What a Slab Leak Is & Why Homes on Slabs Get Them

01What a slab leak actually is

Many homes, especially in the South and West, are built on a concrete slab poured directly on the ground, with the water supply lines run underneath or embedded within that slab before the concrete was poured. A slab leak is simply a leak in one of those pressurized lines, hidden inside or beneath the foundation rather than in an accessible wall or crawl space.

Because the water is under constant pressure, a slab leak does not stop when no fixture is running. It runs around the clock, which is why it can quietly waste thousands of gallons and undermine the soil under the foundation before anyone notices. That hidden, continuous nature is what makes slab leaks more serious than an ordinary drip.

02Why slab homes develop them

The classic cause is abrasion. Pre-1990s homes were often plumbed in soft copper run through the slab, and a copper line that touches concrete, rebar or sharp gravel will slowly wear through as it expands and contracts with hot water and as the house settles. Years of that micro-movement scrub a pinhole, and once it opens, pressure does the rest.

High water pressure accelerates everything. A house running above the 80 PSI that code allows puts excess water pressure on every joint and thin spot in the system, and slab lines are no exception. Add aggressive or acidic water that corrodes copper from the inside, and you have the full recipe: abrasion outside, corrosion inside, and pressure pushing on both.

Hot-water lines fail more often than cold ones, because heat makes the copper expand and contract more, increasing both the rubbing and the internal corrosion. That is also a useful diagnostic clue, covered next.

03Hot line vs cold line: reading the clues

The location of the leak shapes the symptoms. A hot-water slab leak often shows up as a warm or hot spot on the floor, sometimes warm enough to feel through tile or to make a pet favor that patch, along with the water heater seeming to run more than it should because it is constantly reheating water lost to the slab. A cold-water leak skips the warm floor but still shows the shared signs.

The signs every slab leak shares are the ones to watch: an unexplained jump in the water bill, the sound of running water with everything off, a drop in water pressure, damp or warped flooring, or even a faint mildew smell from moisture wicking up. We walk through each of these in detail, and how to confirm them, in our guide to the signs of a slab leak, which is the right next step if you suspect one.

04How a slab leak is found and fixed

You cannot eyeball a slab leak, so detection comes first. A plumber pressure-tests the system to confirm the leak is in the slab lines, then uses electronic acoustic listening gear and sometimes thermal imaging to pinpoint the spot through the concrete, so only a small area has to be opened. This non-destructive locating is its own service; our leak detection cost guide covers what that step runs before any repair begins.

Once located, there are a few repair paths. A spot repair opens the slab at the leak and fixes that section, which is the least invasive when there is a single failure. A reroute abandons the bad under-slab line and runs a new line through the walls or attic, often the smarter call on aged copper likely to leak again elsewhere. A full repipe replaces the system when multiple failures are coming. Which path fits depends on the pipe age and how many leaks you expect; our slab leak repair cost guide breaks down each option and what it costs.