How to Detect a Water Leak Under Concrete

Looking for Signs of Damage

Watch out for a major water bill spike. In most cases, the first indication that your underground pipes are leaking will come from your water or sewer bill. If you see a massive price spike from one month to the next, or if your bill increases each month despite your water usage staying the same, you may be dealing with a leaky pipe. You can also check your water meter—if your meter is spinning and you're not using any water in the house, either your toilet is running or you have a water leak somewhere.

Check for leaky appliances. Some water bill spikes are caused by leaky appliances, especially toilets, so check them first. To check for a toilet leak, add some food coloring to the tank and let it sit for 10 minutes. If the coloring moved to the bowl, you have a toilet leak. For a faucet or water-based appliance, look for pooling or dripping water around the appliance itself or the connected pipes.

Look for pools of water, damp carpet, or warped wood floors. Slab leaks create water build-ups underneath your concrete foundation. To escape, this water may pool on top of your floors, yard, or the concrete itself. If you have carpeted floors, search for areas that look darker or are unexpectedly damp. If you have wood panel floors, watch out for spots where the wood starts to warp.

Sniff out mold or mildew patches. When water sits for a long time, it creates a perfect environment for mold and mildew to grow. Though you most likely won’t be able to see it, be aware of areas in your house that smell musty of funky for a long period of time. Mold and mildew are most likely to grow under carpet or, if the water leak spreads, behind drywall.

Take note if your water fixtures become less powerful. High water pressure is what lets sinks, bathtubs, and showers provide you with plenty of water for your everyday needs. If they suddenly become less powerful, providing smaller or weaker streams, a leaky pipe may be to blame. To check if your fixtures are less powerful, turn off every water-based appliance in the house and test the stream from a single faucet.

Check if your hot water heater is always on. If your hot water heater is running most, if not all, of the time, it may indicate that you have a hot water leak under your concrete. Because hot water is constantly escaping, your heater is working overtime to warm up the new water. If there are no other signs of a slab leak, look for indications that the water heater itself is broken, such as muddy water or loud cracks coming from the tank.

Feel your floor for warm spots. Walk around with bare feet and feel for areas of the floor that are abnormally hot. These will be particularly pronounced on tile or wood floors. If you find one, mark the area with masking tape. If the spot stays hot for over 24 hours, it might be over a broken hot water pipe. If you own a cat, check any areas where it likes to sleep. Cats look for warm spots to curl up on, and their favorite spot may be directly over a hot water leak.

Watch out for foundation cracks indicating a sewer leak. Though most slab leaks are from fresh water pipes, sewer pipes are occasionally to blame. When these pipes break, water escapes to the surface and causes the foundation to crack or warp. Common indications of sewer leaks include: Cracks in your floor, tiles, bricks, or moldings. Rotated, bowed, or separated walls. Uneven or warped floors. Doors or windows that won’t close or have separated from the house.

Testing for a Water Leak

Turn off your faucets and other water appliances. So you can check your water meter for signs of a leak, turn off every faucet in your house and make sure common water appliances like your washing machine and dishwasher are not on. If anything remains on, cutting off your water may damage them or your water meter may give you a false positive reading. EXPERT TIP David Balkan David Balkan Professional Plumber David Balkan is a Professional Plumber, CEO of Balkan Sewer and Water Main Service, and President of Balkan Sewer and Drain Cleaning. As a hands-on owner of these companies for over 40 years, David is knowledgeable about water service lines, sewers, and drain line issues. David is a Committee Chairman of the Master Plumbers Council and has sat on the Executive Committee of the Sub Surface Plumbers Association of New York for over 30 years. His knowledge and solution-oriented approach contributed to Balkan Sewer and Water Main Service being the largest and most trusted service in New York City and the recipient of the 2017 Angie’s List Super Service Award. David Balkan David Balkan Professional Plumber Check your water meter regularly to prevent leaks. Check the water meter in your home from time to time with everything off to catch hidden leaks. If you catch any leaks, supply lines under the floors may be leaking. Also, listen closely for any hissing noises which, could indicate flowing water you can't see.

Find your main water shut-off valve. In cold regions, look for the valve inside your house in places like the basement or garage. In warmer areas, the valve may be located on a pipe on the side of the house or in the ground near your water meter.

Cut off water to your house. If your water valve has only one visible valve wheel, turn it clockwise to shut off your water supply. If you water valve has two visible valve wheels, turn off the one closest to your house and leave the other wheel, connected to the street-side valve, alone. Try turning on a faucet far away from the water valve to make sure the water is cut off. If you have older pipes, your valve may leak or creak when turned off.

Find your house’s water meter. Look for a water meter inside your yard or by the sidewalk surrounding your house. Typically, it will be inside an iron box or covered by a metal plate. Modern water meter boxes are typically labeled as such for clarity. If you can’t find your water meter, call your local public facilities office for advice on where water meters in your area are typically located.

Remove the water meter cover. Some water meter covers can be simply picked up by hand or pried off with a screwdriver, pair of pliers, or crowbar. Others may have a standard or pentagon nut holding them down, in which case you’ll need a standard or pentagon wrench to remove them. If you haven’t examined your meter for a while, watch out for cobwebs and bugs. For safety, wear thick working gloves when prying off the cover.

Check the water meter for activity. Though water meters are all built differently, they will each have a method of tracking water usage. This may be in the form of a sweep hand (a clock-like hand covering the meter), a leak indicator (a small triangle, sailing wheel, or other object), or an odometer (a row of numbers). With your appliances off, see if your pipes are leaking by looking for: A sweep hand that, over the course of several minutes, moves dramatically. A leak indicator that won’t stop turning. An odometer whose numbers continue to increase.

Hiring a Plumber

Search online for local plumbers. Most slab leaks can only be fixed by breaking concrete, so look for general contractors that specialize in remedial plumbing. If possible, go with a plumber that lists slab leak detection or repair directly on their website, indicating that they have experience. Expect to pay around $65 per hour or a set price based on the specific job. Hourly contractors may be cheaper for small fixes, but they can cost significantly more if the plumber encounters unexpected problems. If you can’t find good plumbers online, ask friends, family members, and neighbors for personal recommendations.

Hire a plumber to detect slab leaks. Using specialty equipment, plumbers can confirm that you are actually dealing with a slab leak and will isolate the problem to a specific area. Though some plumbers may offer detection services for free, expect others to charge up to $400.

Ask for a repair estimate. Once you confirm that the problem is a slab leak, ask your plumber for a repair estimate. The estimate will depend largely on your specific problem, but expect a standard repair to cost around $2000. To ensure that the price doesn’t change, make sure you get the estimate in writing.

Compare the estimate with other offers. Before committing to a plumber, call other plumbing companies and ask for repair estimates. Since you’ve already had the leak examined, simply relay the problem as it was described to you. Once you have multiple estimates, choose a plumber that seems reputable and offers their services at a reasonable price. Though the lowest offer may seem enticing, make sure it’s from a plumber that has a legal operating license and good user reviews online.

Pay to have your pipes repaired. Though pricey, pay for the slab leak repairs as soon as possible. Slab leaks can cause far more expensive problems down the road, and a little money now can save a lot going forward. Some homeowner’s insurance policies cover the cost of essential pipe repairs.