The classic combination is a water bill that has climbed with no change in how you use water, a warm or damp patch on the floor, and the sound of running water when every fixture is off. Any one of those alone has other explanations. Two or three together is a slab leak until somebody proves otherwise.
Before you call anyone, there is a ten-minute test you can do yourself that will tell a plumber more than any description over the phone.
What is the test I can do myself?
Turn off every fixture and every water-using appliance in the house. Taps, toilets, washing machine, dishwasher, and the ice maker in the fridge — that last one catches people out. Then walk out to the water meter and look at it.
If the meter is still turning with everything off, water is going somewhere. That is the whole test. It does not tell you where, but it turns "I think something might be wrong" into a fact, and it rules out a lot of expensive guessing.
If the meter is still turning with everything off, water is going somewhere.
What are the signs, in order of how much they mean?
- A meter that turns with everything off. The strongest single sign there is.
- A water bill that climbed with no change in habits. Look at the same month last year, not last month — summer irrigation muddies a month-to-month comparison.
- A warm patch on the floor that was never warm before, in a house with no under-floor heating. That is hot water running under the concrete.
- The sound of running water with everything off. Faint, continuous, usually loudest near a wall.
- Damp carpet, buckling wood, or grout that is dark in one area and nowhere else.
- A hot water heater that runs more than it used to because it is reheating water that is leaving under your floor.
How do you tell a supply leak from a drain leak?
By asking what it has been doing, before anything gets opened. It is the same reasoning we use on a ceiling stain.
On a Baytown job, a two-storey house had a stain on the ceiling below the upstairs bathroom. Before opening anything we asked what the stain had been doing. The tub had not been used in several days and the stain had not grown. That means a drain, not a supply line — a supply line is under pressure all the time and worsens whether anybody is in the bathroom or not, while a drain only leaks when water runs through it. That put the tub shoe at the top of the list, and we opened a single twelve-inch square to find it.
Under a slab the same logic applies. A leak that is constant, regardless of what anyone in the house is doing, is on the pressure side. A leak that comes and goes with use is on the drain side, and drain problems under a slab are a different repair. See leak detection.
The fear is somebody opening half your house hunting for it. That is exactly why we work out what kind of leak it is before we cut. Narrowing it down is not a formality — it is the difference between one small opening and a week of exploratory demolition.
What should I not do?
Do not let anyone start breaking concrete on a hunch. If the location has not been narrowed down, the hole is a guess and the next one will be too.
Do not assume the highest bill is the leak's fault. Check the obvious things first. A toilet flapper that has stopped sealing will turn a meter and inflate a bill by a startling amount, and it costs a few dollars to fix. So will an irrigation valve that is not closing.
Do not wait to see if it settles down. It will not.
Why does finding it early matter so much?
Because the pipe is not the expensive part. Water moving under a slab washes out the soil that supports the concrete, and that is what turns a plumbing repair into a foundation conversation. The repair cost curve on a slab leak is not flat over time — it steps up once the ground underneath has been undermined.
That is also why "it is only a small leak" is not the reassurance it sounds like. A small leak running for a year moves a great deal of soil.
What happens when you come out?
We work out what kind of leak it is, narrow down where it is, and tell you what we found before anything gets cut or broken. If the honest answer is that it is a running toilet and not a slab leak, that is what we will tell you. See what causes a slab leak and whether insurance covers it.
Common questions
How do I know if I have a slab leak?
The classic combination is a water bill that has climbed with no change in how you use water, a warm or damp patch on the floor, and the sound of water running when every fixture in the house is off. Any one of those on its own has other explanations. Two or three together is a slab leak until proven otherwise.
What does a slab leak sound like?
A faint continuous hiss or rush, most audible at night when the house is quiet, and often loudest near a wall rather than in the middle of a room. If you can hear water moving with every tap, toilet and appliance off, something is leaking somewhere.
Can I test for it myself before calling?
Yes, and it is worth ten minutes. Turn off every fixture and water-using appliance in the house, including the ice maker, then go out and look at the water meter. If the meter is still turning, water is going somewhere. That single test tells a plumber more than any description over the phone.
Does a warm spot on the floor always mean a slab leak?
No, but it is a strong sign when it is on a hot water line. A warm patch of floor that was never warm before, in a house with no under-floor heating, usually means hot water is running under the concrete. A cold-side leak often shows up as damp rather than warm, which is why people miss it for longer.
Could it be something other than a slab leak?
Often, and that is the whole point of checking before anyone cuts anything. A running toilet flapper will move a meter and inflate a bill. A leaking irrigation valve will soak a slab edge. A drain leak behaves completely differently from a supply leak, and the difference is diagnostic - a supply line is under pressure all the time and gets worse whether anyone is home or not, while a drain only leaks when water runs through it.
What happens if I leave it?
It does not stop on its own, and the cost of the leak is rarely the expensive part. Water moving under a slab washes out the soil that supports it, and that is what turns a pipe repair into a foundation conversation. Finding it early is the difference.
Last updated · written by a licensed master plumber, MPL 38224