Signs of a Slab Leak and How to Confirm One

Is your house even on a slab?

Start here, because it changes everything. Many older Winston-Salem neighborhoods are mostly crawl space and basement houses, where the supply pipes hang below the floor and a leak can be seen and reached from underneath. A true slab leak happens only where the floor is concrete poured on the ground: slab ranch houses, many newer subdivisions, garages, finished additions and some townhomes. Plenty of local houses are a mix, such as a crawl space under the main house and a slab under a den added later. If you can open a crawl space door and see the floor joists, the leak you are chasing is probably not under concrete.

What are the warning signs?

Slab leaks are usually noticed through a combination of small clues rather than one dramatic one. Most of the signs below appear in Fixr's 2025 cost guide; the warm floor and water heater clues are specific to a leak on the hot side:

• A water bill that keeps creeping up with no change in how the household uses water.
• A warm spot on the floor. A leak on the hot water line heats the concrete above it, which you may feel barefoot on tile or notice where the cat likes to sleep.
• The sound of running water when every fixture is off.
• Musty odors, mold or damp flooring in rooms built on the slab, sometimes following a line across the floor.
• New or widening cracks in the slab or along baseboards.
• Lower water pressure throughout the house.
• A puddle or extra-green strip of grass near the foundation.
• The water heater running constantly or the hot water running out early, which fits a hot side leak.

Several of these have other explanations. A running toilet raises the bill too, and a pressure drop can come from a failing regulator or old galvanized pipe. That is why the next step is a test rather than a guess.

How can you check before calling anyone?

The EPA's WaterSense program describes a simple meter test: read the meter, use no water for two hours, then read it again. If the number has changed at all, you probably have a leak. Make it sharper this way:

1. Turn off the ice maker, irrigation and anything on a timer.
2. Put a few drops of food coloring in each toilet tank. Color in the bowl without flushing means a leaking flapper, which is far more common than a slab leak.
3. Read the meter, wait, and read it again. Many newer meters show a small leak indicator that turns even at very low flow.
4. If it moved, close the house's main shutoff and repeat. If the meter keeps moving with the main closed, the leak is likely on the buried service line in the yard, not under the slab.
5. Feel along the floor for warm areas and listen near the water heater and along hallway floors at night when the house is quiet.

In Winston-Salem, customers should read the meter only if it is visible without opening the box; the city asks people not to open meter boxes because the smart meter equipment is fragile. Salisbury-Rowan Utilities customers can set up leak alerts through the EyeOnWater service.

For context, EPA estimates the average household's leaks waste more than 9,300 gallons a year, and about 9 percent of homes have leaks wasting 50 gallons or more a day. Most of those are toilets, faucets and fixtures, not slabs.

How does a plumber find a leak under concrete?

The goal is a mark on the floor small enough that only one patch of concrete has to come up. A typical sequence:

• Isolation. Closing the water heater inlet or individual valves shows whether the loss is on the hot or cold side.
• Pressure testing. A gauge on an isolated section shows whether it holds or drops.
• Acoustic listening. Ground microphones pick up the hiss of water escaping through concrete.
• Thermal imaging. A thermal camera can trace a warm line across the floor when the hot side is leaking.
• Line locating. Tracing the pipe route keeps the search focused on where the pipe actually runs.

HomeAdvisor's June 2026 guide puts most leak detection bills at $150 to $400. Fixr's 2025 guide gives $100 to $400, depending on how involved the testing is. If you already suspect a hidden leak, ask for non-invasive leak detection before agreeing to any concrete work.

What are the repair options and what do they cost?

Overall, HomeAdvisor reports an average slab leak repair of $2,280 with a typical range of $630 to $4,400, and Fixr gives a national average of $2,500 with most jobs between $1,000 and $4,000. The two guides disagree sharply on rerouting, which reflects how much a reroute depends on the house: one short hot water run through an accessible wall is a different job from replacing every buried line. Flooring replacement, drying and drywall are typically extra and often billed by other contractors.

Which approach fits depends on the pipe. A single leak on otherwise sound pipe is a good candidate for a spot repair through the slab. A second or third leak on the same buried copper usually means more are coming, and a reroute or whole-house repiping stops paying for repeat concrete work. Plumbing permits in Winston-Salem and unincorporated Forsyth County come from the joint City-County Inspections Division, and the plumber normally pulls them.

Why do slab leaks happen?

Fixr lists the main causes as soil movement that stresses the slab and pipes, corrosion of copper, galvanized or cast iron pipe over time, and water chemistry, especially acidic water. For reference, the 2025 Winston-Salem/Forsyth County Utilities water quality report lists an average pH of 7.5 and soft water, with hardness averaging 17.5 ppm. Abrasion where a pipe rubs against concrete or gravel, and poor installation, also play a role. About 43 percent of Winston-Salem homes were built before 1980, so many slab houses in the area have pipe that has been in the ground for decades.

Is a slab leak an emergency?

If water is coming up through the floor, the meter is spinning fast or a floor is lifting, close the main and treat it as urgent, using the steps for the first minutes of a pipe break. A slow leak can usually wait for a scheduled visit, but it keeps wetting the ground under the house until it is fixed. If the meter test pointed to the yard instead, see a plumber about service line repair; the City of Winston-Salem says owners handle breaks on private property and the utility handles mains in the street. If you are not sure what your service line is made of, the utility's inventory at cityofws.org/pipes lets customers look it up and report it.

For insurance, the NC Department of Insurance advises reporting a loss promptly, photographing damage before and after emergency repairs and keeping receipts. Policies differ on whether they pay for the pipe itself, the work to reach it, or only the resulting damage, so ask your agent before the floor is opened. Local plumbers who do slab work are available through this service in towns such as Kernersville and Advance, and the rest of the guides cover related problems such as pressure that has slowly dropped.