A slab leak: water under the floor of a house with no basement.

Close the main shut-off valve, wait fifteen minutes, and look at the water meter. If the meter is still moving with the house isolated, water is leaving the supply somewhere you cannot see, and under a slab is the usual answer.

That one test costs nothing and it is the difference between a plumber locating a leak and a contractor guessing where to break the floor.

What a slab leak is

Most houses without a basement sit on a concrete slab poured on the ground, and in many of them the water supply pipes run in or under that concrete. When one of those pipes fails, the water has nowhere obvious to go. It saturates the ground under the slab, works its way up through cracks and expansion joints, or travels sideways and appears somewhere unrelated.

It is one of the few leaks that can run for weeks before anyone notices, which is why it does more damage per gallon than a burst pipe does.

The signs, in order of how reliable they are

The meter test. Close the main and watch the meter. Still moving means a leak on your side. This is the only one of these signs that is close to proof.

A hot spot on the floor. A warm patch of floor tile or vinyl, especially one you can feel through socks, means a leak on the hot side. This is the classic slab leak sign, and it is fairly specific.

The sound of running water with everything off. Stand in a quiet house at night with every faucet closed. A faint continuous hiss or rush is a supply line running somewhere.

A water bill that jumped for no reason. A useful confirmation, though it tells you nothing about location.

Damp or discolored flooring, or a musty smell in one part of the house. Carpet that is damp in a patch with nothing above it. Vinyl or laminate lifting at a seam. Baseboards that are dark at the bottom.

A crack that has appeared or widened, or a door that has stopped closing. Saturated soil under a slab moves, and the slab moves with it. This is a late sign and an expensive one.

Unexplained wet ground outside next to the foundation, or a patch of lawn that is greener than the rest.

What to do right now

1. Close the main shut-off valve and confirm with the meter. On a slab house the main is usually in a utility closet, the garage, near the water heater, or outside near the foundation or in a box near the street. If it will not turn, there are other ways to stop the water.
2. Leave it off if the leak is significant and you can live without water for a few hours. Every hour of a slab leak is more saturated ground.
3. Get anything absorbent off the affected floor. Rugs, boxes, furniture with wooden feet.
4. Photograph everything, including the meter reading, the warm patch, and the affected flooring, before anyone cuts anything.

Ruling out the things that look like a slab leak

Before anyone talks about breaking concrete, rule out the cheaper explanations:

• A leaking toilet or a running tank. Meter movement with the house isolated will not distinguish a slab leak from a toilet whose flapper leaks continuously. Close each fixture valve one at a time and rerun the meter test.
• An irrigation line or a hose bib, which is on your side of the meter and will show exactly the same way.
• A water heater relief valve quietly discharging.
• Condensation or a plumbing leak above, on an upper floor or in a wall, showing up at floor level below.
• Ground water or drainage, which is not a leak at all. If wet floors follow rain rather than being constant, it is a rain and drainage problem and it is treated completely differently by your insurer.

The constant versus intermittent distinction is the most useful one. Supply leaks do not care about the weather.

How it gets found and fixed

Locating is a specialist job, done with acoustic listening equipment, pressure testing of individual lines, thermal imaging, or tracer gas. Expect the locating and the repair to be discussed separately, because they are separate work.

The repair options are usually three:

• Break and repair. Open the slab at the leak, fix that section, patch the concrete. Cheapest if the pipe is otherwise sound and the access is reasonable.
• Reroute that line. Abandon the section under the slab and run new pipe through the walls or the attic instead. Common where the leak is awkward to reach or the concrete is finished.
• Repipe the house. Where a system has failed once, particularly in older copper in aggressive soil, a second and third leak often follow. Doing it once is sometimes cheaper than three repairs.

A house that has had two slab leaks is telling you something about the whole system, not about that one pipe.

The insurance position, which has a useful wrinkle

The general rule holds: most policies pay for sudden and accidental damage and not for the failed part itself. So the damage to flooring, cabinets, and belongings is usually the covered part, and the pipe repair usually is not.

The wrinkle worth knowing is access. Many homeowners policies contain a provision that pays for tearing out and replacing part of the building in order to reach the leak, even while excluding the repair to the plumbing itself. On a slab leak that access cost, the cutting of the concrete and the making good afterwards, can be a large share of the bill. Ask your insurer about it specifically and by name.

Two things work against a slab leak claim:

• Gradual damage. A leak that has run for months is exactly what the gradual or repeated seepage exclusion is written for. This is the most common reason these claims fail, and it is why a slab leak is worth investigating the week you suspect it rather than the season you get around to it.
• Late notice. Report it as soon as you suspect it, not after the repair.

After the water is off

The ground under the slab stays saturated for a long time, and the moisture keeps moving up into the flooring and the wall base until it dries. That is not a fan and a weekend. Drying under and around a slab needs equipment, and flooring laid over a wet slab traps the moisture rather than solving it.

The 24 to 48 hour window applies to what got wet inside, and it has usually already elapsed by the time a slab leak is found, which is why a moisture reading before new flooring goes down is worth insisting on.

The short version

Close the main, wait, and read the meter. Still moving means a leak on your side. Warm patch on the floor means hot side. Rule out toilets and irrigation before anyone cuts concrete. Report it the week you suspect it, and ask your insurer specifically about coverage for the access work.