A leak under a tiled floor is awkward for one simple reason: tiles are expensive to lift and hard to replace, so you only want to lift the right ones. The good news is that most leaks under tiles can be narrowed down a long way before anyone picks up a chisel. This guide covers the signs, the checks you can do yourself, and how a leak detection engineer pinpoints the leak.
What are the signs of a water leak under a tiled floor?
- Grout that darkens in one area and stays dark long after the floor has been used.
- Cracked or crumbling grout, or tiles that have started to sound hollow when tapped.
- Tiles lifting or rocking, especially in a line that might follow a pipe run.
- Damp or swelling skirting boards, door frames or kitchen plinths next to the tiled area.
- A warm patch on the floor when the heating or hot water is on, which can point to a leaking hot pipe.
- A musty smell, or mould at the edges of the room.
- Staining on the ceiling below, if the tiled floor is on an upper storey.
- A water meter that keeps turning with everything off, or a boiler that keeps losing pressure.
Any one of these can have another cause, such as a spill, condensation or a failing shower seal. Two or three together make a hidden leak far more likely. Our article on signs of a hidden water leak covers the wider list.
What pipes run under a tiled floor?
Knowing what could be leaking narrows the search. Under a typical kitchen, bathroom or hallway you may have:
- the cold mains supply, often entering through a ground floor and rising to the kitchen sink;
- hot water pipes from the boiler or cylinder to taps and showers;
- central heating flow and return pipes feeding radiators;
- underfloor heating pipes, if the room has it;
- waste pipes from showers, baths, basins and appliances.
The floor construction matters too. On a solid ground floor, pipes are usually buried in screed or run in ducts, and water can spread through the screed a long way from the leak. On a suspended timber floor with tiles laid on boards, water tends to drip into the void below, so the tiles may show little until the boards underneath are soaked.
How can you check for a leak under the floor yourself?
These checks do not need any tools beyond a phone camera and a notepad.
- Do a meter test. Turn off every tap and appliance that uses water, make sure no toilet is refilling, and read the water meter. Wait at least 30 minutes without using any water and read it again. If the reading has moved, water is going somewhere.
- Narrow it down by isolating. If you have service valves on the pipes to particular rooms or appliances, close them one at a time and repeat the test. If the meter stops when a valve is closed, the leak is probably downstream of that valve.
- Check the heating separately. On a sealed system, note the boiler pressure when the system is cold, then check it daily. A steady fall with no visible leak points to heating pipework.
- Test the hot side. Turn the hot water on and feel the floor after 20 to 30 minutes. A warm strip or patch that is not near a known heating pipe is worth noting.
- Test the waste. Waste pipes only leak when water flows through them. Run the shower or empty the bath and watch the grout and skirting over the next hour or so.
- Photograph and date the damp area each day, so you can see whether it is growing and in which direction.
A meter test only shows leaks on the supply side. Heating and waste leaks will not move the meter, which is why the other checks matter.
Is it a pipe or the shower waterproofing?
In bathrooms and wet rooms, a common cause of wet tiles is water getting through failed grout, sealant or waterproofing rather than a pipe leak. The tell-tale sign is timing: the damp appears after the shower is used, not when the room is idle. A pipe leak on the supply side tends to continue whether or not anyone showers. A careful engineer tests these separately, because the fix and, often, the insurance position are very different.
How do professionals find a leak under tiles without lifting the floor?
No single method suits every floor. A leak detection engineer usually combines several.
| Method | What it shows | Limits |
|---|---|---|
| Pressure testing | Which pipe or section is losing water | Confirms a leak but not its exact point |
| Moisture mapping | Where the floor is wettest and how the water has spread | Wettest point may not be above the leak |
| Thermal imaging | The route of hot and heating pipes, and warm or cool patches from leaks | Less effective on cold pipes deep in screed |
| Acoustic listening | The sound of water escaping under pressure | Needs a pressurised pipe and a quiet property |
| Tracer gas | Gas escaping at the leak point and rising through the floor | Needs the pipe drained and filled with gas |
A typical sequence is to confirm the leaking section by pressure testing, trace the pipe route with thermal imaging, then pinpoint the escape with acoustic equipment or tracer gas. Tracer gas works well under tiles because the gas finds its way up through grout joints, where it can be picked up by a detector. Our leak detection technology page explains each method in more detail.
If the floor has underfloor heating, the approach is different again, and our underfloor heating leak detection page covers that.
How is finding a leak under floorboards different?
Under a timber floor, the job is usually easier. Boards can often be lifted without damage, the void can be inspected with a torch or an inspection camera, and water tends to show on the joists or the ceiling below. The difficulty comes when boards are covered with a fixed floor finish, such as tiles or engineered wood, or when the void is shallow. In those cases the same non-invasive methods are used to decide which boards to lift.
How many tiles have to come up?
When the leak has been pinpointed, a plumber can often repair it by lifting a small number of tiles and breaking out a small area of screed around the pipe. When it has not, the floor tends to be opened progressively, which can mean a whole section. Before lifting anything:
- check whether you have spare tiles from the original job;
- photograph the floor and note the tile make if you know it;
- if you are claiming, get your insurer's agreement to the opening up and reinstatement.
Wet screed also needs time to dry before new tiles go down, otherwise the adhesive and grout can fail.
What does it cost to find a leak under a tiled floor?
Our under-floor leak detection is priced at £300 to £700, depending on the size of the area, the floor construction and how many systems need testing. A combined survey is £250 to £500, and a single-method thermal or acoustic survey starts from £150. Every survey includes a written report. The repair and retiling are separate and priced by the trades who do them.
When should you call a leak detection specialist?
Call a specialist if your meter test shows water being lost, if the boiler keeps losing pressure, if the damp is spreading, or if you are about to lift tiles and do not know exactly where the leak is. Ring 020 3912 0200 and we can tell you which test is likely to be needed.
Frequently asked questions
Can a leak under tiles be found without breaking them?
In most cases the leak can be located without breaking any tiles, using pressure testing, thermal imaging, acoustic listening and tracer gas. Some tiles usually still need lifting to carry out the repair itself.
How long does it take to find a leak under a tiled floor?
Many surveys are completed in a single visit of a few hours, although larger floors, several suspect systems or slow, intermittent leaks can take longer.
Why is my tiled floor wet only after I shower?
That usually points to the shower waste, tray seal, grout or waterproofing rather than a supply pipe, because those only get wet when the shower is used. It needs testing to be sure.
Will a leak under a concrete floor damage the foundations?
A long-running leak can wash out or soften the ground in some soils, and it keeps the slab and walls damp. It is one reason not to leave a suspected under-floor leak unchecked.
Is a hollow-sounding tile a sign of a leak?
Not necessarily. Tiles often sound hollow because of poor adhesive coverage. A hollow tile together with dark grout, damp skirting or a moving meter is more significant.
