Water running under concrete you cannot see through

A leak in a water line that runs beneath or within the concrete foundation slab your house sits on. Because the pipe is encased in or buried under concrete, the water has nowhere obvious to go. It saturates the ground beneath the slab, wicks up through the concrete, or travels along the outside of the pipe until it emerges at an edge. That is why the symptoms are strange and indirect.

The classic sign is a patch of floor that is warm underfoot with no reason to be. That is a hot-water line. Others include the sound of running water when every fixture is closed, a water bill that has climbed, unexplained damp at the base of a wall or along the edge of a floor, a drop in water pressure, and in bad cases a hairline crack appearing in the slab or a floor covering lifting.

Not usually in the burst-pipe sense, because the water is not filling your living room. But it is not something to sit on either. Water under a slab softens the soil the foundation bears on, and sustained saturation can eventually cause settlement, which is a structural problem rather than a plumbing one, and vastly more expensive. Days are fine. Months are not.

Frequently, yes, and it is often the better option. Rerouting abandons the failed under-slab section and runs a new line overhead through the attic or around through a wall, so nothing gets jackhammered. Spot repair through the slab makes sense when the leak is in an accessible location, the rest of the pipe is in good condition, and the floor covering above is not precious.

The system is isolated and pressurised so the escaping water makes a consistent sound, which acoustic equipment picks up through the slab. Thermal imaging confirms hot-water leaks by showing the warm plume. Between the two, the location is usually narrowed to within a foot or so before anything is opened, which matters enormously when opening means concrete. See leak detection.

It depends on the pipe, not the preference. If the line is copper of a certain age and this is the first failure, a spot repair is reasonable. If it is the second or third leak under the same slab, the pipe is telling you it will keep failing, and rerouting or repiping is the honest answer. Paying to jackhammer the same floor three times is the outcome to avoid.

Several reasons. Copper in direct contact with concrete or soil corrodes over time, particularly where the concrete was poured against it. Abrasion where a pipe rubs against the slab as the house moves. Water chemistry. Excess pressure. Poor installation with too-tight bends. And on the hot side, thermal expansion cycling the pipe against its surroundings thousands of times a year, which is why hot lines fail more often than cold.

Common in newer subdivisions across the state and particularly prevalent in parts of Hampton Roads, where the water table and flat terrain make slab construction the practical choice. Much of the older housing stock statewide is crawl space or basement built, so slab leaks are concentrated in postwar and newer developments.

A reroute leaves it untouched, which is often the deciding factor for houses with tile, hardwood or polished concrete. A spot repair means cutting through whatever is on top, and matching a discontinued tile or refinishing a hardwood patch is sometimes more expensive than the plumbing. That cost belongs in the comparison.

Policies vary considerably and it usually turns on whether the escape was sudden, and whether the policy covers the cost of accessing the pipe as distinct from repairing it. Many policies pay for breaking and restoring the slab but not for the pipe itself. Get the plumber’s findings in writing with photographs and take that to your insurer before committing to an approach.