To detect a water leak in your mains supply, first turn off every tap and appliance and watch the water meter. If it keeps moving, water is escaping. To pinpoint it without digging, specialists use acoustic detection, tracer gas, thermal imaging and leak-noise correlation, usually in combination. The supply pipe from your boundary into the home is yours to fix; the main up to the boundary is the water company's.
A leak on the mains supply pipe, the one feeding your whole home, can run for months without a puddle to show for it. The water often soaks straight into the ground, so the first sign is a higher bill, a soft patch of lawn, or a drop in pressure. Locating it no longer means digging up the garden on a hunch. Here is how to confirm you have a mains leak, then the professional methods that find the exact spot.
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First, confirm it's a mains leak
Before anyone reaches for specialist kit, a simple meter test tells you whether water really is escaping. It's the same check UK water companies recommend, and you can do it yourself in a few minutes.
The water meter test works like this:
- Turn everything off. Close every tap, and make sure no appliance (washing machine, dishwasher, toilet cistern) is drawing water.
- Read the meter. Note the reading, then watch the dial or the last digit for a minute or two.
- Watch for movement. If it keeps ticking over with nothing running, you have a leak somewhere on the supply. One full rotation of the small dial is roughly one litre a minute, per guidance from UK suppliers such as Wessex Water, which gives you a useful gauge of how serious the loss is.
To work out which side of the house the leak is on, find your internal stopcock (usually under the kitchen sink) and turn it off. If the meter stops, the leak is inside your property. If it keeps moving, the leak is on the underground supply pipe between the meter and the house, which is exactly the territory mains detection is built for. Wave Utilities and Wessex Water both set out this stop-tap test in their leak guides. Take care turning an old stopcock: they can be stiff and brittle.
Other tell-tale signs back this up: an unexplained spike in your bill, a patch of ground that stays green or boggy in dry weather, the sound of running water with the supply off, or persistently low pressure. If you've spotted the issue on your bill, our guide to a water leak between the meter and the house walks through what's happening underground.
Whose pipe is it: yours or the water company's?
This matters because it decides who pays. According to Ofwat, the industry regulator, responsibility splits at the boundary of your property:
| Pipe | Runs from | Responsibility |
|---|---|---|
| Water main | In the street | Water company |
| Communication pipe | The main to your boundary | Water company |
| Supply pipe | Boundary into your home | Homeowner (you) |
Where a boundary stop tap has been fitted, that's normally the dividing line. The supply pipe is the property owner's responsibility, including finding, repairing and maintaining any leak on it, as Ofwat and the Consumer Council for Water (CCW) both confirm. So if your stop-tap test pointed to the underground run, that's your pipe, and pinpointing it accurately is what keeps the repair small.
The professional detection methods
Once a mains leak is confirmed, the aim is to find its exact position before a spade touches the ground. Modern detection is non-invasive: specialists locate the leak from the surface, so any excavation is limited to the spot itself rather than the whole pipe run. Three core methods do most of the work, and they are usually used together.
Acoustic detection and correlation
Pressurised water escaping from a pipe makes a noise: a hiss or rushing sound that travels along the pipe wall and up through the ground. Acoustic detection is the art of listening for it. A ground microphone, placed on the surface above the suspected line, amplifies that vibration; the engineer moves along the route and the sound peaks directly over the leak. Ground microphones are sensitive enough to pick out leaks under roads and drives, and work well on pipes buried deeper than around half a metre.
For longer runs, a leak-noise correlator does the maths. Two sensors are clamped to the pipe at accessible points such as stop taps and fittings, and each records the leak sound. Because the noise reaches one sensor a fraction before the other, the correlator uses that time difference, the distance between the sensors and the speed of sound in that pipe material to calculate exactly where the leak is. It is a well-established technique that can pinpoint a leak to within a few inches.
One UK-specific wrinkle: a lot of newer supply pipes are blue MDPE plastic, which carries leak sound less clearly than the old iron and copper. The noise is quieter and dies away faster, so acoustic work on plastic needs high-sensitivity equipment tuned to lower frequencies, and it is often paired with tracer gas to be sure.
Tracer gas
When the pipe is plastic, the leak is tiny, or the ground is awkward, tracer gas takes over. The pipe is isolated and drained, then filled with a safe gas mixture, typically 95% nitrogen and 5% hydrogen. That 5% hydrogen ratio matters: it keeps the blend non-flammable while still using hydrogen's tiny molecules, which slip out through the smallest crack and rise to the surface. Specialist suppliers such as Sewerin and Esders document this exact mix and method.
