How to Tell If You Have a Hidden Water Leak

Start with your water meter. The EPA’s WaterSense program advises reading it before and after a two-hour period when no water is being used; if the reading changes at all, you probably have a leak. A meter only sees water entering the house, so a clean meter test does not rule out a leaking drain, roof or air conditioner, which is why stains, smells and soft spots are the other half of the search.

How common hidden leaks are

Common enough that federal surveys count them. In the 2023 American Housing Survey, run by the Census Bureau for the Department of Housing and Urban Development, 11.2 million of the 133.2 million occupied homes in the United States reported water leaking from inside the home. The survey counts leaks in the 12 months before the interview and groups inside leaks by cause, such as fixtures that backed up or overflowed and pipes that leaked.

The EPA puts the waste in gallons:

• Household leaks can waste nearly 1 trillion gallons of water a year nationwide.
• The average household’s leaks can account for more than 9,300 gallons a year.
• About 9 percent of homes have leaks that waste 50 gallons or more per day.

The EPA’s 2026 leak detection guide lists the common household leaks: worn toilet flappers or malfunctioning fill valves, dripping faucets or showerheads, and damaged irrigation systems. Water from those ends up in a drain or in the yard. The leaks that damage a house are the ones that end up inside a wall, a floor or a ceiling.

The water meter test

It can find leaks you cannot see or hear, and it takes two hours:

1. Turn off all water inside and outside the house, including appliances that use water, as the Portland Water Bureau’s version of the test says. That includes an irrigation system on a timer.
2. Write down the meter reading, or photograph it.
3. Use no water for two hours.
4. Read the meter again. The EPA says that if it changed at all, you probably have a leak.

If the reading moved, the Portland Water Bureau’s guide shows how to split the system in two. Close the main shutoff valve in the house, then open a faucet to confirm that the valve really stops the water. If the meter’s leak indicator or dial still moves, water is flowing between the meter and that valve, possibly through an underground leak. If it stops, the leak is likely somewhere in the plumbing on the house side of that valve.

Your bills are a second check. The EPA says that if a family of four uses more than 12,000 gallons a month in a colder month such as January or February, there could be serious leaks.

Start with the toilets

Toilets are the first suspect because so many of them leak: the EPA’s February 2026 leak detection guide puts the share at about 20 percent. Put a few drops of food coloring in the tank and wait 10 minutes without flushing. If any color shows up in the bowl, the tank is leaking into the bowl. The EPA suggests flushing right after the test so the dye does not stain the tank. The EPA names worn flappers and malfunctioning fill valves as common causes.

A tank-to-bowl leak shows on the meter and on the bill, but the water stays inside the fixture and goes down the drain. If every toilet passes and the meter still moves, look to the supply lines, which run inside walls, under floors and sometimes under a concrete slab.

Leaks the meter will not show

A meter measures water coming in, so it misses leaks that are not part of the pressurized supply. The EPA’s 2026 guide makes the same point about whole-house flow monitors: they cannot identify leaks that are not associated with water use, and it gives three examples:

• condensate leaking from an air conditioner,
• a roof leak,
• water pooling under a sink from a drain pipe.

Drain leaks happen only while water is running, so they show up as a stain that grows after showers, laundry or dishwashing, not as a moving meter. The EPA also advises keeping air conditioning drip pans clean and their drain lines unobstructed and flowing.

When the EPA lists places where hidden mold can grow, it names the back side of drywall, wallpaper or paneling, the top of ceiling tiles, the underside of carpets and pads, the inside of walls around leaking or condensing pipes, inside ductwork, and roof materials above ceiling tiles.

Signs in walls, floors and ceilings

Look for damage that has no other explanation. The National Park Service lists common signs of visible and hidden moisture damage:

• wet stains or salt deposits (efflorescence) on interior and exterior surfaces,
• flaking paint and plaster, peeling wallpaper or moisture blisters,
• musty smells,
• cupped, warped, cracked or rotted wood.

The same guidance warns that water from leaks and condensation can travel great distances down walls and along pipes and conduits, so the source of a stain may sit higher up the wall or farther along a pipe run. Timing helps narrow it down, because a drain carries water only while something is running. A stain that grows after showers points to a drain or the shower itself. One that grows whether or not water is running points to a pressurized supply line. One that appears after rain points to the outside of the building: the roof, a window or a wall.

Leak detectors and automatic shutoff valves

Devices can catch a leak early, but each type has blind spots. The EPA’s 2026 guide sorts them into two groups:

• Point-of-use moisture sensors sit where leaks are likely, such as next to a water heater, behind a washer or under a sink. They alarm when they get wet, but only at the spot where they sit.
• Point-of-entry flow monitors attach to the main line or the meter and watch flow for the whole house. Some include an automatic shutoff valve, and in-line models may require a plumber to install.

The same guide raises three cautions. A shutoff device must not cut the water supply to a home fire sprinkler system. If the device needs to keep working through a power outage, it has to be battery-powered or have a battery backup. And leak alerts from a utility’s smart-meter system can lag by days.

When to call a professional, and what to ask

Call a plumber when the meter test shows a leak you cannot find, when a stain or soft spot keeps spreading, or when a wall near plumbing smells musty. Unusually wet spots or pooling water in the yard are among the signs of an underground leak that the Portland Water Bureau lists; if you find them, also ask your water utility where its responsibility for the line ends and yours begins.

Once materials have been wet, time matters. The EPA advises drying water-damaged areas within 24 to 48 hours to prevent mold growth. If walls, floors or ceilings stayed wet longer than that, or you find more than about 10 square feet of mold, you may need a water damage or mold professional in addition to the plumber.

Questions worth asking the plumber:

1. Can you isolate the supply lines, drains and fixtures to find which one is leaking before opening anything?
2. How will you locate the leak, and how much wall, floor or slab will have to be opened?
3. Is the leak on the house side of the meter or the utility’s side?
4. Once it is repaired, is the surrounding material still wet, and does it need drying before the wall or floor is closed?
5. Does this repair need a permit here?

Keep the meter photographs and their dates. They record when you found the leak and how fast the meter was moving, which is useful to the plumber and to anyone else who looks at the damage later.