What a slab leak is
In a slab-on-grade house, some water lines may run beneath or within the concrete floor. If one of those lines develops a leak, water can spread under flooring, into the soil and sometimes up through cracks. Because the pipe is out of sight, the first clue is often indirect: a warm patch on tile, a damp carpet edge, or a water bill that does not make sense.
Hot water lines are common culprits because heat and expansion add stress. A leak on the hot side can also keep the water heater running more than usual. Whatever the line, the first job is to confirm the leak and its location before choosing a fix.
Clues that point under the floor
- A warm or hot spot on the floor, often in a hallway or near a bathroom.
- Damp or musty carpet, or flooring lifting with no spill above it.
- The sound of running water when everything is off.
- A water meter that keeps moving with all fixtures closed.
- A water heater that seems to run constantly.
- Cracks in the slab or baseboards with moisture nearby.
What to do first
- Run the meter test.Turn off every fixture and appliance, then watch the meter. Movement means water is going somewhere.
- Isolate the hot side.If the floor is warm, closing the cold inlet valve on the water heater can stop flow into a hot water leak.
- Close the main if needed.If water is surfacing or the leak is large, close the main shutoff.
- Get it located.Ask a licensed plumber about leak detection so the location is confirmed before anything is opened.
Comparing the fixes
| Approach | What it involves | When it fits |
|---|---|---|
| Spot repair | Open flooring and concrete at the leak, repair, patch | One isolated leak in otherwise sound pipe |
| Single-line reroute | Abandon the leaking line and run a new one overhead or in walls | A leak in a line that would be hard to reach or likely to fail again |
| Whole-house repipe | Replace all supply lines, usually routed through the attic | Repeat slab leaks or aging pipe throughout |
Questions after a slab leak is found
- Is the leak on the hot or cold side, and how was it located?
- What are the pros and cons of repairing it versus rerouting it?
- Is this the first leak in this line, and what does the rest of the pipe look like?
- If you reroute, where will the new line run and what openings are needed?
Common questions
Is a slab leak an emergency?
If water is surfacing or flowing heavily, close the main and treat it as urgent. A small slow leak still deserves quick attention because damage builds over time.
Will insurance pay for the slab leak?
Coverage varies by policy. Some cover access and damage but not the pipe itself. Check your own policy.
Why reroute instead of repairing?
Rerouting moves the line out of the slab, so that pipe cannot leak under the floor again. It is often chosen when a line has already leaked or is hard to reach.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.