Guide · updated 2026-10-02
Signs of a slab leak, and how to check each one
Slab leaks rarely announce themselves with a puddle. They show up as small, odd things (a warm floor, a sound, a bill) that are easy to explain away. Here's what to look for and how to test each sign.
1. A warm spot on the floor
Walk the house barefoot, especially hallways, the kitchen and bathrooms. A patch of tile or laminate that's warmer than the floor around it (sometimes noticeably hot) is the most specific sign of a hot-water slab leak. Pets often find it first and start sleeping in the same spot.
Check it: turn off the water heater's cold-water inlet for an hour. If the warm spot cools, the leak is on the hot line.
2. The sound of running water
Late at night, with everything off, put your ear to the floor in a quiet room. A faint hiss or trickle under the slab is water escaping under pressure. Sometimes it's audible near the water heater or at a hose bib.
3. The meter moves with everything off
This doesn't prove the leak is under the slab, but it proves there's a leak. Close the house shut-off: if the meter stops, the leak is inside, which in a single-storey slab home often means under the floor.
4. The water heater runs more than it should
A hot-side leak means the heater is constantly refilling and reheating. If the burner seems to fire often when nobody's used hot water, or the gas bill has climbed along with the water bill, take note.
5. Damp flooring or carpet at the edges
Water under a slab finds the easiest way up: cracks, expansion joints, the edges of rooms. Carpet that's damp along a wall, a tack strip that's rusted, or laminate that's started to cup are all signs.
6. New cracks in tile or slab
Settling causes cracks too, so cracks alone don't mean a leak. A new crack combined with any other sign on this list is worth investigating, especially in north Orange County and Inland Empire neighbourhoods with expansive clay soil.
7. A musty smell or mould at the baseboards
Persistent damp under flooring feeds mould. A musty smell in one room that ventilation doesn't clear, or dark spotting at the base of drywall, can point to water coming up from below.
8. Lower water pressure
A large leak diverts enough water to notice at the shower or a far bathroom. On its own, low pressure has many causes; alongside a moving meter, it's another clue.
Which signs mean act today
| Sign | How urgent |
|---|---|
| Water visibly coming up through the floor | Shut off the main now and call |
| Hot floor plus a moving meter | Shut the water heater inlet; book soon |
| Damp carpet or cupping laminate | Book within days; move furniture off it |
| High bill only, nothing visible | Run the meter checks, then book |
| New crack only | Monitor; check the meter |
What a plumber will do
A slab locate combines a pressure test (to confirm and to split hot from cold), thermal imaging, and acoustic listening across the floor to mark the spot. From there, the choice is between a spot repair or a reroute. Typical costs are in what leak detection costs.