Do this first
- Check what is above and behind it before you touch anything. If the patch is on a wall with sockets or switches in it, or a floor with underfloor heating or a cable route, treat the electrics as the first question and turn that circuit off at the consumer unit if the damp is anywhere near a fitting.
- Feel it and photograph it. Cool and soft means water is still arriving. Dry, hard and ringed means it has stopped. Photograph it with something in shot for scale, and date it.
- Mark the edge in pencil. This is the single most useful thing you can do before anyone attends. A pencil line and a date tells an engineer in ten seconds what a description cannot: whether it is growing, and how fast.
- Read the water meter with everything off. No taps, no washing machine, nobody flushing. Read it, wait thirty minutes, read it again. Movement means a live leak on your side.
- Read the boiler pressure gauge cold, and again the next morning.
- Do not redecorate, seal or paint over it. Stain-block over an active leak fails within weeks and destroys the evidence in the meantime.
What it usually is, and how to tell
Damp with no rain overhead is usually plumbing. Where it sits relative to the pipe run, and whether it moves with the weather, sorts the causes out before anything is opened up.
| Pattern | Cause | Confirming test |
|---|---|---|
| Grows in dry weather, worse in the morning, near a bathroom, kitchen or pipe run | Plumbing leak | Meter test with everything off, or a falling boiler gauge |
| Appears during rain and fades days later, on or near an external wall | Penetrating damp: pointing, render, sill, gutter or downpipe | Watch it through one wet week and one dry week |
| A band up to roughly a metre from floor level, tide-marked, on a ground-floor wall | Rising damp or a bridged damp-proof course | Height and the horizontal tide line, both unchanged by weather |
| Black mould in a cold corner, behind furniture, on the coldest wall, no water history | Condensation | Worse in winter, better with ventilation, meter and gauge both static |
| Warm to the touch, on a floor, hours after the heating is off | Hot pipe leaking under screed | Thermal imaging, and a boiler gauge that falls |
Two more distinctions worth making. A patch on a floor near a shower or bath that only darkens after someone washes is a waste or seal failure, not a supply leak, and it stops between uses. A patch on a wall that is directly under a soil stack or a bathroom above is the drainage side, which only wets when something is running, and our guide to finding a water leak in a wall sets out the order the checks are done in. If the floor is warm rather than just wet, start at a warm patch on the floor.
What the repair involves
Detection is layered, cheapest and least invasive first. Moisture mapping across the wall or floor establishes the wet area and, more usefully, where the wettest point is, because the visible stain is almost never centred on the leak. Thermal imaging picks up hot pipe runs and evaporative cooling. Acoustic listening finds pressurised leaks through screed and masonry. Tracer gas goes in last on a drained pipe when the others cannot see it. The written report that comes out of this is what an insurer needs for a trace and access claim.
Then the repair: the wet section of pipe is cut out and remade, or the failed seal, waste or fitting is replaced. After that the wall or floor is dried properly before it is closed, because plastering over a wet substrate blooms through the paint within a month. We cut out, dry, reboard or replaster, reinstate screed, floor and skirting, and redecorate. That is what water leak repair means here.
When it is not us
- It only appears when it rains. That is a roofer, a builder for pointing and render, or a gutter clean. Detection equipment will confirm what the weather already told you.
- A tide mark at low level with no moving meter. That is a damp specialist and possibly a damp-proof course question, not a leak.
- Black mould, cold corner, no water history. Ventilation, heating pattern and extraction. Worth fixing, not by us.
- Wet outside the building line, at a boundary or in the front garden. The supply pipe from the boundary into your home is yours, but the main and the communication pipe are the water company's. Report it to them before paying anyone to dig.
- A wet patch on a shared wall or ceiling in a block. Notify the managing agent in writing first, because the source may sit in common parts you have no right to open.
- You rent. It is the landlord's to investigate and repair, and reporting it in writing protects you if the damage spreads.