The engineer then scans the ground above the pipe with a hydrogen detector. The gas escapes only where the pipe is breached and percolates up through soil, tarmac or concrete, so the highest reading sits right over the leak. Because hydrogen finds gaps water can barely seep through, tracer gas is one of the most precise ways to locate a hard-to-hear leak on a non-metallic supply line.
Thermal imaging
A thermal imaging camera reads surface temperature, not water. A leak shows up because the escaping water is at a different temperature from the floor, wall or ground around it, leaving a tell-tale pattern the camera can map. It's quick and completely non-contact, which makes it a useful way to narrow down where to listen or inject gas next.
The honest limitation, as manufacturers like FLIR explain, is that the temperature difference can be small, sometimes a fraction of a degree for cold mains water in mild weather, so thermal imaging guides the search rather than giving the final answer. A reading is always confirmed with a moisture meter and cross-checked against the acoustic or tracer-gas result, because a warm or cool patch can have other causes. Used as one layer of the picture, it speeds everything up.
Why specialists combine methods
No single method is right for every leak. The pipe material, how deep it's buried, the surface above it and the size of the leak all change what works best. That's why a professional rarely relies on one tool.
| Method | Best for | Watch-out |
|---|---|---|
| Acoustic / correlation | Metal pipes, locating along a run | Quieter on plastic; needs access points |
| Tracer gas | Plastic pipes, tiny leaks, deep runs | Pipe must be isolated and drained first |
| Thermal imaging | Fast scanning, narrowing the area | Reads temperature, not water, so needs confirming |
A typical job starts broad and narrows down: thermal or acoustic to find the area, a correlator to calculate position, then a ground microphone or tracer gas to confirm the precise spot. Combining them is what turns "somewhere under the drive" into a small, confident excavation. The same toolkit underpins our underground water leak detection work, where there is no internal plumbing to read at all and everything is found from the surface.
If you'd rather try the surface checks yourself first, our step-by-step on how to find a water leak underground covers what a homeowner can safely do before calling a specialist.
Mains leak detection in Cornwall & Devon
Across Cornwall and Devon we trace mains supply leaks with these methods every week, on everything from Victorian terraces to modern estates on blue plastic pipe. The aim is always the same: find the leak fast, with the least possible disruption, and give you a clear record of where it is and how it was found. Our mains water leak detection service brings the full kit to the job, so the repair is targeted rather than a guess.
If your water supplier has been in touch about unusually high usage, our guide on what to do when South West Water says you have a leak explains your responsibilities and the next steps.
Frequently asked questions
How do I know if my mains water supply is leaking?
Turn off every tap and water-using appliance, then watch your water meter. If the dial or last digit keeps moving with nothing running, water is escaping somewhere on your supply. Other clues are an unexplained jump in your bill, damp patches in the garden or driveway, or a drop in pressure.
Can you detect a mains leak without digging?
Yes. Professional methods are non-invasive by design. Acoustic sensors listen for the sound of escaping water, tracer gas is injected and detected at the surface, and thermal imaging maps temperature changes. These pinpoint the leak first, so any digging is limited to the exact spot rather than the whole pipe run.
Is the leaking pipe my responsibility or the water company's?
The supply pipe, running from the boundary stop tap into your home, is the homeowner's responsibility to find, repair and maintain, according to Ofwat. The communication pipe and the water main up to the boundary belong to your water company. The boundary stop tap usually marks the dividing line.
Why is acoustic detection harder on plastic pipes?
Plastic (MDPE) pipe carries leak sound less clearly than old metal pipe, so the noise is quieter and travels a shorter distance. Specialists handle it with correlators and high-sensitivity ground microphones tuned to lower frequencies, and often combine acoustic work with tracer gas to confirm the spot.
Does thermal imaging find the water itself?
No. Thermal cameras detect temperature differences, not water. A leak shows up because escaping water warms or cools the surrounding floor or wall. The reading is then confirmed with a moisture meter, because a thermal anomaly alone can have other causes. It's a guiding tool, used alongside acoustic and tracer-gas methods.
How accurate is professional mains leak detection?
When the methods are combined correctly, a leak can usually be pinpointed to within a small area, often a fraction of a metre. Correlation calculates position from the sound's travel time between two sensors, then a ground microphone confirms the exact point, keeping any excavation to a minimum.
Got a mains leak you can't pin down? We'll find it
We trace mains supply leaks across Cornwall & Devon with acoustic, tracer gas and thermal equipment, so there is no guesswork and minimal damage, plus a clear report of exactly where it is. Fast response, no call-out fee.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)