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Slab Leak Detection in UK Homes: Signs, Methods and Cost

Exposed pipe beneath a damp concrete floor

If you suspect a pipe is leaking under a solid concrete floor, shut off the water at the internal stop tap, check your meter, and call a plumber who finds leaks without ripping up half the house. “Slab leak” is the American phrase. In Britain the same problem is a leak under a concrete floor, screed or a ground-floor slab, often with pipes hidden beneath tiles, laminate or timber. This guide explains the signs, the checks you can do in ten minutes, how a professional locates the leak, what drives the cost and when insurance may pay.

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TL;DR

  • Warning signs include a meter that moves with everything off, the sound of water when taps are closed, damp patches, falling pressure and floors that feel warm or cold in one spot.
  • Turn off the internal stop tap first. If the meter stops moving, the leak is inside your property. If it keeps moving, it is likely on the supply pipe outside.
  • Finding the exact point before anyone digs is what keeps repair costs and disruption down.
  • Insurance “trace and access” cover usually only responds when a leak has already caused damage, so read your policy wording before booking.
  • We are available 24/7 across London, with Gas Safe registered engineers and a fixed quote before work starts.

Table of Contents

What a leak under a concrete floor actually is

Most older London houses have suspended timber ground floors, so a leak there often shows up fast as a stain or a soft board. Extensions, conversions, basement flats, newer builds and many garage or kitchen floors are different. Pipes can run in screed or sit in ducts cast into a concrete base. When one of those pipes fails at a joint, a bend or a corroded section, the water has nowhere obvious to go. It soaks the screed, rises through the floor finish or spreads sideways to a wall.

That hidden path is why the water you see is rarely directly above the fault. A damp skirting board in the hall can come from a joint under the kitchen. Guessing and opening the floor in the wrong place is the most expensive mistake in this kind of job, and it is the reason detection is a separate step from repair.

Three kinds of pipe are usually involved:

  • Cold water supply pipes between the stop tap and the kitchen, often the first run to fail.
  • Heating flow and return pipes feeding radiators from a boiler, especially in single-storey layouts where they are buried in the floor.
  • Hot water pipes to bathrooms and utility rooms.

Drains can also leak below a floor, but the symptoms differ: smells, slow drainage and damp that appears after flushing or running a basin rather than constantly.

Signs of a leak under the floor

WaterSafe, the UK plumbing industry’s body for approved plumbers, lists typical indicators of leaks inside a home: loss of water pressure, damp patches or mould on ceilings or walls, rusty or dripping radiators and the constant noise of flowing water in pipes. For a hidden floor leak, these are the signs we would take most seriously:

  1. The meter keeps moving. With every tap, appliance and toilet off, a moving meter means water is going somewhere.
  2. A hiss or trickle you can hear at night. Put your ear near the floor in a quiet house.
  3. A warm patch on the floor. A leak on a hot or heating pipe can warm one area of the floor finish.
  4. A cold, damp patch that does not dry, or tiles that sound hollow or lift at the edges.
  5. Laminate or timber flooring that swells, cups or lifts along a joint.
  6. A musty smell with no visible source, or damp rising up the base of walls.
  7. Low pressure at taps or a boiler that keeps losing pressure and needing to be topped up.
  8. A higher water bill if you are metered. WaterSafe notes that a tap dripping once a second can waste more than 7,000 litres a year, so a pipe leaking continuously under a floor costs far more.

None of these proves the leak is below the floor. A dripping radiator valve or a failing washing machine hose can produce the same symptoms. That is why the next step is a simple test that separates the possibilities.

What to do in the first hour

This follows WaterSafe’s advice on checking for leaks and takes about twenty minutes.

