How do you find a water pipe leak under a floor slab? (2024)

Here’s 5 signs that you might have a water pipe leak under the floor slab:

1. The small triangular spinner in the dial of your water meter that indicates water flow does not stop slowly spinning, even with everything (including the ice maker) shut off.
2. You discover a wet spot in the floor.
3. You notice a warm spot in the floor.
4. You hear the faint sound of water running. This is more likely to happen at night when the house is quiet.
5. Your water bill is unusually high.

We locate water leaks using an infrared camera during a home inspection, and other construction professionals use the same technique. The camera sees differences in temperature across surfaces as we scan the floor of a home. Because water begins evaporating within seconds of leaking from a pipe, and the evaporation cools the wet area, it will show up as a blue area in floor, as part of a color-coded image that runs from deep blue to yellow. A hot water leak will appear as a orange or yellow area, like in the infrared image above that shows a hot water leak under a floor and next to a wall.

Once you have determined that there is definitely a pipe leak under the floor, there are two solutions for the repair:

1. Break open the concrete slab, remove the damaged pipe, replace it, and then replace the area of removed concrete and whatever floor covering was above it. This is the obvious way to do it.
2. An alternative is to abandon the damaged section of pipe and reroute a new pipe through the attic to replace it. If your water pipe is copper, pitting of the pipe from acidity in the water or soil underground may be what is causing the leak, and you can expect there will be more leaks in the future. Old galvanized steel or PB (polybutylene) pipes under the slab may have the same potential for recurring leakage. In that case, abandoning all your underground pipe and running new pipe in the attic may be the sensible solution. A local plumber that has experience with below-slab leaks in your neighborhood will the best person to advise you on which way to go.

Even a small water pipe leak under a floor slab can pump an amazing amount of water into the ground, as this infographic shows.

Another type of under-slab leak is caused by fractured drain pipe. It is slower to manifest itself as a wet spot under the slab and is best analyzed by a plumber with a video borescope. The leakage might also be located between the house and your septic tank or connection to the municipal sewer system near the street.

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house?

• How can I tell if I have cast iron pipes in my house?

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection?

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains?

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water?

• Is a hot water faucet handle required to be on the left?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater?

• If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?

• How can I tell what type of plumbing pipe I have?

• Why is there a flexible accordion pipe under the sink?

• What is the difference between PVC and ABS plumbing pipe?

• What is the difference between water service pipe and water supply pipe?

• What are the pipes on my roof?

•How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

• What is a P-trap?

• Why is old galvanized steel water pipe a problem for homebuyers?

• What does polybutylene pipe look like? Why is it a problem?

•Which water pipes are an insurance problem and possibly uninsurable?

•Can you connect CPVC pipe directly to a gas water heater?

Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.