Slab Leak Signs: How to Tell if a Pipe Is Leaking Under Your Foundation

What a slab leak is

In a slab-on-grade home, water supply lines (and drain lines) run through or under the concrete foundation. A slab leak is a leak in one of those pipes. Because the pipe is buried, the water has nowhere obvious to go. It soaks into the soil or finds its way up through the slab.

Two kinds matter:

• Supply line leaks are under pressure, so they leak all the time. They show up on the water meter and your bill. Hot water line leaks can also create warm spots on the floor.
• Drain line leaks only leak when water goes down the drain. They don't show on the meter, and are usually found through slow drains, odors or a camera inspection. See sewer line problems.

Signs of a slab leak

Listed roughly from most to least telling:

1. The water meter moves with everything turned off. The most reliable homeowner check. See the test below.
2. A warm or hot spot on the floor, often noticed barefoot or by pets lying in one place. That points to a hot water line.
3. Sound of running water when nothing is running, heard near the floor or in a quiet room at night.
4. A water bill that climbs without explanation, month after month.
5. Damp carpet, lifting or cupping wood floors, loose tiles, or musty odors without a visible source.
6. Water heater running more than usual. A hot-side leak means the heater keeps refilling and reheating.
7. Low water pressure throughout the house. See low water pressure for other causes first.
8. New cracks in walls, floors or the foundation. On their own, cracks are weak evidence. Soil movement alone causes cracks. Together with the signs above, they're worth mentioning to a plumber.

The meter test (10 to 30 minutes)

The City of McKinney's water meter usually sits in a box near the street. You're allowed to open the box lid to read the meter. Don't operate the valve inside, which the City says only its staff may use.

1. Turn off everything that uses water inside and out: faucets, showers, the dishwasher, the washing machine, the ice maker, water softener regeneration, and the irrigation system. Make sure no toilet is refilling.
2. Find the leak indicator on the meter. Most meters have a small triangle, star or dial that spins with even a tiny flow. Take a photo of the reading.
3. Wait 15 to 30 minutes without using any water. Take another photo.
4. Compare. If the indicator is spinning or the reading has changed, water is going somewhere.

Narrowing it down

• Close your main house shutoff (see how to shut off your water) and watch the meter again.
• Meter stops: the leak is inside the house (including under the slab).
• Meter keeps moving: the leak is between the meter and your house valve, which is usually the underground service line in the yard, or an irrigation line tapped before the house valve.
• Check toilets with the dye test in our leaking toilet guide. A running toilet is by far the most common hidden leak.
• Close the irrigation shutoff and repeat. Irrigation leaks are common and often mistaken for slab leaks.
• Close the cold inlet to the water heater. If the meter stops, the leak is likely on the hot side, which fits a warm spot.

If toilets, irrigation and fixtures are all ruled out and the meter still moves with the house valve open, a slab leak or hidden supply leak is a real possibility.

What not to do

• Don't start breaking up flooring or concrete to look for it. Professional leak detection narrows the location first, so repairs open the smallest area possible.
• Don't ignore it because the bill increase seems small. Water under a slab can affect the soil and flooring over time.
• Don't assume foundation cracks mean a slab leak, or that a slab leak means foundation failure. They're related questions, but different trades answer them.

What happens next

A plumber with leak detection equipment typically:

1. confirms the leak with a pressure test, isolating hot and cold lines,
2. locates it using acoustic listening equipment, thermal imaging for hot lines, or other methods,
3. recommends a repair.

Repair options generally include:

• Direct spot repair: opening the slab at the leak and repairing that section.
• Rerouting: abandoning the leaking line and running a new one through walls or the attic, avoiding the slab.
• Tunneling from outside the foundation, sometimes used to avoid disturbing interior floors.
• Repiping if the system has had repeated failures.

Which makes sense depends on where the leak is, the pipe material, the flooring above it, and the age of the plumbing. Our slab leak repair page explains the trade-offs, and leak detection covers how leaks are located without guesswork.

What to tell the plumber

• Results of your meter test, with before and after photos
• Whether the meter stopped when you closed the house valve
• Any warm spots, sounds or damp flooring, and where
• Your home's approximate age and any previous slab leaks or repiping