  1. Find your water meter. It is usually in a chamber near the boundary of the property, or inside under the kitchen sink or in a utility area.
  2. Make sure no water is being used. Turn off taps, check cisterns are not refilling, and pause the dishwasher and washing machine.
  3. Wait about 15 minutes and read the meter again. If the reading has gone up, there is a leak.
  4. Turn off the internal stop tap and check the meter once more. If it stops moving, the leak is on the pipework inside your property. If it carries on, the leak is probably on the supply pipe outside, between the meter or boundary and your home.
  5. Contact your water company and your insurer. WaterSafe notes that many water companies offer free leak detection and sometimes repairs, which is worth asking about if the leak seems to be on the outside supply.

If you cannot stop the water, or it is near electrics, turn off the electricity at the consumer unit if you can reach it safely and call an emergency plumber. If you smell gas at any point, do not use switches or flames: open windows and call the National Gas Emergency Service on 0800 111 999.

How a professional finds a hidden floor leak

A good detection visit works through the building in stages, starting with the cheapest and least intrusive checks and narrowing down. The usual stages are:

  • Questions and a visual check. When did you notice it, which areas are affected, is the heating losing pressure? The pattern of damp often points to a pipe run.
  • Pressure testing sections of pipework. Isolating the heating, hot and cold circuits tells the engineer which system is losing water before anything is opened up.
  • Moisture meters. These map where the floor and walls are wet, showing the spread of the water.
  • Acoustic detection. Sensitive microphones pick up the sound a pressurised leak makes in a pipe or the surrounding floor.
  • Thermal imaging. A camera shows temperature differences, which can reveal a leaking hot or heating pipe or wet material cooler than its surroundings.
  • Tracer gas. A harmless gas mixture is introduced into the pipe and detected at the surface, which can narrow a leak to a small area under tile or timber.

No single tool solves every case. Heavy concrete can muffle sound, a plastic pipe is quieter than a copper one and water can travel a long way before surfacing. Experienced engineers combine methods and cross-check them. The aim is a marked spot on the floor, so any opening-up is a small, neat access hole rather than a trench.

If you are in London and want to see how we approach this, our leak detection service page covers what we do, and our guide to leak detection in London goes into the hidden-leak process in more detail.

Repairing a leak under a concrete floor

Once the leak is located, there are generally three routes. Which one fits depends on the pipe, the floor and how many joints are failing.

Three routes for repairing a floor leak

Spot repair. If a single joint or short length has failed, the floor is opened over that point, the pipe is cut out and replaced, and the floor is made good. This is the common outcome when detection has been accurate.

Re-routing the pipe. Where a pipe is corroded along its length or buried in a place that is hard to reach, the plumber can abandon the buried section and run a new pipe above the floor or along a wall, skirting or ceiling void. Floors stay intact and future access is easier. It is common with heating and hot water pipes in older properties.

Replacing the circuit. If leaks keep appearing at different points, it can make sense to replace the whole run rather than chasing individual failures.

After the plumbing is fixed, the floor needs to dry and be reinstated. Screed and timber hold moisture for a long time, so a floor that looks dry at the surface may need time and monitoring before new flooring goes on. Ask whoever repairs the floor how they will check it is dry.

If the leak affects a gas or heating system, such as a boiler circuit that keeps losing pressure, the repair touches the heating. Our plumbing work team handles pipework, and if the boiler itself is old and the heating circuit keeps failing, boiler installation may be worth discussing as part of the repair.

What affects the cost of detection and repair

We do not publish a price list for leak detection, because the cost depends on the job, and we would rather quote a fixed price once we understand it. These are the factors that move the price most:

  • How hard the leak is to find. A leak on an accessible pipe with a clear damp patch is a shorter job than a slow leak somewhere under a large open-plan floor.
  • Which methods are needed. A moisture check is quick, whereas combining pressure testing, acoustic work, thermal imaging and tracer gas takes longer.
  • Access. A fitted kitchen, tiled floor or underfloor heating makes it harder to reach the spot, and adds to make-good work.
  • The type of pipe and floor. Copper, plastic, heating pipework, screed depth and floor finishes all change the approach.
  • The repair route. A spot repair is smaller than re-routing a run of pipework.
  • Reinstatement. Drying time, re-screeding, retiling and redecorating are often separate from the plumbing.

