How to tell if you have an underground water leak

An underground water leak usually shows itself through a handful of reliable signs: an unexplained rise in a metered water bill, a drop in pressure at the cold kitchen tap, damp or boggy ground in dry weather, and the sound of running water when everything is switched off. On a buried supply pipe or main, water can escape for months with nothing visible at the surface, so knowing what to look for is what turns a hidden problem into one you can act on.

A leak on a pressurised underground pipe is a different thing from a dripping tap or an overflowing cistern. The pipe is buried, often under a lawn, driveway or car park, and the water soaks away into the ground rather than pooling where you can see it. That is why the earliest signs are indirect: your meter, your pressure and your bill notice the leak before your eyes do. McFadden Utilities carries out professional leak detection on commercial sites, supply pipes and private networks, and the checks below are the same first steps we work through before any equipment comes off the van.

The signs of an underground water leak, and what each one means

A steady or sudden rise in a metered bill, with no change in how much water you are using, is one of the clearest signs of a leak. Water escaping a buried pipe is still measured by the meter, so it lands on the bill even though none of it reaches a tap.

A drop in pressure or flow, most tellingly at the cold kitchen tap, points to water escaping before it reaches the building. The cold kitchen tap usually runs straight off the incoming mains supply, so weak flow there in particular suggests the loss is on the supply pipe rather than the internal plumbing.

Wet, soggy or boggy ground that stays damp in dry weather is a strong outdoor sign, especially along the line the pipe takes from the boundary to the building. A leak underground has to surface somewhere, and saturated ground with no rain to explain it usually means water is coming up from below. A strip of grass that is greener or growing faster than everything around it can sit directly over a leaking pipe that is quietly watering and feeding it.

New cracks, dips or small sinkholes in paving, a driveway or a car park can be caused by water washing away the sub-base beneath the surface. This one is worth taking seriously, because the same erosion that cracks the surface can undermine what sits on top of it. The sound of running, hissing or trickling water when every tap and appliance is off, heard near where the supply pipe enters the building or at an outside stop tap, is a classic sign of a live leak on a pressurised pipe. Inside, damp patches, mould or a musty smell low down with no obvious cause above can be water rising from a leak below the floor.

No single one of these is conclusive on its own, because each can have another explanation. Several of them together point firmly to an underground leak, and that is the point to start confirming it.

What these signs look like on a commercial site or larger property

On a commercial site, an estate or any larger property, the same signs appear at a different scale, and some are easier to miss. A leak on a private network serving several buildings often shows up first as a jump in sub-metered consumption rather than a single obvious bill, so the reading that flags it is a facilities or estates figure rather than a household one. Pressure loss can be felt across a whole site rather than at one tap, and wet patches may appear across grounds, verges or a car park rather than a back garden.

The scale also raises the stakes. On a large private supply, water lost to a leak is charged just as it is to a household, so the running cost of an undetected leak on a commercial network mounts up quickly. And because these pipes often run under car parks, access roads and hard standing, the erosion a leak causes can undermine surfaces that are expensive and disruptive to lift and relay. Catching the signs early on a site of any size is worth far more than it first looks.

Two tests to confirm an underground leak

Two simple checks confirm whether the signs add up to a real leak, and roughly where it is. Neither needs specialist equipment.

The first is the water meter test. Turn off every tap, appliance and outside tap so that no water is being used anywhere on the property, then read the meter, either the analogue dial with its small low-flow indicator or the digital display. If the meter is still registering flow with everything off, water is leaving the system somewhere, which means a leak.

The second test tells you whether that leak is inside the building or outside on the underground supply pipe. Find and close the internal stop tap, or stopcock, usually under the kitchen sink or where the pipe enters the building, and check the meter again. If the meter stops, the leak is inside the property, in the internal plumbing. If it keeps running with the stop tap closed, the leak is between the meter at the boundary and the building, on the underground supply pipe. That distinction matters, because it decides both who is responsible and how the leak needs to be found.

Is it your leak, or the water company’s?

Where an underground leak sits decides who has to deal with it, and it is not always the property owner. The water supply is split into three sections with different owners.

In broad terms, the water company looks after the mains and the communication pipe up to the boundary, and the owner is responsible for the supply pipe from the boundary onwards, along with all the internal plumbing. So a leak on the supply pipe, the section the stop tap test points to, is normally the owner’s to find and fix. Ofwat, the water regulator, sets this out, and confirms that maintaining your supply pipe includes leak detection, repair and replacement.

There is help available, though. Many water companies offer a free leak detection visit of up to half an hour on the external supply pipe, and metered customers can usually claim a leak allowance that credits the bill for water lost once the leak is repaired, so it is worth a call before commissioning anything. It cuts the other way too: if a company records leakage in an area and traces it to a private pipe, it can issue a legal notice under the Water Industry Act 1991 requiring the owner to repair it within a set time. The full question of who is responsible for a water leak is worth understanding before the digging starts.

What to do once you suspect a leak

Once the meter and stop tap tests confirm a leak on the underground supply, the instinct to start digging is the one to resist. A buried leak gives almost no clue at the surface about where along the pipe it actually is, so digging speculatively means a run of exploratory holes and a torn-up driveway with no guarantee of hitting it.

The professional approach is to pinpoint the leak precisely first, then dig once, in the right place. Specialists locate a buried leak without excavation using the range of detection methods, acoustic listening, leak noise correlation, tracer gas and thermal imaging, chosen to suit the pipe and the ground. The result is a single targeted hole at the confirmed leak position rather than a trench across the site. What that survey costs depends on access, pipe length and whether a written report is needed, which our guide to water leak detection costs in the UK sets out in full.

If the leak is yours to sort, use a qualified contractor. You can find an approved one through WaterSafe, the national accreditation scheme, or speak to a specialist directly. McFadden Utilities locates and repairs leaks on commercial sites, supply pipes and private networks, and reinstates the ground afterwards, so the leak detection and repair runs start to finish with one contractor rather than three.