What is actually under your floor
Two very different jobs get described the same way, and they are found differently.
- Pipework in the screed. Underfloor heating loops, or heating flow and returns and hot and cold feeds run in a screed layer over a slab or over insulation. Plastic barrier pipe in most modern installations, copper in older ones. The depth is usually modest, which helps.
- Pipework under or through the slab. An incoming supply crossing beneath a solid floor, or an old feed buried in the concrete itself, sometimes in lead or in copper that was never sleeved. Deeper, less accessible, and the most expensive to guess at.
Which of the two you have changes the whole approach, so it is the first thing an engineer establishes, usually from the age of the property and where the pipes emerge at the walls.
The symptoms that point under the floor
- A warm patch underfoot on a floor that has no underfloor heating, or a warm patch in the wrong place on one that does.
- Boiler pressure that drops steadily with no visible wet anywhere in the house.
- The water meter turning with every tap and appliance off.
- Damp appearing at the bottom of skirting boards or at the perimeter of the room, because water under a slab reaches the edges first.
- Tiles lifting or drumming, laminate cupping, or a vinyl floor lifting at a seam.
- A musty smell in a room with no visible damp, which is water in the build-up rather than on it.
There is a symptom walkthrough for a warm patch on the floor if that is what brought you here.
Prove the loss before locating it
Two free tests, and an engineer will do both anyway.
- Meter test. Everything off, photograph the meter, wait two hours with no water used, photograph it again. Movement means a live loss on the supply side.
- Pressure test. Note the boiler gauge cold and check it cold the next day. A drop means the sealed system is losing water, and if nothing is visible it is under the floor or inside a wall.
Closing the internal stopcock and re-running the meter test splits the search: still moving means the leak is on the buried supply between the boundary and the house, which is supply pipe work; stopped means it is inside.
Why the usual methods struggle here
Acoustic
Leak noise needs to reach a surface a microphone can sit on, and concrete and screed absorb a great deal of it. Worse, most pipe in a modern screed is plastic, and leak noise in plastic pipe attenuates sharply with distance and survives mainly at low frequencies, where the building's own noise lives. On a copper supply crossing under a slab in a quiet house, acoustic can still work well. On a plastic underfloor loop, it usually does not. See the guide to acoustic leak detection.
Thermal imaging
A thermal camera reads the temperature of the floor surface. If the leaking pipe is hot and the slab is cold, that is a strong, obvious signature, and this is the fastest way to find a hot feed leaking under tile. If the pipe is cold, or the whole floor is warm because the underfloor heating is running, there is nothing to see. See thermal imaging.
Moisture mapping
Useful for sizing the damage and proving it is active, poor at locating, because water travels a long way through a floor build-up before it reaches the surface. It answers how much floor has to come up, not where to cut.
Why tracer gas is the primary method on solid floors
The circuit is drained and refilled with a mixture of 5% hydrogen in 95% nitrogen. Hydrogen is the smallest molecule available and it is buoyant, so it escapes at the leak and works its way upward. Manufacturers of the detection equipment state that the gas reaches the surface through asphalt, concrete and soil, which is precisely the property a solid floor takes away from every other method. A hydrogen probe then sweeps the floor on a grid and finds the peak.
It works equally on copper and plastic, it works on a system with no pressure left in it, and it works on a leak too slow to make any noise. That combination is why it is the method for screed and slab.
The honest caveat: gas follows the path of least resistance. On a concrete slab it can surface at a crack, an expansion joint, a service penetration or the slab edge rather than directly above the leak, and detector manufacturers say so openly. A good engineer treats the strongest reading as a small search radius, sweeps outwards to establish how the signal falls away, airs the room and repeats, and confirms with a second method before a disc cutter comes out. The full method and its other limits are in tracer gas leak detection explained.
Opening the floor, once
Once the point is confirmed, the access is cut small: a section of tile or board lifted and the screed broken back far enough to expose the pipe and work on it. On a screed floor with underfloor heating this is typically one opening. On a buried supply under a slab, re-routing the pipe above ground or around the slab is often cheaper and better than chasing it through concrete, and that is a decision worth having before anyone starts.
Then the part most firms do not do: the screed is made good, the insulation and membrane reinstated where they were cut, and the tile, board or vinyl relaid so the floor matches. See leak damage repair and reinstatement, and underfloor heating leak repair if the leak is on a loop.
Cost, and the insurance point
Detection is £150 per hour, fixed before we attend. If we do not find the leak, you do not pay for the time we spent looking. The total depends on how long the job takes, and a solid floor takes longer than an accessible one because the circuit has to be drained and charged before the sweep begins.
Many UK buildings policies include trace and access cover, which is designed for exactly this: the cost of finding the leak and of making good the damage caused by getting to it. The pipe repair itself is often excluded, so check your schedule. Our detection report is written to be read by a loss adjuster.