We give a fixed quote before work starts, so you know the figure before you agree to anything. Be wary of any quote for a hidden leak that does not say what is included: detection only, repair, floor reinstatement or all three.

Does home insurance pay for finding a leak under the floor?

Sometimes, and the wording matters. Many buildings policies include “trace and access” cover: the cost of finding the source of a leak and repairing the damage made during the search. The AA’s Home Silver policy, for example, includes trace and access cover up to £5,000, and Legal & General’s policy booklet lists a £5,000 limit for tracing and accessing leaks. Limits and terms differ by insurer and policy, so yours may not match.

The catch is that cover often depends on damage having already happened. In one Financial Ombudsman Service case, the insurer did not reimburse a leak detection report fee because no access works were carried out and no claim for water damage was made, and the complaint was not upheld. In another, the ombudsman confirmed that trace and access cover under a buildings policy only applied once an escape of water had damaged the building, so a claim for finding a suspected leak under a concrete floor was properly declined.

In practice:

  1. Call your insurer before the work starts, tell them what you have found, and ask whether trace and access applies.
  2. Photograph any damage such as stains, lifting flooring and damp walls, and keep the meter readings you took.
  3. Keep invoices and any detection report, which can support a claim.
  4. Check your excess against the likely cost of the work.

If you want to understand how this kind of water damage protection is generally structured, this overview of insurance cover for water damage shows the sort of risks such policies list. Your own policy document is what decides what you can claim.

Our perspective

Most people who call us about a floor leak have already lost a day to guesswork. They have moved furniture, lifted a corner of carpet or been told to break out a section of floor “to see”. The best thing we can do is work out where the water is going before anyone makes a hole.

We run emergency callouts around the clock across London, because a leak under a floor does not wait for office hours. A pipe losing water into a screed damages more the longer it runs. So the first job is to stop the water safely, then locate the fault and explain it in plain terms.

We quote a fixed price before work begins. For hidden leaks, that matters even more, since the worry is always that the bill will grow once the floor is open. Our engineers are Gas Safe registered, which counts when a leak sits close to heating or gas pipework and the repair has to be done properly and safely.

If you think you have a leak under the floor, do the meter test, turn off the stop tap and call us with what you found. That information alone often tells us which system to look at first.

the author

Book leak detection in London

If your meter is moving, your floor is damp for no obvious reason, or the heating keeps losing pressure, we can come out any time of day or night. We cover London’s boroughs, including areas such as Fulham, Kensington and Acton, and give you a fixed quote before we start.

Evoplumbers

See our leak detection page or, if water is actively escaping, our emergency call out service. Tell us what you have seen and what the meter did, and we will take it from there.

FAQ

How can I tell if I have a leak under my concrete floor?

Start with the meter test: with all water off, a meter that moves after 15 minutes means a leak somewhere. Warm or damp floor patches, the sound of running water, falling pressure and a heating system that keeps losing pressure all point the same way. A plumber then confirms where it is.

Is a leak under the floor an emergency?

It can be. Turn off the internal stop tap straight away to limit damage, and call an emergency plumber if water is spreading, near electrics or you cannot stop it. A slow leak with a dry, stable floor is less urgent but should still be investigated promptly, because damp spreads and screed holds water.

Do I have to dig up the floor to find the leak?

No. Detection is about locating the fault with moisture readings, pressure tests and methods such as acoustic detection or thermal imaging before any opening-up. The aim is a small, targeted access point instead of breaking out large areas.

Will my insurer pay for leak detection?

Possibly, if your policy includes trace and access cover and the leak has caused damage. Ombudsman decisions show insurers can decline when no water damage has occurred. Call your insurer before work starts and check the limit and excess.

Who is responsible for a leak on the supply pipe?

A leak inside your property is yours to fix. For the supply pipe outside, start with the meter test and contact your water company, since WaterSafe notes many offer free leak detection and sometimes repairs. A plumber can confirm which side the leak is on.

Sources

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