Why is the stain not directly under the leak?

Water runs along the top of the ceiling and down the joists until it finds the lowest point or a gap. A stain a metre or more from the actual leak is completely normal, which is why cutting a hole where the stain is often finds nothing.

The stain has dried out and stopped growing. Is it fixed?

Not necessarily. It may be an old leak, or an intermittent one from a fitting nobody has used lately. Mark the edge in pencil and date it; if it grows again, it is live.

Can I just paint over it?

Only once the leak is fixed and the plasterboard is dry, and it will need a stain-block first or the tannins come through. Painting a live leak simply hides it while the ceiling keeps soaking.

Is it safe to use the lights below?

No, not while that area is wet. Turn the lighting circuit off at the consumer unit and have it checked before those fittings go back into use.

Caution

Updated 30 August 2026

Damp Patch On The Ceiling Below The Bathroom

A stain, a bulge or a drip below an upstairs bathroom — something up there is leaking into the floor.

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Bathroom fitting work, illustrative image

What's happening?

A ceiling stain under a bathroom is the end of a journey, not the start of one. Water leaves a joint or a fitting, runs along the top of the ceiling or down a joist to the lowest point, and shows itself somewhere that may be well away from the leak. That is why guessing is expensive: the stain tells you where the water stopped, not where it began. The useful information is timing and pattern. A stain that grows only while someone showers is a shower fault; a stain that grows whether or not the bathroom is used is a supply pipe under pressure; a stain that appears after a bath is filled and emptied is the bath, its waste or its seal. A brown, sharply edged ring that has stopped growing is usually an old leak that has already been fixed or has dried up.

What usually causes it?

A failed seal or missing waterproofing behind a shower, letting water past the tiles into the floor
A shower tray moving or unsupported, breaking the joint every time it is used
A leaking waste or trap on a bath, shower or basin, which only leaks while water is running through it
A failed soil connector or floor seal on a toilet, leaking on each flush
A slow leak on a hot or cold supply pipe under the floor, which runs constantly
Grout and sealant failure round the bath, so water is tipped into the floor at the rim
Water running along a joist from a leak metres away and dropping through the first available gap

Is it dangerous?

Yes, more than it looks. A saturated plasterboard ceiling becomes heavy and loses its fixing strength, and a bulging or sagging ceiling can come down in one piece — keep out from underneath it. Where the water is near a light fitting, a downlight or a loft hatch with cabling, treat it as an electrical hazard: switch that circuit off at the consumer unit and do not use those lights. Long-term saturation rots joists and grows mould in the void where nobody sees it.

Can I fix it myself?

The diagnosis, not usually the repair. Stop using the bathroom fittings one at a time and mark the edge of the stain with a pencil and the date, so you can see whether it is still growing and what makes it grow. Test in order: no water used for a day; then only the basin; then only the toilet; then only the bath; then only the shower. That sequence points at one fitting in a couple of days and saves paying someone to do the same thing. Switch off lighting circuits below a wet ceiling and put a container under any active drip.

When should you call a bathroom fitter?

Call as soon as you have narrowed it down, and immediately if the ceiling is sagging, if water is dripping rather than staining, or if the stain grows with the bathroom unused — that last one is a pressurised supply leak and it does not stop on its own. A bathroom fitter deals with the leak where it is a tray, screen, seal, tiling, toilet or waste fault; a plumber deals with a supply pipe under pressure. If a light fitting is in the wet area, get the circuit checked before it is used again.

What will the bathroom fitter actually do?

Confirm which fitting is leaking by isolating and water-testing each one in turn
Open a small inspection hole in the ceiling or lift a floorboard above to see the leak at its source
Repair the failed component — tray support, tanking, seal, waste, connector or tiled surface
Dry the void and check the joists and decking for rot before anything is closed up
Make good the ceiling and the bathroom finishes disturbed by the investigation
Water-test the repaired fitting under normal use before signing it off

The elimination test, in order

Mark the edge of the stain in pencil and write today's date beside it.
Use no water in that bathroom for 24 hours. Growth now means a supply pipe under pressure.
Use only the basin for a day and check.
Use only the toilet for a day and check.
Fill and empty the bath, then check.
Shower only, with the screen door closed, then check.
Whichever step moves the pencil line names the fitting.

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Leaking Shower TrayCaution Toilet Rocking Or Leaking At The BaseCaution Shower Screen LeakingLow risk Wet Room Floor LeakingHigh risk

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