How to check for a hidden water leak

Why a hidden leak matters

The leaks you can’t see do the most damage. A pipe inside a wall, a line under the slab or a slow drip behind a cabinet can soak drywall, carpet pad and flooring for weeks before a stain shows up.

The Smart Home Water Guide, made by the Arizona Municipal Water Users Association with the City of Phoenix, warns that water-using devices “can leak and cause damage to walls and floors, potentially creating an environment for mold or mildew.” The City of Phoenix adds that unexplained increases in water use may indicate a leak.

This guide covers two questions. Is water escaping somewhere? And has it reached the house? The repair itself belongs to a plumber or appliance technician. If wet materials turn up, that part is water damage cleanup.

Before you start: turn off everything that uses water

The meter test only works if nothing is using water. The Smart Home Water Guide says not to use water or run any water-using device in or around the home during the test.

Go through the house and the yard:

• Faucets, showers and toilets. Don’t flush during the test.
• Washer, dishwasher and refrigerator icemaker.
• Irrigation. Turn the controller off so a cycle doesn’t start mid-test.
• Pool autofill, water softener, reverse osmosis system and evaporative cooler.

The guide lists pool autofill, cooler and icemaker refills, and softener and reverse osmosis regeneration as normal reasons for small amounts of water use. It suggests turning off their water supplies before the test.

Step 1: Read the water meter

The Smart Home Water Guide says most residential meters sit in a concrete box near the curb or sidewalk at the front of the property. Lift the lid with a long screwdriver, and watch for insects, reptiles and other small animals that sometimes live in these boxes. Read the meter, but don’t touch the valve inside the box. The City of Phoenix asks residents not to shut water off at the meter.

On an analog meter, watch two things:

• The sweep hand. If it moves while everything is off, you have a continuous leak. One full turn equals 10 gallons or 10 cubic feet, depending on your meter.
• The low-flow indicator. This is a small triangle, star or gear on the dial. If it’s turning, water is flowing somewhere.

On a digital meter, you may need to shine a flashlight on the display to wake it up. Watch the flow rate screen for at least 10 flashes. A number above zero on any flash means a continuous leak. Some digital meters also store past use, which can show when a leak started.

Step 2: Give slow leaks time to show

A very slow leak can barely move the dial. Write down the full reading, wait, and read it again. Agencies suggest different waits:

• 20 minutes. The Smart Home Water Guide’s meter test uses two readings 20 minutes apart. If the number went up, you have a continuous leak.
• 30 minutes. The City of Scottsdale says to use no water for at least 30 minutes before the second reading. If the reading changed, it says, something on the property is pulling water through the meter.
• Two hours. The EPA’s WaterSense program suggests checking the meter before and after a two-hour period with no water use. If it changes at all, you probably have a leak.

If the meter didn’t move, you likely don’t have a continuous leak, though an appliance can still leak while it runs.

Step 3: Close the house valve to split outside from inside

If the meter moved, the next question is which side of your house valve the leak is on. The Smart Home Water Guide calls this the isolation method.

1. Leave the meter valve alone. The guide says the service line must stay full and pressurized during the check.
2. Close the house shut-off valve. It’s generally at the front or side of the house below a hose bib, where the pipe comes up from the ground. Turn it by hand. The guide warns that old or corroded valves can break.
3. Look for lines that bypass it. Irrigation and pool lines sometimes branch off before the house valve. If you see one, close its valve too.
4. Confirm and read. Turn on a faucet, start the irrigation controller and press the pool autofill. Nothing should flow. Then read the meter again.

If the meter still moves, the leak is in the service line, the buried pipe between the meter and your house. The Smart Home Water Guide says it’s the homeowner’s job to find and repair leaks on that side of the meter box, and to call a plumber. If the meter stops, the leak is past your valve: in the house, under the slab or on a branch line. Our shut-off valve guide shows where the valve usually sits in Valley homes.

Step 4: Check the usual suspects one at a time

Open the house valve again and isolate the systems below one by one. After each, read the meter. When it stops, you’ve found the leaking system.

• Toilets. The Smart Home Water Guide calls toilet leaks the most common leak inside the home, and notes they can be silent. Try its dye test: lift the tank lid, add a few drops of food coloring, and wait 15 minutes without flushing. Color in the bowl means a leak. The guide says a leaking toilet can waste over 6,000 gallons a month.
• Irrigation. Its shut-off is usually next to the backflow prevention device. If closing it stops the meter, the guide says the leak is most likely in the pipe between the backflow preventer and a valve box, or in a valve inside a box.
• Pool autofill. Close the pool supply valve, also usually next to a backflow device. If the meter stops, the guide suggests leaving the supply off and filling the pool with a hose until it’s repaired.
• Evaporative cooler. University of Arizona Water Wise notes that if the float valve sticks, water runs continuously. The City of Chandler lists coolers among the things that can keep a meter moving.
• Softener, reverse osmosis system, icemaker and water heater. Each usually has its own supply valve. Close them one at a time.

The City of Mesa’s list of common sources adds hose bibs. Irrigation and pool leaks usually soak the yard. Leaks inside walls or under floors are the ones that cause water damage.

Step 5: If the meter still moves, look closer

If every system is isolated and the meter still turns with the house valve open, the leak is probably in the supply pipes themselves. The Smart Home Water Guide suggests:

• Outside: look for wet spots, small holes and depressions in the yard along the pipe routes to the pool and irrigation valves.
• Inside: look for damp spots and water stains on ceilings and walls.
• On the floor: run hot water at nearby fixtures, then feel for warm spots. A hot spot could indicate a leaking hot water pipe under the concrete slab. The guide notes that most interior pipe leaks happen on hot water lines, because of expansion and contraction from temperature changes.

If you still can’t find it, the guide says you may need a plumber or a contractor who offers leak detection. If a leak under the slab is confirmed, see slab leak water damage.

Signs the water has already reached the house

A clean meter test doesn’t rule everything out. The Smart Home Water Guide notes that some leaks only happen while a device runs, so run the washer and dishwasher and watch. Then walk the house and look for:

• Warm spots on tile or concrete floors.
• Stains or damp patches on ceilings and walls, and bubbling or peeling paint.
• A musty smell, especially under sinks. The guide also lists wet spots and bowed cabinet bottoms.
• Swollen baseboards, or laminate and wood flooring that has started to cup or lift.
• Carpet that feels damp along one wall, with no spill to explain it.

The EPA says you may suspect hidden mold if a building smells moldy but you can’t see the source. Our guide to the signs of hidden mold covers where it tends to grow.

If you find wet materials

A hidden leak has often been running for a while, so don’t let the next steps wait.

1. Stop the water. Close the fixture’s valve or the house valve and leave it closed until the plumber has made the repair.
2. Record what you found. Photograph the meter readings with the time, the stains and the damp flooring. Keep your recent water bills. Policies generally treat a sudden leak differently from one that seeped over time, so the record matters. Our insurance guide explains the general rules.
3. Don’t wait on the drying. The EPA advises drying water-damaged materials within 24 to 48 hours to prevent mold. Joint EPA and CDC guidance says that if a home wasn’t dried in that window, you should assume mold growth.
4. Get the wet area measured. A restoration provider uses moisture meters and often a thermal camera to map how far the water spread. Water from a slow leak tends to travel sideways under flooring and up into walls, so the wet area is usually bigger than the stain.
5. Plan the drying and the repair together. The plumber fixes the pipe. The provider removes water, sets up structural drying and takes out what can’t be dried. For a pipe inside a wall, see burst pipe water damage.

Before anyone starts work, look up their contractor license with the Arizona Registrar of Contractors and get the scope of work in writing.