What is the supply pipe, and who looks after it?
Water reaches your home from the water main in the street. A short pipe, the communication pipe, runs from the main to the boundary stop tap, which is usually under a small cover in the pavement or at the edge of your front garden. From there, a second pipe runs under your garden, path or driveway and into the house, ending at your internal stop tap. That second pipe is the supply pipe.
In the Thames Water area, the water company looks after the main and the communication pipe, and the property owner is responsible for the supply pipe from the boundary into the home, along with all the plumbing inside. Where one supply pipe serves several homes, as it does with many terraces and converted houses, the owners share responsibility for the shared section. Other water companies work on the same broad principle, but the details and any help they offer vary, so check with your own supplier.
Thames Water's guidance asks customers to arrange a repair promptly once a leak on their pipework is confirmed, and if you are on a meter you may be able to claim a leak allowance for the wasted water once the leak is fixed. The terms, including deadlines and a limit on how often an allowance can be given, are set out on Thames Water's own website and are worth reading before you start. Our older post on water mains leak signs covers the symptoms in more depth.
How do I know if I have an underground water leak?
Underground leaks rarely announce themselves with a puddle. The signs tend to be indirect:
- A water bill that has jumped with no change in how the household uses water.
- A letter from the water company saying its meter readings suggest a leak.
- Lower water pressure or flow than you used to have, especially at upstairs taps.
- A patch of lawn that stays greener or softer than the rest, or a spot on a path or driveway that never dries.
- A faint hissing or running sound at the internal stop tap when nothing is being used.
- Paving that has sunk or a small hole that keeps reappearing in the same place.
None of these proves a supply pipe leak on its own. An overflowing toilet cistern or a dripping tap inside can also push a bill up. That is why the next step is a meter test.
How do you do a water meter leak test?
If you have a water meter, you can usually check for a leak yourself in under an hour. The meter is most often in a chamber at the boundary, under a small cover in the pavement or front garden, although some are fitted inside near the internal stop tap.
- Stop all water use. Turn off taps, the washing machine, dishwasher and anything that fills automatically. Wait until toilet cisterns and any cold water storage tank have finished filling.
- Read the meter. Note the full reading, or watch the small dial or flashing indicator that shows water is flowing.
- Wait at least 30 minutes without using any water, then read it again.
- If it has moved, close the internal stop tap and repeat the test.
- Interpret the result. If the meter still moves with the internal stop tap closed, water is escaping between the meter and the stop tap, which usually means the underground supply pipe. If it stops when the internal stop tap is closed, the leak is inside the property.
This test assumes the meter is at the boundary and the internal stop tap closes fully. Old stop taps often do not, and a meter fitted inside the house will not register a leak on the pipe before it. If you have no meter, a specialist can still check the supply pipe by other means, including listening at the stop tap and measuring flow.
How deep is an underground water supply pipe?
The Water Supply (Water Fittings) Regulations 1999 guidance sets a depth for new underground pipes of not less than 750mm and not more than 1350mm, unless the pipe is in a duct. That protects the pipe from frost and damage. Older supply pipes, particularly lead and early copper ones, were not always laid to that standard and can be shallower or deeper, and pipes often change depth where they pass under steps, walls or extensions.
Depth matters because every detection method loses some sensitivity as the pipe gets deeper, and because the depth decides how big the hole for the repair needs to be. If you need the route of the pipe as well as the leak, our guide to finding where your water pipe runs underground covers the locating methods.
How do you detect a water leak underground?
