Signs of a Slab Leak, Ranked From Strongest Clue to Weakest

Most slab leak signs are indirect. You never see the pipe, only what the escaping water does to the floor, the bill or the meter. Some of those clues point straight at a leak. Others have three or four innocent explanations. Here they are in order, strongest first.

Near-certain: the meter moves with everything off

Here's the test: turn off every faucet, the dishwasher, the washing machine, the ice maker line if you can, and the irrigation controller. Then find the water meter, usually in a concrete box near the street, and lift the lid. Most meters have a small dial, a triangle or a star, that spins when even a little water flows. If it turns while nothing in the house is using water, water is leaving the system somewhere.

That doesn't prove the leak is in the slab. A running toilet or a leaking sprinkler valve will spin the dial too. But it proves there is a leak, and from there it's a process of elimination. The full method, with the toilet and irrigation checks, is in the water meter leak test.

Strong: water you can hear but can't find

A hiss or a trickle under the floor, loudest at night when the house is quiet, is one of the most reliable slab clues. Pressurized water escaping from a pinhole makes a steady sound that carries through concrete. Callers describe it as "it's still going, so there has to be a leak."

Rule out the usual suspects first: a toilet refilling, a water softener regenerating, an irrigation line. If the sound stays steady with all of those off, the slab is the likely place. More on what that sound can mean is in running water under the floor.

Strong: a warm patch of floor

A warm spot on tile or wood, in a room with no heat source under it, usually means the hot water line below is leaking. Hot water spreads into the slab and warms the surface. Pets tend to find these spots before people do. If the warm patch sits between the water heater and a bathroom or the kitchen, that's the route the hot line usually takes.

The hot side has its own page, hot water slab leaks, because the first move is different: closing the cold inlet on the water heater.

Strong: water coming up through concrete

Water seeping up at the edge of the slab, in the garage, or through a crack in the floor is about as direct as slab clues get. The exception is a slab that sits over high groundwater or was poured without a moisture barrier. In those houses, damp concrete can be ground moisture rather than a pipe. The meter test separates the two: a still meter with a damp floor points away from the supply lines.

Moderate: damp carpet or baseboards with no visible source

Carpet that stays damp along a wall, a baseboard that swells, or a musty smell in one room can come from a slab leak. It can also come from a leaking shower pan, a dishwasher hose, or a window that lets rain in. Check what sits on the other side of the wall before blaming the slab.

Moderate: a water bill that jumps

A bill that doubles or triples with no change in habits is a classic slab clue. One caller put it simply: "he's getting almost triple the water usage." But a stuck toilet flapper, a broken sprinkler head or a new pool fill can do the same. Use the bill as the reason to run the meter test, not as the diagnosis. High water bill, no leak found walks through the order of checks.

Weak on its own: low pressure, cracks, mildew

A sudden drop in water pressure, a new crack in tile, or a mildew smell can all appear with slab leaks. Each has plenty of other causes. They matter when they show up alongside a stronger sign.

What to do once two or more line up

If the meter moves and you have one of the strong signs, the leak is probably under the slab. The next step is finding the exact spot before anyone opens the floor. Slab leak detection explains how plumbers narrow it down, and the home page covers the repair choices for Westminster houses.

Why Westminster-area houses show these signs often

Westminster grew from about 10,800 residents when it became a city in 1957 to about 91,000 by 2020. Much of that growth was single-story housing on concrete slabs, and houses from that period commonly have copper water lines under the slab. After several decades, that copper can thin and wear where it touches concrete. The same pattern shows up in Midway City and Fountain Valley.