Slab Leak Warning Signs: 7 Signs and How to Test Them
Each sign has a test you can do in a few minutes, and the results tell a plumber where to start.
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The short answer
The most reliable warning sign of a slab leak is water use you cannot explain: a bill that climbs or a water meter that moves when everything is off. Warm spots on the floor, the sound of running water and damp flooring come next. None of these proves a slab leak by itself, but two or three together are a strong reason to test.
This guide lists seven signs, then shows how to test for each one at home. A slab leak is a leak in a pipe that runs in or under a concrete foundation. If your house sits on a raised foundation with a crawlspace, the same symptoms usually point to a different kind of leak.
Seven signs and a quick test for each
Use the test column to decide whether to call.
| Sign | What may be happening | Test you can do |
|---|---|---|
| Water bill climbs with no change in habits | A leak is losing water around the clock | Compare the last three bills, then do the meter test below |
| Meter moves with all water off | Water is escaping on your side of the meter | Read the meter, wait two hours with no water use, read it again |
| A warm patch on the floor | A hot water line is leaking under the slab | Feel the floor by hand in the morning before the heater runs much |
| Sound of running water at night | A pressurized line is flowing somewhere | Stand in the quietest room with all appliances off and listen |
| Damp carpet or lifting floor | Moisture is coming up from below | Press a paper towel on the spot and check if it comes away wet |
| Musty smell or new mold at baseboards | Constant moisture behind or under materials | Look along the base of walls near the affected area |
| Lower water pressure | A leak reduces pressure downstream | Compare pressure at several taps, and check aerators first |
Test one: the meter test
This is the test that matters most, and it costs nothing. Turn off every tap and appliance that uses water, including ice makers and irrigation. Find the meter, note the reading or the position of the small low-flow indicator, and wait two hours without using water. If the number has moved or the indicator is spinning, water is escaping somewhere on your side of the meter.
Our separate guide to reading your water meter explains the different dials and units in more detail.
Test two: hot line or cold line
To learn whether the leak is on the hot side, close the cold water valve above the water heater, then repeat the meter check. If the meter stops moving, the leak is on the hot side of the house, between the heater and the fixtures. If it keeps moving, the leak is on the cold side or the line from the meter to the house.
Turn the valve back on when you finish, and do not leave a gas or electric heater running with the valve closed. Knowing which line is leaking saves the plumber time and can keep the search area small.
Test three: the yard line
If the meter still moves with the house main valve closed, the leak is between the meter and the house, not under the slab. That is a job for water line repair, not for slab work. Closing the main and repeating the meter check separates the two cases before anyone opens a floor. See water line repair for how those leaks are fixed.
What else to note before you call
- Where in the house the warm spot or dampness is, and how large
- Whether the water heater is nearby and whether the spot is warm all day or only when hot water runs
- The dates of your last few water bills and the amounts
- Whether your floor is tile, wood, laminate or carpet
- Whether you have had plumbing repairs, or a neighbor has had a slab leak
- Photos of any cracks, stains or lifting flooring
What happens if you wait
A slab leak does not fix itself, and the water it loses keeps flowing. Damp flooring can grow mold, and long-running leaks can wash soil out from under the slab. The longer the leak runs, the more you pay for water and the more there is to dry and repair.
That does not mean panic. It means testing this week rather than next season. If you confirm a leak, the next step is detection, which our slab leak repair page explains, and a leak detection visit can locate it without cutting into the floor.
Slab leaks in rentals
If you rent, a slab leak is the owner's to repair, and it is important to give notice in writing. Include the meter readings, the bills and photos. About half of occupied homes in North Highlands are rented, according to the 2020 census, so it is a situation many households face.
Questions about this topic
A rising water bill, a meter that moves with everything off, a warm floor spot or the sound of running water are the main signs. A meter test is the best first check.
You can confirm that a leak exists and narrow it to hot, cold or the yard line. Locating the exact spot takes listening or thermal equipment.
No. Some homes have radiant heat, and sun on a floor can warm it. A warm spot that appears with no heating system and comes with a high water bill is more concerning.
If you see water or the floor is soaking, yes, close the main. If it is only a suspected leak, do the meter test and call.
Long-running leaks can wash away soil under the slab, which is a reason to check promptly.
Related guides
How to Read Your Water Meter to Find a Hidden Leak
Your meter is a free leak detector. This is how to use it in about two hours.
Does Homeowners Insurance Cover Plumbing Leaks in California?
Sudden leaks are often covered for the damage they cause. Slow leaks and worn pipes usually are not.
Burst Pipe: How to Find and Turn Off Your Water Shutoff
Most flooding is made worse by the minutes spent hunting for a valve. Find yours now.
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