Running Water Under the Floor When Nothing Is On

A steady hiss or trickle under the floor in a quiet house is one of the clearest hints of a slab leak. It's also one of the easiest to misread, because several normal fixtures make the same sound. Work through the list below in order before deciding it's the slab.

Rule these out first

1. A toilet refilling. A worn flapper lets water seep from the tank, and the fill valve tops it up every few minutes with a short hiss. Lift the tank lid and listen. The food-coloring test in the water meter leak test confirms it.
2. The water softener. Softeners regenerate on a timer, often at night, and the flow sounds like a running pipe. Check the unit's display or its schedule.
3. Irrigation. A valve that fails to close fully can run water to the yard all night. A line that passes near the house can sound like it's under the floor.
4. The water heater. Heaters tick, pop and hiss as they heat. That's normal. A steady flow sound into the heater with no hot water in use is not.
5. The neighbor's side. In condos and attached homes, the sound may come from the unit next door.

When the sound is the slab

If the sound stays steady with every fixture off, and the meter's leak indicator moves, the water is escaping from a pressurized line. Under a slab, that means a hole in a copper supply line, and the sound you hear is water forcing its way out into soil and gravel.

Plumbers use the same sound to find the leak. An acoustic ground microphone placed on the floor picks up the hiss, and the leak locator moves it across the room until the sound peaks. That's why the sound you hear in the hallway at night is useful to describe on the phone: it tells the plumber where to start listening.

Is it hot or cold?

Close the cold inlet valve on top of the water heater. If the sound fades and the meter slows or stops, the leak is on the hot side. Hot lines fail more often under slabs because they expand and contract with every use. If the sound continues, the leak is probably on the cold side or before the heater. Hot water slab leaks covers what to do next for the hot side.

What to write down before calling

• Where in the house the sound is loudest
• Whether it's constant or follows hot water use
• What the meter did with everything off
• Whether any floor spot is warm or damp

That information shortens the search. The full process is on the slab leak detection page.

Don't wait it out

A slab leak that makes noise is usually a steady leak, and steady leaks move a lot of water into the soil under the house over weeks. That can wash out soil under the slab and push up the water bill. Will a slab leak damage the foundation explains why that matters.