How do you know the heating leak is under the concrete floor?
Very often you do not know for certain until someone looks. What usually happens is a process of elimination. The boiler pressure keeps dropping, the relief valve discharge pipe is dry, the radiators and visible valves are fine, and the ground floor is solid concrete with pipes disappearing into it. At that point a buried leak becomes the most likely explanation.
Some signs make it more likely still:
- A patch of floor that feels warm underfoot, especially in a line or a spot that does not match where a radiator is.
- Flooring that is lifting, bubbling or staining, or laminate that has started to swell at the joints.
- Skirting boards or the bottom of walls going damp, with a tide mark rising from the floor.
- A musty smell in one room, or mould behind furniture on an outside wall.
- Pipes to one or more radiators that enter the floor rather than running along the skirting.
- Pressure loss that is worse when the heating is on, because warm pipes expand and small defects open up.
If you are still at the stage of working out whether you have a leak at all, start with our guide on what to do when your boiler keeps losing pressure.
Why are heating pipes buried in concrete in so many London homes?
Solid ground floors became common in houses built from the middle of the last century onwards, and many flats and maisonettes have concrete floors on every level. When central heating was installed or added, the easiest route to radiators on internal walls or the far side of a room was often through the floor screed. Copper pipes, sometimes small bore microbore pipe, were laid on the slab and the screed poured over them.
Plenty of these installations have run for decades without trouble. The problem is that buried pipes are hard to inspect and impossible to maintain, and when one does fail, the water has nowhere visible to go.
What causes heating pipes under concrete to leak?
The causes are fairly consistent from job to job:
- External corrosion. Copper laid directly in screed, without a protective sleeve or wrapping, can corrode from the outside, particularly where the screed stays damp or has been contaminated. Pitting eventually goes right through the pipe wall.
- Internal corrosion. Systems that have been topped up often, or have little inhibitor, corrode from the inside. Pinholes can form anywhere, including in buried sections.
- Thermal movement. Pipes expand and contract every time the heating cycles. Where a pipe is locked tight in the screed, especially at a bend or where it rises out of the floor, that repeated movement can wear or crack it.
- Joints in the screed. Good practice avoids joints in buried pipework, but older installations sometimes have soldered or compression joints under the floor, and these are common failure points.
- Later building work. Fixings for kitchen units, door thresholds, partition walls or new flooring can hit a pipe nobody knew was there.
How is a heating pipe leak under a concrete floor located?
A buried heating leak is found by combining several methods, because each one covers the weaknesses of the others.
- Establish the pipe routes. The engineer works out where pipes enter and leave the floor and uses thermal imaging to trace their paths across the slab with the heating running.
- Isolate the circuits. If the radiator circuits can be valved off, the pressure on each is watched to find which buried run is losing water.
- Thermal imaging along the suspect run. Warm water escaping into the screed produces a warm area that spreads away from the line of the pipe, often with a hot centre where the leak is.
- Acoustic listening. A ground microphone on the floor surface picks up the sound of water escaping under pressure. The sound is usually loudest directly above the leak.
- Tracer gas if the picture is unclear. The circuit is drained and filled with a hydrogen and nitrogen mix. The gas escapes at the leak and works up through the screed, joints and cracks, where a detector picks it up.
- Moisture mapping. A moisture meter across the floor and wall bases shows how far the water has travelled, which matters for drying and for an insurance claim.
- Mark and record. The leak is marked on the floor, photographed and written up with thermal images in the survey report.
This is what our central heating leak detection surveys are built around, and it is why an opening can usually be kept to the size needed for the repair.
Can thermal imaging see through a concrete floor?
A thermal camera does not see through anything. It reads the temperature of the surface. What makes it work on concrete floors is that heat from a hot pipe conducts up through the screed and warms the surface above it. That warmth appears as a line on the image, and a leak appears as a warm area that breaks away from the line.