A supply pipe survey normally uses more than one method. The order below is typical, although the engineer will adapt it to the pipe material and the site.
| Method | What it does | Where it struggles |
|---|---|---|
| Pipe locating | Traces the route of the pipe from the boundary to the house, using a signal applied to a metal pipe or a flexible tracer pushed into a plastic one | Plastic pipes cannot carry a signal on their own |
| Ground microphone | Listens at the surface along the pipe route for the noise of water escaping under pressure | Soft ground, deep pipes, plastic pipe and background traffic noise |
| Leak noise correlator | Uses two sensors on the pipe at either end and calculates the leak position from the difference in the time the noise takes to reach each one | Needs access points at both ends and accurate pipe length and material |
| Tracer gas | Fills the isolated pipe with a hydrogen and nitrogen mix and detects the gas where it reaches the surface | Very deep pipes, saturated clay and wide sealed surfaces that divert the gas |
| Thermal imaging | Occasionally shows a temperature difference where water has soaked the ground | Cold mains water often gives little contrast, so it is rarely the main method outdoors |
For a walk-through of each of these tools and what they can and cannot find, see our guide to leak detection equipment. Our earlier article on underground leak detection without digging describes a survey visit from arrival to marking the spot.
Why does the pipe material matter?
Metal pipes carry sound well, so a leak on an old lead or copper supply pipe can often be heard from some distance along the pipe, and a correlator works well. Modern blue MDPE plastic pipe, which most supply pipes laid in recent decades use, absorbs sound. The leak noise does not travel far along it, so ground microphones have to be very close to the leak and correlation over long runs is harder. On plastic, tracer gas is often the more reliable choice.
It is also common to find a mix: an old lead pipe partly replaced with plastic, or a plastic pipe joined to an old section under a path. Joints between different materials are frequent leak points, and knowing where they are helps the survey.
What happens after the leak is found?
The engineer marks the leak position on the ground, usually with paint or a marker, and records the depth estimate and the method that confirmed it. The written report includes photographs and a plan of the pipe route, so the plumber or groundworker doing the repair knows exactly where to dig. A single excavation over the leak is generally enough for a repair, rather than a trench along the whole run.
If the pipe is old lead, or shows several weak points, the repair contractor may recommend replacing the supply pipe instead. The survey still helps, because the route plan tells them where the pipe runs. Keep the report and your repair invoice together, as both are useful if you apply for a leak allowance or make an insurance claim.
When should you call a leak detection specialist?
Call a specialist if your meter test shows water moving with the internal stop tap closed, if the water company has written to you about a suspected leak, or if there is a wet area outside that you cannot explain. Speed matters on a metered supply because you pay for every litre lost, and a leak under a path or foundation can wash out the ground around it.
Our water mains leak detection surveys use acoustic equipment from £150 and tracer gas from £200, with a combined survey at £250 to £500 where more than one method is needed. The report is included and every survey is no find, no fee. Call 020 3912 0200.
Are underground supply pipes covered by home insurance?
Sometimes, and it depends on the policy. Many buildings policies include cover for underground services, meaning the pipes, drains and cables you are legally responsible for, and some add trace and access cover for the cost of finding a hidden leak and making good afterwards. Cover is often limited to accidental damage, such as a pipe broken by ground movement or tree roots. Wear and tear, corrosion and gradual deterioration are common exclusions, which matters for old lead and iron supply pipes. Some policies only include underground services or accidental damage as an optional extra.
Check your policy schedule and wording for underground services, accidental damage, escape of water and trace and access, and note any excess and limits. Speak to your insurer before work starts, as many want to agree the investigation first. A detection report showing where the leak is, how it was confirmed and the pipe material gives them the evidence they need to assess the claim.
Frequently asked questions
Can you detect a water leak underground without digging?
Yes. Pipe locating, ground microphones, correlators and tracer gas together can pinpoint most supply pipe leaks from the surface, so only one small excavation is needed for the repair.
Who pays for a leak on my supply pipe?
In the Thames Water area, the property owner is responsible for the supply pipe from the boundary stop tap into the house, and for sharing the cost where a pipe serves several homes. Tenants should report leaks to their landlord.
How do I check if my supply pipe is leaking?
With all water use stopped and the internal stop tap closed, watch the water meter for 30 minutes. If it moves, water is escaping between the meter and your stop tap.
Can a supply pipe leak affect my water pressure?
It can. Water escaping from the pipe before it reaches the house reduces the flow available at your taps, especially when several are used at once.
Does home insurance cover underground supply pipe leaks?
Some policies include underground pipes and trace and access cover, and some do not. Check the wording for underground services and escape of water, and speak to your insurer before work starts.
