Leak in the Ceiling Below a Bathroom: How to Find the Source

Updated July 2026 · Emergency Plumber Chelsea

Make it safe before you investigate. If the wet patch is near a ceiling light, a downlight or a smoke alarm, switch off that lighting circuit at the consumer unit from a dry spot and do not touch the light switch. A ceiling that is sagging or bulging is holding water and can come down, so keep people out from underneath it and put a bucket below the lowest point. Our guide to water coming through a light fitting covers the electrical side in more detail. Photograph the ceiling and note the time before anything is moved, because insurers usually ask for this.

Decide whether the leak is constant or only when something is used. This single question narrows it down more than anything else. Stop using the bathroom above completely for an hour or two. If the ceiling keeps dripping, or the wet patch keeps spreading, the water is coming from something that is always under pressure: the cold mains, the hot water supply, or a central heating pipe feeding a radiator or towel rail. Turn off the main stopcock, and if the drip slows and stops over the next hour, you have confirmed a supply leak. Our guide on how to turn off a stopcock shows where to look. If the ceiling dries out while nobody uses the bathroom, the leak is on the waste side or from water escaping during use, which is less urgent but still needs finding.

Check the water meter if you have one. With every tap, appliance and toilet in the house off and not refilling, note the reading or watch the small dial for ten minutes. If it moves, water is escaping somewhere on the supply side. Heating pipes will not show on the meter because a sealed heating system is not drawing from the mains, so on a combi or system boiler also watch the pressure gauge. A gauge that keeps falling over a day or two, with a wet ceiling under a radiator or towel rail, points to a heating leak. Our guide on how to read your water meter walks through the test.

Test the shower first, because it is the most common source. Shower leaks usually come from the silicone seal around the tray, cracked or porous grout, a tray that has cracked or moved, or the waste fitting under the tray. Run the shower with the head pointing straight down into the middle of the tray, not at the walls, for five minutes. If the ceiling stays dry, now spray the walls and the seal along the tray edge. If it drips only when the walls and edges get wet, the water is getting past the seal or the grout rather than through the waste. Our guide to a leaking shower tray explains how to tell a seal leak from a waste or tray leak.

Remember that a shower can also leak behind the tiles. The mixer valve, the pipes feeding it and the connection to a fixed head or riser are often buried in the wall. A leak there can run whether or not anyone is showering if it is on the supply side of the valve, or only while the shower is running if it is on the outlet side. Water from a concealed valve often shows as damp on the wall on the other side, loose tiles or a stain that tracks down to the ceiling below. That kind of leak needs the valve area opened up or accessed from behind, so it is a job for a plumber.

Test the bath next, in stages. Put the plug in and run a few centimetres of water, then pull the plug and watch the ceiling below while it drains. Then fill it until the water just reaches the overflow hole. Then splash water along the seal where the bath meets the wall. A drip after draining points to the waste or trap, a drip only when it is filled high points to the overflow, and a drip after splashing points to the seal. Our guide to a leaking bath covers each of those tests in detail, including how to get the bath panel off.

Then the toilet. Flush it several times and watch the ceiling, then leave it alone for an hour. A leak only when flushing usually comes from the joint between the pan and the soil pipe, the flush pipe or seal between cistern and pan, or on a close-coupled toilet, the washers on the bolts that hold the cistern to the pan. A leak that continues without flushing is on the fill side: the flexible connector or isolation valve feeding the cistern. If the toilet has a concealed cistern hidden behind a panel or wall, the fill valve, flush valve and connections are out of sight, and our guide to concealed cistern problems explains the warning signs. Beads of water on the outside of a cold cistern can also drip steadily in humid weather, which our guide to toilet cistern condensation covers. Our guide to a toilet leaking from the back explains how to tell a supply leak from a seal or pan connector leak.

Then the basin. Run the hot and cold taps into the basin with the plug out for a couple of minutes, then fill it and let it drain. Most basin leaks come from the flexible tap connectors, the waste fitting or the trap, and they usually show inside a vanity unit or behind the pedestal before they reach the ceiling. Our guide to a leak under the bathroom sink goes through each connection.

Where the water shows is not always where it starts. Water runs along joists, pipes and the underside of floorboards before it drips, so the stain can be a metre or more from the fixture that is leaking. In Chelsea's period houses and mansion blocks, lath and plaster ceilings can hold a lot of water before they show a mark, and bathrooms have often been moved or added over the years with pipes run through old joists. If the tests above do not give a clear answer, guessing and cutting holes wastes time and money. Our water leak detection service uses moisture mapping, thermal imaging and acoustic equipment to pinpoint the source with as little opening up as possible.

If you live in a flat and the bathroom above is not yours, the steps are different, because the leak has to be found and fixed in someone else's home. Read our guide on a ceiling leak from the flat above for how to deal with the neighbour and the managing agent. If the ceiling is stained but dry and you are not sure the leak is still active, our guide to a water stain on the ceiling but no leak explains what an old stain can and cannot tell you.

Let the ceiling dry fully before it is repaired. Once the leak is fixed, a wet plasterboard ceiling can often be dried and redecorated with a stain-blocking primer, but a ceiling that has sagged, softened or come away from the joists usually needs that section replaced. Escape of water damage is often covered by buildings insurance even when the repair to the leaking fitting is not, so keep your photos and the plumber's report. Our guide to an escape of water insurance claim explains what insurers usually want.

If water is coming through the ceiling below your bathroom and you cannot find the source, or it will not stop with the stopcock off, call Emergency Plumber Chelsea on 020 3002 6667. We trace and repair shower, bath, toilet, basin and pipe leaks in houses and flats across Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham, and our bathroom plumbing service covers the repair once the source is found. There is no call-out fee on jobs over 3 hours.

Quick tips

• Switch off the lighting circuit at the consumer unit if the leak is near a light or downlight.
• Stop using the bathroom for an hour: a ceiling that keeps dripping means a supply or heating pipe.
• Test one fixture at a time, shower first, then bath, toilet and basin, and watch the ceiling after each.
• A drip only after showers usually means the seal, grout or tray waste, not a burst pipe.
• Photograph the ceiling before anything is moved; insurers usually ask for it.

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