A wet patch in the middle of a solid floor comes from one of four places: a pipe leaking in or under the screed, a missing damp-proof membrane, groundwater on a low-lying plot, or condensation on a cold slab. A water meter check and a taped-down square of polythene usually tell you which, without breaking the floor open.
You have pulled back the carpet and there it is: a dark, damp patch in the middle of the room, nowhere near a wall, a sink or a radiator. It does not dry out, and the vinyl has started lifting. The question everyone asks is the right one: where is the water coming from?
On a solid floor there are four realistic answers, and they need four different fixes. Treat the wrong one and you pay twice. This guide covers each cause, the clues that point to it, and the tests you can do at home. If they point to a pipe, our underground water leak detection service finds the escape point through the slab without breaking it up on a guess.
The four places the water can come from
A concrete floor is porous, it sits on the ground, and in most Cornish homes built after the 1960s it has a plastic membrane underneath and a layer of screed on top with pipes laid in it. Any of those layers can be the source.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Patch is warm to the touch, or the boiler keeps losing pressure | Heating pipe leaking in the screed | Log the boiler pressure daily, book a survey |
| Patch is cold, wet in all weathers, water meter creeps with everything off | Supply pipe leaking under the slab | Overnight meter test, then a survey |
| Damp across a wide area, tide marks and white salts low on the walls, older house | No membrane, or a failed one | Polythene test, then a damp survey once a leak is ruled out |
| Wet after heavy rain, dries in a dry spell, house on low ground | Groundwater | Check outside drainage and ground levels |
| Wet on cold mornings, windows streaming too, dry by afternoon | Condensation on a cold slab | Heat and ventilate, then re-test |
A pipe leaking in the screed
When the patch is in the middle of the room rather than along a wall, a buried pipe is the first thing to rule in or out. Heating pipes are often run across the floor in the screed, and copper laid that way can pit and pinhole after a few decades. A heating or hot water leak gives itself away with a patch that feels warmer than the floor around it. A cold mains leak is sneakier: the slab just stays wet.
The reliable tells are away from the floor. If the boiler needs topping up more often than it used to, the leak is on the heating circuit. If the water meter moves when nothing in the house is running, it is on the supply. Our guide to the overnight water meter test shows you how to prove a supply leak in one night, and if the patch is warm, our post on a warm patch on the floor covers the heating side.
One thing catches people out: the wet patch is not always above the leak. Water under a slab follows the easiest route, often a gap in the membrane or a crack in the screed a metre or two away. That is why we never advise breaking the floor where the damp shows.
A missing or failed damp-proof membrane
Solid floors laid before the 1960s were often poured straight onto the ground with nothing underneath. Membranes came later, but they get torn during the build, punctured by later work, or never joined to the damp-proof course in the walls. Without that barrier, ground moisture wicks up through the concrete.
Membrane failure usually looks different from a pipe leak. The damp is spread rather than concentrated, and it comes with company: tide marks and powdery white salts low on the walls, plaster crumbling at skirting level. You see it in the older housing stock around Redruth and Camborne, in granite terraces and cob cottages where a concrete floor went in at some point and nobody thought about a membrane.
The catch is that a slow pipe leak under a membrane-free floor looks much the same, and damp-proofing firms are not set up to tell the difference. Rule out the pipe first. A membrane laid over an active leak does not cure anything; it pushes the water into the walls instead.
Groundwater on low-lying Cornish plots
Cornwall has plenty of homes on wet ground: converted mills beside rivers, cottages at the bottom of a valley, bungalows on reclaimed land around Hayle. On those plots, water can push up through a slab under real pressure after a wet week, sometimes bubbling through hairline cracks.
Groundwater has a rhythm. The floor gets wet a day or two after heavy rain and dries out in a dry spell. It shows first at the lowest point of the floor. If your patch follows the weather, check gutters, downpipes, land drains and ground levels outside before you think about the floor itself. A raised path or flower bed against the wall is a common culprit.
Condensation on a cold slab
The fourth cause has nothing to do with water in the ground or the pipes. An uninsulated concrete floor sits at ground temperature all year, well below the warm, moist air of a lived-in room. On a cold morning, or after a shower, that air gives up its moisture onto the coldest surface it can find, and on tiles or vinyl it sits in a film that looks exactly like a leak.
Condensation is worst in corners and against outside walls, comes and goes within the day, and pairs with streaming windows. It responds to heat and ventilation, which a leak never does. In a holiday let left unheated between winter bookings it is very common, so do not assume the worst the first time a cleaner reports a wet floor in January.
How to tell them apart in a weekend
You can narrow this down a long way before anyone comes out. Four checks, in this order.
- Read the water meter with everything off. Note the reading last thing at night, use no water overnight, read it again in the morning. Any movement means the supply is losing water; none rules out a supply leak.
- Watch the boiler pressure. A sealed heating system normally sits at about 1 to 1.5 bar when cold. Write the reading down each morning for a week with no top-ups. A steady fall points to the heating circuit.
- Tape down a square of polythene. Dry the patch with a fan heater, tape a piece of thick plastic about 60 cm square over it with all four edges sealed, and leave it for two or three days. Water on the underside means moisture from below (a pipe, the membrane or the ground). Water on top means condensation from the room.
- Draw round it in pencil and feel it. A patch that grows past the pencil line over a few days, whatever the weather, is being fed by something. A patch warmer than the floor beside it is almost certainly a heating or hot water pipe.
Keep the readings and photos. If it turns out to be a pipe and you claim on your buildings insurance, that record shows when you noticed the problem.
Why we do not need to break the floor to find it
The old way of finding a leak under a slab was to dig where the damp showed and keep digging. The modern way is to find the escape point first and open the floor once, in the right place. We do that with three pieces of equipment, used together.
A thermal imaging camera shows the warm plume from a heating pipe spreading through the screed, and on a cold leak it can show the wet area as a cooler zone. Acoustic listening equipment picks up the sound of pressurised water escaping through the slab. Where the readings need confirming, we put tracer gas into the isolated pipe: a safe hydrogen and nitrogen mix that rises through screed, concrete and tiles to a probe directly over the leak. You get a marked spot and a written report, and the repair becomes one small hole rather than a trench.
If you are wondering what the visit involves, we have written up what happens during a leak detection survey step by step. We provide leak detection across Cornwall, and most surveys on a solid floor are finished in a single visit.
Frequently asked questions
Can water really come up through a concrete floor?
Yes. Concrete is porous, so ground moisture wicks up through a slab that has no membrane, and groundwater can push through cracks on wet, low-lying ground. Just as often, though, the water is coming from a pipe laid in or under the screed, which is why a meter check comes before any damp treatment.
How do I know if the wet patch is a leak or rising damp?
Check the water meter and boiler pressure first. If either is losing water with everything off, you have a leak. Rising damp through a floor tends to be widespread, slow, and paired with tide marks and white salts at the base of the walls, whereas a leak is usually a defined patch that keeps growing.
Will you have to dig up my floor to find the leak?
No, not to find it. Thermal imaging, acoustic listening and tracer gas locate the escape point from the surface and we mark it. The repair does need a small opening at that spot, but it is one targeted hole rather than exploratory digging across the room.
Does home insurance cover a leak under a concrete floor?
Most UK buildings policies include trace and access cover, which pays towards finding the leak and making good the floor opened to reach it, usually up to a limit. It does not pay to repair the failed pipe itself. Check your own wording, and keep your meter readings and photos for the claim.
Find out what is under the floor
If the meter or the boiler is losing water and the patch will not dry, we will locate the leak through the slab without breaking it open on a guess, and give you the insurance-ready trace and access report to go with it.
