is that the flat below will normally report damage to the managing agent, the managing agent will want to know the cause and the date, and somebody will ask whether the leak came from your demise. Exactly how liability lands depends entirely on the wording of your lease and the policy — you will need to check both rather than assume.
Two practical consequences. First, evidence is worth having. Photograph the leak, the wet area, the fitting you suspect, and anything the plumber removes. Note the date you first noticed it and the date it was stopped. Second, act quickly and be seen to act quickly. In our experience of working in mansion blocks and conversions across Marylebone and Maida Vale, the disputes that get ugly are the ones where a leak was known about and left running. A leak stopped the same day is a repair. A leak left for a month is an argument.
If the water is not slowing and it is heading downwards, treat it as urgent rather than something to book for next week. Our guide to calling an emergency plumber in Central London sets out how that process works.
Getting to the bath — the part nobody plans for
Almost every bath in Central London is boxed in, and how it is boxed in decides how hard the job is.
A removable side panel with screws is the best case. Take it off, and everything except the far end is reachable. Half an hour of work and the panel goes back.
A tiled bath panel is the common case and the difficult one. Somewhere along the line a bathroom fitter built a stud frame around the bath, tiled it, and grouted it in. There is no access. To reach the trap you have to cut the panel out, and the tiles will not survive. That is why we always ask about the panel before quoting: the plumbing repair might be twenty minutes, and the making-good afterwards might be the bulk of the job. If your tiles are discontinued, and in period stock they usually are, you are looking at either a patch that will always be visible or a wider re-tile. Our tiling page covers how we handle matching and reinstatement.
The far end — the tap end, against the wall — is the worst case. If the leak is on the tap tails behind a tiled panel with no hatch, the honest options are to open the panel, open the wall, or come at it from the room next door. All three are decisions with cost attached, and it is better to make them consciously than to discover them halfway through.
Whenever we do open a boxed bath, we put an access hatch back in. It costs very little at the time and it means the next repair is minutes rather than a demolition. If you are already planning a bathroom refurbishment, ask for hatches at every trap, valve and connection. It is the single cheapest thing you can specify.
What the repair usually involves
Most bath leaks are genuinely small repairs once you can see them. A trap reseated with a new washer. A waste connector replaced. An overflow reseated with fresh sealant behind the escutcheon. A flexible tap tail swapped for a new one. A compression joint remade properly. These are the everyday jobs behind our plumbing service, and our plumbing call-outs start from £80 with a fixed price agreed before work starts.
Some are not small. If the pipework under the floor is failing rather than a fitting, the question becomes how much of it to replace while the floor is up, and the sensible answer is usually more than the minimum. Opening a Central London bathroom floor twice is a waste of everyone's money. If the pipe is old lead, that is a separate conversation covered in our guide to lead pipe replacement in London.
And some are not plumbing at all. If we test the room and the leak only appears when the shower runs, the fix is stripping and reforming the seal properly, or addressing the tiling, not changing a trap. We would rather tell you that on day one than sell you a repair that will not stop the water.
Constraints particular to Central London stock
Three things reliably complicate this job here.
Undocumented pipework. A Victorian house in Bayswater converted into flats in the 1970s and refitted twice since has pipework nobody has a record of. Runs cross under floors into other rooms. Isolation valves are in unexpected places. Occasionally a pipe under your floor serves a flat that is not yours. This is why we lift and look rather than commit to a scope from a photograph alone.
Fabric you cannot cut. In listed buildings and in the older townhouse stock around Mayfair and Fitzrovia, chasing into original fabric, lifting original boards carelessly, or altering historic joinery can engage listed building consent. Most bath repairs are far too minor to trouble anyone, but the moment the work grows into altering the room, check your position rather than assume. Similarly, most leases require freeholder or managing-agent consent for altering waste and soil runs — confirm before you plan around a new layout.
Access and timing. Blocks with porters often have set contractor hours and a booking procedure, and noisy work is frequently restricted to weekday daytimes. A leak that is actively running is usually treated differently to planned work, but the only way to know is to ask your porter or managing agent. It is worth knowing your building's rules before you need them.
Stopping it happening again
Bath leaks are boring and preventable. Re-seal the bath-to-wall junction properly when it starts to discolour or lift rather than running a new bead over an old one. Do not overfill. Check under the bath twice a year if you have a panel that comes off. Do not let a slow weep on a tap tail run for months on the basis that it is only a drip — under a bathroom floor, a drip is a wet joist.
If you own several flats or manage a building, this is exactly the kind of thing that belongs on a schedule rather than in an emergency. Our maintenance service exists for that: look at seals, traps, valves and stopcocks on a planned visit and most of these calls never happen.
How we handle it
Send a photo of the wet area and the bath, tell us what you found in the test sequence above, and we will tell you what we think it is and what it will cost to put right. Plumbing starts from £80, the price is fixed before we start, and if opening a panel is going to mean tiling work afterwards we will say so upfront rather than discover it with your bathroom in pieces. We work across W1, W2, NW8 and W9 — you can reach a plumber in Marylebone, Bayswater or the surrounding areas through the same quote form.