Slab Leak Signs and What to Do About Them

Could your home even have a slab leak?

A lot of houses around Washington have basements, and in a typical basement house the water supply pipes run along the basement ceiling or walls where you can see them. That makes a true supply-line slab leak less common than in regions built mostly on slabs. It does not make it impossible. Slab leaks show up in:

• Slab-on-grade houses and townhouses, especially those with the water heater and laundry on the main floor.
• Basement floors, where the incoming service line, a hot water line to a basement bathroom, or a buried line to a floor-level fixture runs under the concrete.
• Additions and sunrooms built on a slab and tied into the original plumbing.
• Older homes where past renovations buried supply lines in a new concrete floor.

If every supply pipe in your house is visible from the basement, a leak that looks like a slab leak may really be a leak inside a wall, a failing water heater connection, or a break in the service line outside. Ruling those out first saves money.

What are the signs of a slab leak?

HomeAdvisor lists warm or damp floors, standing water, sudden shifts in pressure, higher bills and the sound of running water as the common warning signs. None of them alone proves a slab leak. Several together make it likely.

How can you check at home?

1. Do a meter test. Turn off every faucet, the dishwasher, the washer, the ice maker and any irrigation. Find your meter and note the reading or watch the small leak indicator. Wait 30 minutes to an hour without using water. If it moved, water is leaving the system.
2. Take toilets out of the equation. A running toilet is the most common silent leak. Close the valve behind each toilet and repeat the test. If the meter now holds still, the problem is a toilet, not the slab.
3. Separate the house from the service line. Close the main shutoff inside the house and watch the meter again. If it still moves, the leak is between the meter and the house, which is outside the slab entirely.
4. Check hot versus cold. Close the cold water inlet valve on top of the water heater. If the meter stops, the leak is on the hot side, which matches a warm floor.
5. Walk the floor barefoot. Warm patches on tile or concrete, especially along a path between the water heater and a bathroom or kitchen, point to a hot line.

Where the meter sits depends on your provider. In Arlington the County owns the line up to and including the meter, and the owner has everything from the meter into the house. Fairfax Water customers own the underground lateral from the meter to the main shutoff in the house. DC Water handles the service pipe in public space while the private-side section belongs to the owner. In WSSC Water territory, the customer owns the line from the property line into the home. If you cannot find or read your meter, your utility's customer service number on the bill can help.

How do plumbers pinpoint the leak?

Good leak work starts with finding the exact spot so only a small section of floor has to be opened. Plumbers combine pressure tests that isolate hot and cold lines, acoustic listening equipment that picks up the hiss of escaping water through concrete, and thermal cameras that show warm spots from hot water leaks. Some also use line tracing to map where buried pipes run. HomeAdvisor puts most leak detection visits at $150 to $400, with an average of $280. Fixr's range is $100 to $400, depending on how involved the search is. For this step, ask for professional leak detection rather than a general service call.

What are the repair options?

Once the leak is located, there are a few ways to fix it. The right one depends on the pipe's age and material, how much of the floor is finished, and whether this is the first leak or one of several.

For the overall job, HomeAdvisor reports a national average of $2,280 for slab leak repair, with most homeowners paying between $630 and $4,400 in its 2025 figures, and its range for rerouting runs $600 to $4,000. Fixr puts the 2025 national average at $2,500. Local prices in the Washington area can run higher or lower; your quote will reflect the access, the flooring that has to be removed, and the permit. Plumbing permits come from the DC Department of Buildings in the District, from WSSC Water in Montgomery and Prince George's counties, and from the county or city building office in Northern Virginia.

Rerouting means abandoning the buried pipe and running a new line through walls, ceilings or the attic. It costs more up front but avoids opening the floor and gets the pipe out of the concrete for good. When pipes are old enough that a second leak seems likely, it is worth comparing a spot fix with a repipe of the house. You can also be connected with a plumber for slab leak repair in the District.

How urgent is a slab leak?

It depends on what you see. A meter that creeps slowly with no visible water is a leak to schedule promptly, within days, not one to ignore for months: water under a floor softens soil, feeds mold under flooring, and runs up the bill every day. Water coming up through the floor, a large warm area, or a sharp pressure drop deserves a same-day call. If water is spreading, close the main shutoff and follow the steps in our burst pipe checklist. If you are weighing whether to pay after-hours rates, our guide on when a plumbing problem can wait lays out the trade-offs.

Find a plumber near you

Slab and hidden-leak calls come from newer townhouse communities and older neighborhoods alike. Homeowners in Montgomery County can start with leak detection in Gaithersburg or plumbers in Germantown. In Prince William County there is help with slab leaks in Woodbridge, and in Loudoun, Ashburn leak detection. Low pressure can be a sign of a leak too, and our low water pressure guide covers the other causes.