Rule out the easy causes first
Short answer: before you assume a leak, check for radiant heat, sunlight, appliances and ducts, then confirm with the water meter.
A few things can warm a floor without any leak at all:
- Afternoon sun through a window or glass door
- A refrigerator, dishwasher or dryer venting warm air near the floor
- A heating duct running under or beside that spot
- Radiant floor heating, if the house has it
If none of those explains it, and the warm spot is there in the morning too, move on to the meter test.
The meter test
- Turn off every faucet, appliance and irrigation line in the house.
- Find your water meter, usually in a box near the street.
- Watch the small leak indicator, which may be a dial, a star or a triangle.
- If it moves while everything is off, water is leaking somewhere.
Next, turn off the cold supply valve on top of your water heater and watch the meter again. If the indicator stops, the leak is on the hot side, which matches a warm floor.
Other signs that point to the slab
- You can hear water running when everything is off
- The water heater runs far more often than it used to
- The carpet is damp or the floor smells musty
- Tile cracks or the wood flooring starts to cup
- The water bill has jumped and stayed high
Why it’s common in slab tracts here
Many of the post-war homes around Whittier, Norwalk and Pico Rivera were built on concrete slabs, with copper water lines run in or under the concrete. After decades of heat, pressure and contact with the soil, that copper can wear through in pinholes. Hot lines tend to fail first, which is why so many slab leaks announce themselves as warmth rather than water.
What to do while you wait for a plumber
Shut the cold supply valve on top of the water heater, which stops it from feeding the leak and reheating water that’s escaping into the ground. You’ll lose hot water, but you’ll stop the damage and the gas or electric bill. Take photos of any damp flooring for your insurer.
How it’s fixed
A plumber pinpoints the leak with pressure tests, listening equipment and sometimes a thermal camera. Then you have three choices: open the floor for a spot repair, reroute the line overhead so the slab stays closed, or repipe the house if it has already had leaks.