The short answer
A slab leak is a water pipe leaking under a concrete floor. In Northern Virginia that’s mostly townhouses, ramblers and additions built on a slab, plus basements where the water line comes up through the concrete. The signs are a warm spot on the floor, the sound of water running with everything off, a water meter that never stops, a water heater that runs all the time, and floors that get damp, cup or lift for no reason. Two of those together, and it’s worth a test today.
Is it worth fixing fast? Yes. A slab leak runs day and night, it’s often hot water you’re paying to heat, and the water has to go somewhere: into the soil under the house, up into the floor, or out through a wall. Unlike the lights and codes in error codes and warning lights at home, nothing in the house flashes for this one. These signs are all you get.
The signs
- A warm spot on the floor. Tile or wood that’s warm in one patch, in a room with no heating pipes, is a hot water line leaking under it. Bare feet find it first.
- The dog or cat picks one spot. Pets find a warm patch before people do.
- Water running with everything off. A hiss or rush from the floor or the base of a wall (hearing water running in the walls).
- The water heater never rests. It keeps heating new water to replace what’s leaking, and the gas or electric bill climbs with the water bill.
- Damp carpet, cupping wood, lifting vinyl or loose tile in one area, with no spill to blame.
- Cracks in the floor that are damp or crusted white.
- Lower water pressure all over the house (low water pressure).
- A musty smell that stays in one room, or water pooling outside along the foundation.
- A water bill that jumped with no change in how you use water (why the water bill is so high).
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The five minute test
- Rule out the toilets first, because a running toilet fools this test: put a few drops of food coloring in each toilet tank, wait 15 minutes without flushing, and look in the bowl. Color in the bowl means the flapper is leaking.
- Turn off all water and watch the leak indicator on the water meter (how to read the water meter leak indicator).
- If it moves, close the cold water valve on the pipe going into the water heater, on top of a tank or under a tankless unit.
- Watch the meter again. If it stops, the leak is on the hot side. If it keeps moving, shut the main valve at the house: if it stops then, the leak is on a cold line inside.
- Open the water heater valve again when you’re done, and don’t run hot water while it’s closed.
Why pipes leak under a slab
Hot lines leak under slabs more often than cold ones. They grow and shrink every time they heat up, and where they touch gravel, rebar or the concrete itself, they slowly wear through. Water chemistry pits copper from the inside over decades. A kink or a nick from when the house was built can take 30 years to become a hole. And Northern Virginia’s clay soil swells when it’s wet and shrinks when it’s dry, which moves everything sitting in it.
The three ways to fix it
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| Fix | What it is | Best when |
|---|---|---|
| Spot repair | Open the floor over the leak, repair the pipe, patch the concrete | One leak in pipe that’s otherwise in good shape |
| Reroute | Leave the old line under the slab and run new pipe through the walls or ceiling | The pipe under the floor is old, or the leak sits under something you don’t want torn up |
| Repipe | Replace the water lines in the house | It’s the second or third leak, or the pipe is failing everywhere |
Finding the exact spot first runs $200 to $500, and it’s what keeps a spot repair to one small opening instead of a trench across the kitchen (how leak detection works). A repipe runs $1,000 to $12,000, depending on the size of the house and how the lines run. From about $17 a month at 0% APR for 60 months, the whole price financed, on approved credit.
Holding a quote to jackhammer the floor or repipe the house? Our free second opinion looks at it first. You don’t owe us anything for looking.
Will insurance pay?
Often for part of it. Many homeowners policies pay to tear out and put back the floor to reach a leak, and for the water damage, but not to fix the pipe itself, which counts as wear. Policies differ, so call your agent before the floor comes up, take photos of every sign, and keep every invoice.
SwiftPro’s plumbers find and fix slab leaks across Northern Virginia. If you already know something’s leaking, a plumbing visit is $49 in regular hours, and the price of the fix is in writing before we start. Booking a plumbing repair? The whole-house plumbing inspection is included, so the next drip gets caught too. Our Water Test and Plumbing Safety Check is $29: a licensed plumber walks your plumbing with photos, checks the water heater and tests your water’s hardness at the kitchen table.
Get a plumber on it
Book a plumber online, call (703) 997-9757, or book the $29 plumbing safety check. We answer day or night. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What are the first signs of a slab leak?
A warm spot on the floor, the sound of water running with everything off, a water meter that keeps moving, and a water heater that seems to run all the time. Damp or cupping floors and a jump in the water bill often come next.
How do I know if a slab leak is on the hot or cold side?
With all water off and the meter’s leak indicator moving, close the cold water valve going into the water heater. If the meter stops, the leak is on the hot side. Open the valve again when you’re done.
How much does it cost to find a slab leak?
In Northern Virginia, leak detection at SwiftPro runs $200 to $500. Finding the exact spot keeps the opening in the floor small.
Is a slab leak an emergency?
It isn’t a flood, but it won’t stop on its own and it gets worse under the floor where you can’t see it. Have it found within days, not weeks. If water is coming up through the floor, shut the main valve and call now.
