Signs of a leak under a concrete shop floor
Warm patches, lifting flooring, a meter that never stops: the signs a pipe is leaking beneath your shop's slab, what else causes them, and what to do before anyone digs.
Most shop floors are a concrete slab under a screed and a finish, with fixtures, stock and customers on top. The pipes that feed the toilets, the staff kitchen and sometimes the heating often run through or beneath that slab. So a leak down there can carry on for weeks before anyone sees a drop of water, and by the time it shows, it has usually been running for a while.
The good news is that a leak under concrete leaves clues. Some are on the floor, some are on the water meter, and some are in the boiler cupboard. Here is what to look for, what else can cause the same symptoms, and what to do next.
Why there are pipes under the slab at all
In many older units, supply pipes were laid in or under the floor slab to reach a WC, a tea point or a sink at the back of the shop. Heating pipes were sometimes run the same way, and some units have underfloor heating loops set into the screed.
The rules have tightened since. In England and Wales, Schedule 2 of the Water Supply (Water Fittings) Regulations 1999 says no water fitting shall be embedded in any wall or solid floor, and that fittings with joints must not be concealed. Scotland’s equivalent is the Water Supply (Water Fittings) (Scotland) Byelaws 2014, which have been extended to 2029. In newer installations, pipes under a floor usually run in a duct or sleeve so they can be reached. Plenty of retail units still have pipework buried by an earlier fit-out, though, and nobody on the current team knows the route.
The signs on the floor
Walk the shop floor, the stock room and the back-of-house corridor, and look for these:
- A warm patch. A floor that feels warm underfoot in one spot, or a place where colleagues say “it’s always warm here”, often points to a hot water or heating pipe leaking under the screed.
- Lifting or bubbling flooring. Vinyl, safety flooring and luxury vinyl tiles can lift, bubble or lose their grip when moisture comes up from below. Carpet tiles may feel damp at the edges or smell musty.
- Damp patches that keep coming back. If a patch reappears in the same place after it has been dried, water is arriving from somewhere, and mopping will never fix it.
- White powdery deposits. Salts left behind as water evaporates through concrete or tiles can show as a white bloom on the surface.
- Damp at the base of internal walls. Damp creeping up from the skirting on a wall nowhere near an outside wall can be water spreading from under the floor.
- A hissing or running sound. In a quiet stock room, a leak under pressure can sometimes be heard through the floor, especially near where the supply enters the building.
The signs away from the floor
Some of the best evidence is nowhere near the wet patch.
The water meter. Turn off every tap, cistern, urinal and water-using appliance, read the meter, wait ten minutes without using water, and read it again. If it has moved, water is leaving the pipework somewhere, and a buried pipe under the slab is one of the usual suspects.
The water bill. A bill that jumps with no change in trading, staffing or equipment deserves a meter check before you start querying the tariff.
The boiler pressure. A sealed heating system should hold its pressure. If the gauge keeps dropping and someone keeps topping it up, water is escaping from the heating circuit. When that circuit runs under the floor, the slab is a likely place to look.
What else causes the same symptoms
Before you blame a pipe, rule out the usual lookalikes. They are common, and they are usually cheaper to fix.
| Symptom | Could also be | How to tell |
|---|---|---|
| Damp patch near the entrance | Rain tracked in, or a badly sealed door threshold | Worse on wet days, dries out in dry spells |
| Damp at the base of walls | Condensation, or water running down from a roof or gutter | Check higher up the wall and the outside face |
| Wet floor by chillers or freezers | Condensate or defrost water from the cabinet | Clean, cold water that follows the defrost cycle |
| Smell and damp near a floor gully | A cracked or blocked drain | Sewage smell, slow draining sinks and toilets |
| Lifting floor after a deep clean | Cleaning water getting under the finish | Appeared straight after cleaning and does not return |
If the meter moves with everything switched off, most of these lookalikes drop out, because rain, condensation and cleaning water never pass through your meter. Neither does a leaking drain, which is why a drain problem needs a different kind of survey.
What to do if you think it is under the slab
- Record the evidence. Photograph the patch, note the date, take two meter readings with the time between them, and write down the boiler pressure if you have heating. That record helps the engineer and your insurer.
- Keep the area safe. Wet or lifting flooring on a shop floor is a slip and trip risk. HSE’s slips and trips guidance covers the causes and how to prevent them, which is useful while you get the leak found. Cone the area off or move the display if you need to.
- Do not break out the floor. The wet patch is often some distance from the break, because water runs along the pipe route and across the top of the membrane before it surfaces. Digging where it is wet can mean a closed section of the shop and no leak found.
- Check the building’s age before anyone disturbs the floor. HSE advises that if a building was built or refurbished before 2000, you should assume asbestos is present, and that work disturbing the fabric of the building needs a more intrusive survey first. Have your asbestos register to hand before any floor finish is lifted.
- Get the leak located. A non-invasive survey pins down the break from above, so any opening is usually small and in the right place.
If the fault turns out to be a simple dripping fitting you can see, a WaterSafe approved plumber can repair it directly.
How a leak under concrete is found
A leak detection survey usually combines three methods:
- Acoustic listening, which picks up the sound of water escaping from a pressurised pipe through the floor.
- Thermal imaging, which shows the heat pattern of a hot water or heating leak spreading through the screed.
- Tracer gas, where the isolated pipe is filled with a hydrogen and nitrogen mix that finds its way up through the slab at the break.
Once the point is marked, the repair is usually a small, targeted opening rather than a trench across the shop. There is more on the process on how it works, on floors in particular on underfloor leak detection, and on heating circuits on heating leak detection.
About ADI Leak Detection
ADI has worked on leaks for over a decade, with 25 engineers across England, Scotland and Wales. We are accredited by CIPHE, WIAPS, WaterSafe and SPATA, carry £5m public liability cover, and provide insurance-ready reports. We aim to find and repair on the same day, although that depends on what we find.
To book a survey, call 0333 567 2615.
Frequently asked
Can a leak under concrete really go unnoticed for months?
Yes. Water can drain into the ground beneath the slab or run along the pipe route, so the floor may stay dry while the meter keeps turning. A rising bill, or a meter that moves with everything switched off, is often the first clue.
Should we lift the floor where the damp patch is?
Not yet. The wet patch is often some distance from the break, so get the leak located first. If the building was built or refurbished before 2000, HSE advises assuming asbestos is present, which matters before anyone disturbs a floor finish or screed.