How well this works depends on the depth of the pipe, the thickness and type of screed, what is on top, and how long the heating has been running. A pipe a few centimetres down in a thin screed under tiles shows very clearly. A pipe deep in a thick slab, under carpet and underlay, may show weakly or not at all. That is why thermal imaging is paired with acoustic and tracer gas methods rather than used alone.
What if the floor is tiled, carpeted or has timber on top?
Tiled floors are the easiest to survey and the most expensive to repair badly, so a precise location matters most there. With carpet, lifting the carpet and underlay along the suspect pipe route often gives a much clearer thermal image and costs nothing but a little time. Floating timber and laminate floors can hide water spreading underneath them across a wide area, so the damp may show up metres away from the actual leak. Tracer gas is often useful here, because the gas tends to come out at the edges and joints of the floor finish and can be followed back to the source.
Should a buried heating pipe be repaired, rerouted or replaced?
Once the leak has been found, there is a decision to make. The right answer depends on the condition of the pipe and how the room is used.
| Option | When it tends to make sense | Disruption |
|---|---|---|
| Repair at the leak point | A single defect, such as a fixing through the pipe, on pipework that is otherwise sound | One small opening, then make good the screed and floor finish |
| Abandon the buried section and reroute | Pipe is corroded along its length, or a second leak is likely soon | New pipe run along skirting, through walls or via the ceiling void above. The floor stays intact |
| Replace the buried run in a new sleeve or duct | The route must stay under the floor and the pipe is failing | A trench along the route, so the most disruptive |
A heating engineer is best placed to advise on which to choose, but the survey report makes that conversation much easier. If a small opening shows the pipe is heavily pitted around the leak, that is a good indication that repairing one spot will only move the problem along the pipe.
Will insurance pay for finding a heating leak under the floor?
Many home insurance policies include trace and access cover, which pays towards finding the source of an escape of water and opening up and making good the structure to reach it. What is covered, the limits and the excess vary between policies, and some policies exclude repairing the leaking pipe itself or damage from a leak that has run for a long time. Check your policy wording, and speak to your insurer before any opening-up work if you intend to claim.
Insurers generally want to see evidence of where the leak is and how it was found. Our surveys include a written report with thermal images and marked locations for that purpose. Our page on insurance leak claims explains how the report fits into a claim.
When should you call a leak detection specialist?
If you have a sealed heating system that keeps losing pressure with no visible leak, and pipes run through a solid floor, call a specialist sooner rather than later. Screed and the ground under it can absorb a lot of water, and by the time damp shows at the surface or the skirting, the affected area is often much larger than the leak itself. Call straight away if water is showing at the floor surface or the floor finish is lifting.
Leak detection under solid floors is priced at £300 to £700 depending on the floor area and the methods needed, with the report included and on a no find, no fee basis. Call 020 3912 0200 to arrange a survey.
Frequently asked questions
How do you find a central heating leak under a concrete floor?
By tracing the pipe route with thermal imaging while the heating runs, isolating circuits to find the leaking run, then pinpointing the spot with acoustic listening or tracer gas. The position is marked so only a small opening is needed.
Can a heating pipe under a concrete floor leak without showing?
Yes. Water can soak into the screed and the ground beneath for a long time before any damp reaches the surface, which is why pressure loss is often the only early sign.
Is it better to repair or reroute a leaking pipe in a concrete floor?
A one-off defect on sound pipe is usually repaired where it is. If the pipe is corroded along its length, rerouting it above the floor avoids repeated openings.
Can I turn off the radiator that the leaking pipe feeds?
Closing both valves on that radiator stops flow to it, but it only isolates the leak if the leaking pipe is the one serving that radiator and nothing else. Check the pipe route first, and keep the rest of the system running if a survey is booked.
Does a heating leak under the floor cause damp on the walls?
It can. Water in the screed spreads outwards and can rise into the base of walls, leaving tide marks and damp skirting that are easy to mistake for rising damp.
