A water main leak is often working quietly before it becomes obvious. You may notice a water bill that no longer makes sense, a boiler that repeatedly loses pressure, damp ground near the property, or the sound of water moving when every tap is off. Knowing how to check for water main leaks can help you act before water damages floors, foundations or neighbouring property.

The key is to separate a genuine underground supply-pipe leak from normal water use, a leaking appliance or a drainage issue. Start with simple checks you can carry out safely. If the evidence points to a concealed leak, specialist trace & access testing can locate it without lifting large areas of flooring or excavating blindly.

First, understand which pipe may be leaking

In most homes, the water main or incoming supply pipe carries clean mains water from the network into the property. It may run beneath a front garden, driveway, communal area or ground floor before reaching the internal stopcock. A leak on this pipe can occur outside, under a concrete slab, behind finishes or within a service void.

Responsibility is not always straightforward. The water company is generally responsible for its network and, in many cases, the pipework up to the boundary. The homeowner, freeholder or managing agent may be responsible for the private supply pipe beyond that point. In flats, the arrangement can be more complicated, particularly where several properties share a supply. Establishing where the leak sits is therefore as important as proving that a leak exists.

Do not confuse a supply-pipe leak with a leaking drain. A water main leak usually causes a continuous loss of clean water and can keep a meter moving even when the property is unused. Drainage leaks tend to show symptoms only when water is being discharged from baths, showers, sinks or appliances.

How to check for water main leaks with your meter

Your water meter is the most useful first test. It measures all water entering the property, so it can reveal a continuous loss that is not visible indoors.

Choose a time when nobody needs water for at least 30 minutes. Switch off appliances that may draw water automatically, including washing machines, dishwashers, irrigation systems, water softeners and ice makers. Do not run taps, flush toilets or use the boiler’s filling loop during the test.

Take a clear photograph of the meter reading, including the smaller leak indicator if your meter has one. Leave the supply untouched for 30 minutes, then check it again. If the reading has increased or the leak indicator is turning, water is passing through the meter.

A moving meter does not automatically mean the incoming main is leaking. A toilet cistern may be silently overflowing, a faulty pressure relief valve may be discharging, or an appliance could still be drawing water. The next isolation test helps narrow this down.

Use the internal stopcock to narrow the fault down

Locate the internal stopcock, commonly found beneath the kitchen sink, near the front door, in a utility cupboard or under floorboards. Turn it clockwise carefully until the water supply is off. Avoid forcing an old or stiff valve, as it may fail or begin to leak.

With the internal stopcock closed, return to the meter. If it continues to register water use, the likely loss is between the meter and the stopcock. That strongly suggests a leak on the private incoming supply pipe, often referred to as the water main.

If the meter stops moving once the internal stopcock is closed, the issue is more likely within the property after the stopcock. Check toilets, taps, visible pipework, appliances, tanks and heating equipment before assuming an underground main leak.

This test is highly useful, but shared supplies can affect the result. In converted houses and blocks of flats, one meter or stopcock may serve more than one dwelling. Do not shut off a communal valve without permission or without warning affected residents.

Look for the physical signs of a concealed supply leak

An underground leak will not always create a puddle. Water follows the path of least resistance and may emerge some distance from the damaged pipe. It can also drain into surrounding soil without reaching the surface.

Look for a persistently wet patch on a path, driveway or lawn during dry weather. A strip of greener, faster-growing grass can be a clue, especially if it follows the likely route from the boundary towards the house. Sunken paving, loose ground, unexplained pooling and water appearing near an external wall also warrant investigation.

Inside, watch for a musty smell, damp skirting boards, lifting floor finishes, peeling paint or localised mould. These signs can indicate a leak beneath a solid floor, but they can also result from condensation, penetrating rain or a failed heating pipe. The pattern matters. Continuous dampness alongside a moving water meter is more suspicious than a patch that appears only after heavy rain.

You may also hear a faint hiss or rushing sound in a quiet property. Listen near the internal stopcock, where the supply enters the building and along ground-floor walls. Acoustic signs are helpful, but they are not proof on their own. Modern homes contain many normal noises from boilers, pumps and appliances.

Check whether pressure and water use support the diagnosis

A noticeable fall in water pressure can accompany a significant supply-pipe leak. Taps may run weakly, showers may lose force or the flow may vary at different times of day. However, low pressure alone is not a reliable diagnosis. It may be caused by high demand on the local network, a partially closed valve, a blocked filter or work being carried out nearby.

Compare your water bills with the same period last year where possible. An unexplained and sustained increase is more useful than one high bill, particularly if your household routine has not changed. Meter readings taken over several days can show whether the loss is constant. Record the date, time and reading so there is a clear evidence trail.

If you have a combi boiler that loses pressure, treat that as a separate but urgent line of enquiry. Boiler pressure loss commonly points to a heating-system leak, a faulty expansion vessel or a pressure relief issue, not a leak on the cold-water main. Do not repeatedly top up the system without finding the cause, as this can worsen hidden damage.

When not to excavate or start opening up floors

It is tempting to dig where the ground looks damp or lift flooring where a stain appears. That approach often creates unnecessary damage because the visible symptom may be far from the failed pipe. It can also disturb electrical cables, gas services, drainage runs and waterproofing.

A trace & access specialist should locate the leak before targeted repairs begin. The right method depends on the pipe material, depth, access, noise levels and whether the system can be isolated. Thermal imaging can identify abnormal temperature patterns where water affects building materials. Acoustic testing can detect the sound created by pressurised water escaping from pipework. Tracer gas detection is particularly effective for locating small leaks beneath floors and hard surfaces, while moisture mapping defines how far water has travelled.

No single tool is right for every property. A competent engineer uses the results together, isolates sections where possible and confirms the likely leak location before recommending access. This is the difference between evidence-led investigation and guesswork.

What to do if your tests suggest a water main leak

If the meter keeps moving with the internal stopcock closed, or if water is causing visible damage, turn off the supply if you can do so safely. Open a cold tap briefly to relieve remaining pressure, then avoid using water until the problem is assessed. If water is approaching electrical sockets, consumer units or wiring, keep clear and arrange urgent professional help.

Take photographs of the meter readings, damp areas and any damage before repairs start. Keep copies of water bills and note when the issue was first noticed. Many home insurance policies include trace & access cover, which may contribute towards locating the source and accessing it for repair. Insurers commonly require a clear cause, testing method, leak location, photographs and an engineer’s signed declaration before considering a claim.

For homeowners, landlords and managing agents, the practical next step is a leak detection visit that identifies the source, confirms the affected pipework and sets out the least disruptive repair route. Ignite Plumbing Heating & Gas can provide non-invasive testing, targeted access, repair and insurance-ready reporting, helping keep the investigation and the remedial work under one controlled process.

A moving meter is your strongest early warning. Record it, isolate the internal supply where appropriate, and get the leak precisely located before anyone starts digging or breaking into floors. That protects the property, the repair budget and the evidence you may need for an insurance claim.