If you have a warm spot on the floor, a water bill that jumped for no reason, and the sound of water running when nothing is on, you have a slab leak until proven otherwise. Shut the water off at the main and call someone.
Here is the part people underestimate: the damage is a function of time, not severity. A small leak found in a day is a plumbing repair. The same leak found in a month is a plumbing repair plus flooring, plus baseboard, plus whatever the moisture did to the cabinets.
The signs
Roughly in order of how reliable they are:
- A warm patch on the floor. The most specific sign there is. It means the leak is on the hot side, and hot-side leaks are the majority — hot water is harder on copper than cold.
- Running water you can hear with everything off. Stand in a quiet house at night. If you can hear water moving, water is moving.
- The water bill jumped and nothing changed. A leak runs continuously, so it shows up as a step change rather than a gradual drift.
- Pressure has dropped. Especially at fixtures furthest from the heater.
- Damp carpet, warped flooring, or a musty smell with no spill to explain it.
- Cracks appearing in flooring or drywall. A late sign — that is the soil under the slab moving.
The meter test you can run yourself
This takes ten minutes and costs nothing, and it is genuinely worth doing before you call anyone.
- Turn off every fixture in the house. Every tap, the ice maker, the irrigation, anything that draws water automatically.
- Find your water meter — usually a concrete box near the street.
- Look at the low-flow indicator. On most meters it is a small triangle or dial that spins on the tiniest flow.
- Watch it. If it is turning with everything off, water is going somewhere it should not.
To narrow it further: close the shut-off valve on the house side and check the meter again. Still turning means the leak is between the meter and the house — that is a service line problem, not a slab leak. Stopped means it is inside, and now a slab leak is very much on the table.
Tell whoever you call what the meter did. It saves diagnostic time and it saves you money.
Break the slab, or reroute
Once the leak is located there are two honest options and one wrong answer.
Open the slab and repair in place. Cut the concrete, expose the pipe, fix the failed section, patch. Contained and relatively cheap when the leak is accessible and the rest of the plumbing is in good shape. You are betting the rest of that line has life left.
Reroute. Abandon the run under the slab and re-run that line overhead through the attic or through walls. More disruption to the finished house, no concrete work, and the new run is accessible forever after. You are buying out of any future leak on that line.
Which one is right depends on the age and condition of the rest of the piping, where the leak sits, and how many times this has happened. On a first leak in otherwise sound copper, a spot repair is reasonable. On a second or third leak in the same house, spot-repairing again is throwing money at a system that is telling you what it plans to do next.
Here is the opinion: if you have had two slab leaks, stop repairing individual leaks and price a repipe. The third one is coming and it will not wait for a convenient month.
The wrong answer is letting someone jackhammer the floor before the leak has been properly located. Location comes first, always.
What it probably is not
Not every wet floor is a slab leak, and it is worth ruling out the cheap causes first.
- A failed wax ring under a toilet puts water on the floor near the base and can wick a surprising distance.
- A shower pan leak shows up as damp flooring just outside the shower and only after someone has used it.
- A water heater seeping quietly at the base can soak an area of garage or closet floor with no other symptom.
- Irrigation running against the foundation. Common here, and it will happily fake most of the outward signs.
The meter test separates these from a real slab leak faster than anything else, because none of them run continuously with every fixture shut.