Slab Leak Signs: What to Look For and How to Check

Could your house even have a slab leak?

Only if water lines run under or through a concrete floor poured on the ground. Many older Durham neighborhoods are built over crawl spaces, where supply pipes hang under the floor and a leak can be seen and reached from below. Slab foundations show up in ranch houses, many newer subdivisions, townhomes, garages and additions. Durham has seen heavy construction since 2000, with a median year built of 1995 in Census survey data, so plenty of local homes sit on concrete. Some houses are a mix, with a crawl space under the original rooms and a slab under a den or garage added later. If you can open a crawl space door and see joists, start your search there before assuming the worst.

Which signs point to a leak under concrete?

A slab leak rarely announces itself all at once. It shows up as a cluster of small clues, and each one has a more ordinary explanation you should rule out first.

Most of these signs appear in Fixr's 2025 slab leak cost guide. A water heater that seems to run constantly, or hot water that runs out early, fits the same pattern when the leak is on the hot side. Before blaming the slab, put a few drops of food coloring in each toilet tank and wait without flushing. Color in the bowl means a leaking flapper, a far more common problem and an easy toilet repair.

How do you test for a leak yourself?

EPA's WaterSense program describes the basic test: read the water meter, use no water for two hours, and read it again. Any change means a leak somewhere. To narrow it down:

1. Turn off the ice maker, irrigation and anything on a timer, and make sure nobody runs water during the test.
2. Note the reading, or watch the small leak indicator many meters have, which moves even at very low flow.
3. If water is moving, close the main shutoff at the house and watch again.
4. If the meter stops with the main closed, the loss is inside the house, under the slab or in a wall. If it keeps moving, the buried line between the meter and the house is the likely culprit.
5. With the house quiet at night, listen along hallway floors and near the water heater, and feel tile floors barefoot for a warm strip.

For scale, EPA estimates the average household's leaks waste more than 9,300 gallons a year, and about 9 percent of homes have leaks losing 50 gallons or more a day. Most of that comes from toilets, faucets and fixtures rather than slabs. During Durham's drought restrictions this year, a leak running under the floor wastes water the whole community has been asked to save.

How does a plumber pinpoint the leak?

The point of professional detection is a mark on the floor precise enough that only one small patch of concrete, if any, has to come up. A typical visit combines several methods:

• Splitting hot from cold: shutting the valve feeding the water heater tells the plumber which side is losing water.
• Gauge tests: a sealed section either holds pressure or drops, which confirms or clears it.
• Listening gear: sensitive ground mics hear the hiss of escaping water through tile and concrete.
• Infrared cameras: a hot side leak shows up as a warm streak on the screen.
• Pipe locators: mapping the actual route keeps the search on the pipe instead of guesswork.

Expect a detection visit alone to run $150 to $400 by HomeAdvisor's June 2026 numbers, or $100 to $400 by Fixr's 2025 figures. When a hidden leak is the worry, request non-destructive leak locating before agreeing to any concrete work.

What are the repair choices, and what do they cost?

Looking at every method together, HomeAdvisor's figures center on $2,280, with most homeowners spending somewhere from $630 to $4,400. Fixr lands at $2,500 nationally and says the bulk of jobs fall from $1,000 to $4,000. Where the two part ways is rerouting, and the gap is understandable: swapping one short hot line through an open wall bears little resemblance to replacing all the buried piping in a large house. Budget separately for new flooring, drying equipment and drywall, which other trades usually bill.

The right choice depends on the pipe more than the floor. One leak on otherwise sound pipe is a good candidate for a targeted slab leak fix. A second or third leak on the same buried line usually means more are on the way, and a reroute or full replacement of the supply piping stops the cycle of opening the floor. Durham City-County Building & Safety issues plumbing permits for both the City and the County and requires them for replacement work; the plumber normally pulls the permit, and the department's number is 919-560-4144.

What causes slab leaks?

Fixr lists expansive soils, excessive water pressure, unsuitable pipe materials and water chemistry as the main causes. Abrasion where a pipe rubs against concrete or gravel and poor original workmanship also contribute. On the chemistry side, Durham's 2024 water quality report lists an average pH of 7.5, and the City adds orthophosphate and sodium hydroxide as part of its corrosion control treatment. In a newer slab home, a bad joint or a pipe rubbing against the concrete is a more likely culprit than worn-out pipe.

Hot water lines tend to fail first, which is why a warm floor is such a useful clue. If the water heater has been working overtime, the guide on fixing versus replacing a water heater is worth a look once the leak is found, since a hidden leak can make a healthy tank seem to be failing.

Is it urgent, and who pays?

Water seeping up between tiles, a meter dial racing, or flooring that has started to buckle all mean you should shut the main now and use the steps for the first minutes of a pipe break. A gradual leak with none of those symptoms can generally be booked for a normal appointment, but every day it runs adds moisture to the ground beneath the foundation.

If the meter test pointed to the yard, the problem is the service line. Durham's 2024 water quality report shows the owner responsible for the building side and the City for the main side, without stating the exact split point, so a yard service line repair is usually the owner's job. The same report says homes built before 1988 may have a lead service line, and the City is inventorying lines and offers lead testing. That is worth checking if the line is being dug up anyway.

On the insurance side, the NC Department of Insurance wants losses reported right away, with photos taken both before and after any emergency work and every receipt saved. Coverage varies a lot: one policy may pay to access the pipe, another only for the water damage, and few cover the worn pipe itself. Call your agent before anyone cuts concrete. Plumbers who handle slab work are available through this service across the area, including a Pittsboro slab leak repair and homes in Creedmoor. The full set of homeowner guides covers related problems.