A wet carpet with no obvious leak that returns after drying is fed from below. A warm patch points to a heating pipe in the floor, a cold one to a supply pipe or moisture through the slab. A water meter test confirms whether water is escaping, and the source can be located before the floor is opened.
You have dried it with towels, borrowed a dehumidifier, maybe even lifted the corner and left a fan heater running for a weekend. The carpet came up dry. A week later you walked across the room in socks and there it was again, the same cold, spongy patch in the same place. Nothing has been spilled and there is no pipe on show anywhere near it. This guide explains why the patch keeps returning, how to read what it is telling you, and how the source is traced without turning the room into a building site.
Why drying the carpet never fixes it
Drying treats the symptom. The carpet and underlay are only the last stop for water arriving from somewhere else, and the moment you remove the moisture, the supply carries on. If the patch comes back, water is being replaced at least as fast as it evaporates.
Water under a floor does not sit where it escapes. It runs along the top of a slab, follows a pipe or a joist, pools in the lowest point it can find, then rises through the screed into the underlay. So the wet patch marks where water surfaces, which can be a metre or more from the failing pipe. That is why guessing where to lift the floor so often goes wrong.
The four usual reasons a patch keeps returning
Across Cornwall a recurring damp patch in a carpet nearly always comes down to one of four things. Two are leaks, one is a building problem, and one is not a leak at all.
A heating pipe leaking in the screed
Many houses built or extended from the 1970s onwards have the heating flow and return buried in the floor screed. Copper in contact with cement can corrode over decades, and a pinhole or a weeping joint releases warm water straight into the floor. The giveaway is temperature: the patch feels warmer than the floor around it, it is wettest after the heating has been on, and the boiler pressure gauge keeps slipping. If that last part sounds familiar, our guide on why a boiler keeps losing pressure explains what the gauge is telling you.
A cold water supply pipe under the floor
The incoming main or an internal supply pipe often runs under the ground floor on its way to the kitchen. These pipes are under mains pressure all day, so even a tiny split pushes out a surprising amount of water. The patch will be cold, it will not change with the heating, and a water meter will keep ticking with every tap off. This is the classic case for water leak detection because there is nothing to see above the floor.
Moisture coming through the slab
Older solid floors, and plenty of cottage extensions, were laid without a damp-proof membrane or with one that has since failed. Ground water then finds its way up through the concrete. This shows as a wider, softer area rather than a sharp patch, it is worse after wet weather, and it does not move the water meter. The answer is a floor repair rather than a pipe repair, and telling the two apart before anyone digs is the point of a proper survey.
Condensation under the underlay
A cold slab, a foam underlay and an unventilated room can produce condensation on the underside of the underlay. It feels damp rather than wet, it is usually against an outside wall, and it comes and goes with the season. Rooms shut up for weeks at a time get it most, which is why coastal holiday lets and second homes see it so often.
The touch and temperature test
Before you call anyone, spend five minutes with your hand on the floor. It narrows the cause more reliably than most people expect. Do it when the heating has been running for an hour, then again the next morning after it has been off overnight.
One more clue: if the patch is only wet after a bath, a shower or a washing machine cycle, the water is tracking in from another room and the source is probably not in this room at all.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Patch is warm, boiler pressure drops | Heating pipe leaking in the screed | Note the pressure daily, keep the heating on low, book a survey |
| Patch is cold, water meter moves with everything off | Supply pipe leaking under the floor | Close the stopcock overnight if you can, book a survey |
| Wide damp area, worse after rain, meter still | Moisture through the slab | Improve ventilation, get the floor construction checked |
| Damp rather than wet, along an outside wall | Condensation under the underlay | Lift the carpet to air it, heat and ventilate the room |
Two checks that confirm a leak before anyone lifts a floor
The temperature test points you in a direction. These two checks tell you whether water is genuinely escaping, which is what an insurer will want to know first.
The water meter. Turn off every tap and appliance, wait half an hour for cisterns to settle, then look at the meter. Most have a small dial or star that turns with the slightest flow. If it moves with nothing running, water is leaving the system somewhere. The full method is in our guide to the overnight water meter leak test. Then close the internal stopcock and look again: if the meter stops, the leak is inside the house, which fits a wet carpet.
The boiler gauge. A sealed heating system holds its pressure, typically around 1 to 1.5 bar when cold. If you are topping it up every few days with no drip at a radiator valve, the water is going somewhere hidden. A carpet that is warm and wet in the same period is not a coincidence.
If neither check shows anything, the patch is more likely a building or condensation issue.
How the source is found without lifting the whole floor
Once a leak is confirmed, the job is to narrow it to a small area before a single board or tile comes up. We use several tools together, because each one sees something different.
- Moisture mapping. A calibrated meter gives readings across the whole floor, not just the wet patch. The readings usually climb towards the source, which is often nowhere near where you can feel it.
- Thermal imaging. A heating pipe leaking in the screed shows on a thermal camera as a warm bloom along the pipe run, and an escaping cold main as a cool shape. The camera does not see through concrete, but it sees what the water does to it.
- Acoustic listening. Water leaving a pressurised pipe makes a noise. Sensitive ground microphones pick that up through screed and tiles, and the loudest point is normally within a short distance of the fault.
- Tracer gas. For a small leak on a plastic pipe that makes little sound, a safe hydrogen and nitrogen mix is introduced into the drained pipe and a detector finds where it surfaces through the floor.
The result is a marked spot and a written, photographed explanation of what was found. The repair then opens one small area rather than the room.
What to do while you wait
Pull the carpet and underlay back from the patch so air gets underneath. Do not run a heater on the floor for days, because drying the surface makes the moisture readings harder to interpret on the day of the survey. If the meter test pointed to a supply leak, closing the stopcock overnight limits the water going into the floor. If the heating is the suspect, keep an eye on the gauge so the boiler does not run dry.
Check your buildings insurance too. Most UK policies include trace and access cover, which pays for finding the leak and getting to it, though not for repairing the failed pipe. Our guide to what trace and access insurance covers explains how it usually works, and our survey report is written for the insurer as much as for you.
Whether it is a slab-floored bungalow in Newquay or a converted barn near Bodmin, the pattern is the same: confirm the leak, read the clues, then locate before you lift. We carry out leak detection across Cornwall, and a recurring carpet patch is one of the most common calls we take.
Frequently asked questions
Can a carpet be wet with no leak at all?
Yes. Condensation under the underlay and ground moisture coming through an old slab both wet a carpet without a pipe being involved. They tend to cover a wider area, feel damp rather than wet and change with the weather. A leak gives a defined patch that comes back quickly and, on a metered supply, keeps the meter moving.
Why is the wet patch warm?
Warmth almost always means a central heating pipe is leaking in the floor. The escaping water carries heat from the system into the screed, so the patch feels warmer than the floor around it, especially after the heating has been running. A boiler that keeps needing topping up in the same period makes it near certain.
Will the leak be under the wet patch?
Often not. Water runs along the slab or a pipe route and rises where the floor is lowest or most porous, so the patch can be a metre or more from the split. Lifting a corner of carpet to look is sensible. Cutting into the screed on a guess is not, which is why the leak is located first.
Does insurance cover finding a leak under the carpet?
Most UK buildings policies include trace and access cover, which pays to locate the leak and gain access to it, and to make good afterwards. The repair to the pipe itself is normally excluded. Check your own policy wording and keep photographs and meter readings from the start.
Stop drying it, find it
If the same patch of carpet keeps coming back wet, we can confirm whether it is a leak and pinpoint the source without lifting the whole floor. You get a marked location and a report written for your insurer.
