How to Find a Hidden Water Leak in Your House

Small hidden leaks waste a lot of water and do a lot of damage: rotted subfloors, mold in walls, ruined ceilings. Home inspectors look for the clues. Here's how to check your own house.

01Clues something's leaking

• A water bill higher than usual with no change in use
• Stains on ceilings or walls, bubbling paint or warped flooring
• A musty smell or mold in a cabinet or closet
• The sound of running water when everything's off
• Soggy spots in the yard over the water line
• A toilet that refills on its own

02Step 1: The water meter test

1. Turn off every faucet and water-using appliance, including the ice maker and irrigation.
2. Find your water meter (often in the basement, a utility room or a box near the street).
3. Look for the leak indicator, a small triangle or dial that spins with even tiny flows, or write down the reading.
4. Wait 30 minutes to an hour without using water and check again.

If the indicator moves or the reading changed, you have a leak somewhere after the meter.

03Step 2: Narrow it down

• Check toilets first, the most common culprit. Put a few drops of food coloring in the tank and wait 15 minutes without flushing. Color in the bowl means the flapper is leaking.
• Shut off the main valve inside the house (leaving the meter running). If the meter still moves, the leak may be in the line between the meter and the house, often underground.
• Check under every sink, behind the washer, around the water heater, the dishwasher and the refrigerator line.
• Check outdoor spigots and irrigation.

04Step 3: Look for the hidden ones

• Ceilings under bathrooms: a shower pan or tub drain leak shows up below.
• Slab leaks: warm spots on the floor, the sound of water, cracks.
• Behind walls: a moisture meter or thermal camera helps; a plumber can use acoustic leak detection.

05When to call a plumber

Leaks you can't find, leaks in walls, under slabs, or in the main line, and any active leak you can't stop. Know where your main shutoff valve is and that it works before you need it.

06Prevent the next one

Replace old supply hoses (braided steel hoses are sturdier), insulate pipes in cold spots, and consider a water leak detector or automatic shutoff valve.

Leaks are one of the items on our fall home inspection checklist.

Photo: BCA pipe repair 2 in Cabacú by Sarang, Public domain.