Slab Leak Signs: How to Check for a Leak Under Your Floor
By Mike Holt, NJ Master Plumber · June 11, 2024 · 4 min read · Updated November 1, 2026
Quick answer
The usual signs of a slab leak are the sound of running water when nothing is on, a warm or damp spot on the floor, a water bill that jumps for no reason, and new cracks in floors or walls. To check, turn off every fixture and watch your water meter. If it keeps moving, water is leaking somewhere.
- Difficulty
- Easy
- Time
- 30 to 60 minutes
- You need
- Flashlight, food coloring, your water meter
Rather not risk it? A licensed plumber can handle this for you, with written upfront flat-rate pricing options before any work starts.
A slab leak is a leak in a water pipe that runs under a concrete floor. Many Central Jersey homes have basements, but ranch houses on slabs, additions, finished garages and many condos sit on concrete, and their pipes run underneath. The leak is out of sight, so the clues are indirect.
What are the signs of a slab leak?
- The sound of running water when every faucet and appliance is off
- A warm or hot spot on the floor, often on tile or hardwood (a hot-water line)
- A water bill that jumps while your use hasn't changed
- Damp carpet, swollen baseboards, or flooring that lifts or buckles
- New cracks in the slab, floor tile or walls
- Low water pressure, or a water heater that seems to run all the time
- A musty smell or mildew near the floor
One sign alone can have another cause. Two or more together are worth checking.
Check for a leak with your water meter in 6 steps
Step 1: Turn off everything that uses water
Close every faucet. Make sure the dishwasher, washing machine, ice maker, water softener and lawn sprinklers aren't running, and that no toilet is refilling.
Step 2: Find your water meter
In most Central Jersey homes the meter is in the basement or crawl space where the water line comes in, next to the main shut-off. Some towns put it in a covered pit near the curb.
Step 3: Look at the leak indicator
Most meters have a small triangle or star-shaped dial that spins when any water flows. If it's turning with everything off, water is going somewhere. If your meter has no indicator, write down the full reading.
Step 4: Wait an hour and check again
Don't use any water for 30 to 60 minutes, then read the meter again. If the number went up, you have a leak.
Step 5: Rule out the toilets
Toilets are the most common hidden leak. Put a few drops of food coloring in each toilet tank and wait 15 minutes without flushing. Color in the bowl means the flapper is leaking. Fix that, then repeat the meter test. Our guide to a toilet tank that won't fill covers the parts.
Step 6: Check for a warm spot
Walk the floors barefoot, especially near the water heater, kitchen and bathrooms. A warm patch on a slab floor points to a leak in a hot-water line under it.
If the meter still moves after you've ruled out the toilets and every fixture you can see, the leak is likely under the floor, in a wall, or in the line from the meter to the house.
How do plumbers find a slab leak?
We start by confirming the leak and testing the hot and cold lines separately. Then we trace it with a thermal camera, which shows warm and cold spots that water leaves behind, and a moisture meter, which reads how wet the floor and walls are. The photos here are from our own leak searches.
Finding the spot first means only a small patch of floor gets opened, or none at all if rerouting the line makes more sense. See our leak detection page and what leak detection costs.
How is a slab leak fixed?
There are two common ways. A spot repair opens the floor right above the leak, fixes the pipe and patches the concrete. A reroute runs a new line through the walls or ceiling and abandons the old one under the slab. Rerouting often makes sense when the pipes are old and a second leak is likely. Our repiping page explains when we recommend it. We do dig and excavate indoor sewer and water lines in the slab, but we don't do outside digging or excavation. Contact us to confirm whether we handle your scope and materials.
Think you have a slab leak? Call 908-873-6686 or request service online. Written upfront flat-rate pricing options. You choose.
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Stop and call us if
- The meter keeps moving with every fixture turned off.
- You feel a warm spot on the floor or hear water running under it.
- Water is coming up through the floor or soaking carpet or flooring.
- Your water heater runs much more often than it used to.
- The water bill went up and you can't find a leak anywhere you can see.
Call 908-873-6686.
Michael Holt
Owner and operator of Plumbing Doctor, NJ Master Plumber (Lic. #36BI01345600) and U.S. Marine Corps veteran.
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