Signs to look for
- A damp or stained patch on a ceiling, wall or floor that grows or keeps coming back
- Peeling paint, bubbling plaster, lifting floor or a musty smell in one area
- The sound of running water when nothing is in use
- A higher metered water bill without a change in how you use water
- Wet patches outside in dry weather, or a noticeable drop in water pressure
What to do first
Do a meter check
If you have a water meter, Thames Water suggests this test: stop using water, close the inside stop tap and take a reading, then read it again after 30 minutes. If the numbers differ, water is being lost between the meter and the stop tap, on the supply pipe.
Check inside the property
With the stop tap open and every tap, toilet and appliance off, a meter that still moves suggests water is being used or lost somewhere inside. Check toilets first, because a running toilet is a common culprit.
Mark and photograph the damp
Draw lightly round the edge of a stain with a pencil and take a dated photo. If it spreads, you have clear evidence of an active leak.
Note what is nearby
Write down what sits above or next to the damp patch: a bathroom, shower, radiator, kitchen or tank. That narrows down where to look.
If a damp patch is near light fittings or sockets, or a ceiling is sagging, keep clear and switch off the electricity at the mains if you can reach it safely from somewhere dry. Do not cut into walls or ceilings to look for pipes, as cables and pipes often run together.
Possible causes
These are common causes, not a diagnosis. The actual fault is confirmed on site.
Supply pipe leak
The pipe from your boundary to the house can leak underground. Often there is little sign beyond the meter, a wet patch outside or lower pressure.
Concealed pipework or joints
Pipes under floors, inside boxing or behind kitchen units can leak at a joint for a long time before showing.
Shower or bath seals
Failed sealant, grout or a tray seal can let water escape only when the shower is used. This often gets mistaken for a pipe leak.
Heating pipework
Leaks on radiator pipes or valves can show as damp. On a sealed heating system, the boiler pressure gauge falling repeatedly can be another sign.
A running toilet or tank overflow
Not a hidden leak in the pipe sense, but it can make a meter keep moving and is worth ruling out first.
How WHP would assess it
- Start with the evidence you have gathered: the meter readings, photos and what sits near the damp.
- Rule out the simple causes first, such as toilets, seals, visible joints and tanks.
- Test sections of pipework where practical to narrow down the area.
- If the source cannot be confirmed without opening up, explain the options first. That may include a specialist leak-detection survey before any work begins.
We explain any call-out or diagnostic charge before booking, and agree the repair and its cost with you before carrying it out.
What affects the price
Every repair is different, so we do not publish fixed prices for this work. These are the things that make the most difference:
- How long it takes to trace the source, which varies with the evidence available
- Whether floors, boxing or plaster need to be opened, and who will make good afterwards
- Whether the leak is on the underground supply pipe or inside the property
- Whether a specialist survey is needed
Before you call
Useful to send us
- Photos of the damp patch, with the pencil line if you have marked it
- Your meter readings and the times you took them, if you did the meter check
- What is above or next to the damp area
- How long you have noticed it and whether it changes when a shower or the heating is used
Get professional help when
- The meter keeps moving with everything turned off
- A stain is growing, or a ceiling is sagging
- You can hear water running when nothing is in use
- The damp is affecting a neighbour or a flat below
Common questions
I do not have a water meter. Can I still check?
Yes, though less precisely. Turn everything off and listen near the stop tap and the pipes for running water. Watch whether a marked stain spreads, and check for changes in water pressure. A plumber can test sections of pipework directly.
Who pays for a leak on the supply pipe?
Thames Water says the supply pipe from your boundary into the home is the homeowner’s responsibility, and asks for confirmed leaks to be repaired within four weeks. Metered customers who fix a leak promptly may be able to claim a leak allowance. Check the current terms with your water supplier.
Will my insurance cover finding the leak?
Some buildings policies include “trace and access” cover for finding and reaching a leak. Check your policy before work starts, and keep your photos and readings.
Sources
Safety and responsibility details checked against these sources on 12 October 2026. Water supplier guidance can change. Check your own supplier’s current advice.