Slab Leak: Signs, Detection and Repair Options Explained

A pipe can fail under your concrete floor for months before you ever see a drop of water. A slab leak is a break in a water supply or drain line buried beneath or inside a concrete foundation, and the first clue is often a water bill that jumps $40 to $150 with no change in your habits. Other early signs include a warm patch on the floor, the sound of running water when every fixture is off, and a water meter that keeps moving.

If you suspect one, the quick answer is this: confirm the leak with a water meter test, shut off the main if the meter is spinning fast, and call a leak detection company or licensed plumber to pinpoint the break. Repair then comes down to three options: opening the slab for a spot repair, abandoning the buried line and rerouting new pipe through walls or the attic, or lining the existing pipe with epoxy. Each has a different price range and a different effect on your flooring.

Below you will find how to recognize the warning signs, how to run the meter test yourself, how professionals locate a slab plumbing leak without jackhammering half the house, and how to choose the right repair.

Slab Leak Warning Signs to Watch For

Most homes built on a slab since the 1960s have copper or galvanized supply lines running through or under the concrete. Copper can develop pinhole leaks from aggressive water chemistry, and pipes can abrade where they rub against concrete, gravel or rebar as the house expands and contracts. When a line fails, the symptoms depend on whether it carries hot water, cold water or waste.

• Warm or hot spots on the floor. A leaking hot water line heats the slab above it. Barefoot, you may notice a patch of tile or vinyl that feels noticeably warmer than the rest of the room.
• Unexplained jump in the water bill. Compare the last three months against the same period last year. A steady 20 to 50 percent increase is a red flag.
• Sound of running water. With all fixtures, the irrigation system and the ice maker off, a faint hiss in a quiet room points to pressurized water escaping.
• Damp carpet, cupping hardwood or lifting laminate. Moisture wicking up through the slab swells wood and makes carpet pad smell musty.
• Cracks in the slab or walls. Water can wash out the soil under a foundation, leading to new cracks in tile grout, drywall or the slab itself.
• Low water pressure or a water heater that runs constantly. A hot-side leak forces the heater to keep reheating fresh water.
• Mold or mildew odor near baseboards, often along an interior wall where supply lines run.

Drain line leaks behave differently. They are not under pressure, so the meter will not move, but you may notice slow drains, sewage odors, or soggy soil near the foundation edge. Those usually call for a camera inspection rather than pressure testing.

How to Test Your Water Meter for a Hidden Leak

The meter test is free, takes about 30 minutes, and tells you whether water is escaping somewhere on the supply side. You will need a flashlight, a meter key or large screwdriver to lift the lid, and a notepad.

1. Turn off every water user. Shut off all faucets, the dishwasher, washing machine, ice maker, irrigation controller and water softener regeneration. Make sure no toilet is refilling.
2. Locate the meter. It is usually in a concrete box near the street or sidewalk. Lift the lid and wipe off the dial face.
3. Watch the leak indicator. Most meters have a small red or black triangle or star that spins with even a trickle of flow. If it is turning, water is moving.
4. Record the reading. Write down the full number, then wait 30 to 60 minutes without using any water and read it again.
5. Isolate the house. If the meter moved, close the main shutoff valve at the house. If the meter stops, the leak is inside the home. If it keeps moving, the leak is in the service line between the meter and the house.
6. Rule out toilets. Add a few drops of food coloring to each toilet tank. If color shows in the bowl within 15 minutes, a flapper is leaking and may explain the flow.

If toilets and fixtures check out and the meter still moves with the house valve open, a slab leak becomes a strong possibility. A spinning dial that moves several gallons per hour means you should keep the main off between uses until the leak is found, because water under a slab can undermine the soil and damage floors quickly.

How Professionals Pinpoint a Slab Plumbing Leak

Finding the exact spot matters because concrete removal is the expensive part. Leak detection specialists typically charge a flat fee in the low-to-mid hundreds of dollars and combine several methods to narrow the location to within a foot or two.

Pressure Testing

The technician isolates the hot and cold sides of the system and pressurizes each line with a gauge. A line that loses pressure over a set period confirms which side is leaking and rules out fixtures.

Acoustic Listening

Sensitive ground microphones pick up the sound of water escaping a pinhole at 40 to 80 psi. The tech moves the sensor across the floor, listening for the point where the sound is loudest.

Thermal Imaging

An infrared camera shows the heat bloom from a hot-water leak through tile, vinyl or thin carpet. It is fast and non-invasive, though it works poorly on cold-side leaks.

Line Tracing and Tracer Gas

Electronic line locators map where the pipes actually run, since they rarely match the original plans. For stubborn leaks, a harmless hydrogen-nitrogen gas mix is pushed through the drained line, and a detector picks it up where it rises through the concrete.

For suspected drain leaks, a plumber runs a sewer camera through a cleanout and may perform a hydrostatic test, plugging the line and filling it with water to see if the level drops.

Slab Leak Repair Options Compared

Once the leak is located, you and your plumber choose between three main approaches. The right one depends on pipe age, how many leaks you have had, the flooring on top, and your budget.

Spot Repair Through the Slab

The plumber cuts and breaks out a section of concrete, usually about 2 by 2 feet, exposes the pipe, splices in a new section, pressure tests it, then backfills and pours new concrete. It is the most direct fix and makes sense when the pipe is otherwise healthy. The downside is flooring: tile, hardwood or luxury vinyl over the opening has to come up, and matching discontinued flooring can cost more than the plumbing itself.

Rerouting the Line

Instead of chasing the leak, the plumber caps the damaged buried line and runs new PEX or copper through the walls, ceiling or attic to the affected fixtures. Many plumbers favor this for homes with 30-plus-year-old copper, because once one pinhole appears, others often follow. Rerouting leaves your floors intact but requires drywall patching, and in cold climates attic lines need proper insulation.

Epoxy Lining

The line is drained, dried with heated air, cleaned with an abrasive blast, and coated inside with an epoxy that seals pinholes and forms a new barrier. It avoids both concrete and wall demolition. It works best on small leaks in pipes that are still structurally sound, and it is not suitable for fully separated joints or collapsed sections. Ask about the warranty period and whether local code permits lining for potable water in your area.

Protecting Your Floors During and After Repair

A slab leak is also a flooring problem. Water trapped under vinyl plank, engineered wood or carpet encourages mold and can ruin adhesive bonds. After the pipe is fixed:

• Pull back carpet and pad in the wet area and run dehumidifiers and fans for three to seven days.
• Use a concrete moisture meter or calcium chloride test before reinstalling any flooring. Most manufacturers list a maximum moisture emission rate, and installing over a wet slab voids warranties.
• Photograph damage and keep all receipts. Many homeowners policies cover the cost of access and resulting water damage even if they exclude the pipe itself.
• If you are replacing flooring anyway, consider a floating floor with a vapor barrier underlayment so future access is easier.

Troubleshooting Common Situations

The meter moves only a little. A very slow drop can be a running toilet, a dripping hose bib or a softener valve. Eliminate those before assuming the worst.

The warm spot moves. Heat can travel along the pipe run, so the warmest point is not always the leak. That is why acoustic and line tracing methods are paired with thermal imaging.

Second leak within a year. Repeat failures usually signal pipe-wide corrosion. At that point, rerouting or a partial repipe is almost always better value than another hole in the floor.

Water pooling outside near the foundation. This can be a service line leak between the meter and the house rather than a true under-slab break. The house-valve test in the meter procedure helps separate the two.

When to Call a Licensed Professional

Homeowners can safely run the meter test and shut off water, but repair of a slab leak belongs to a licensed plumber. Always shut off water at the main valve first if the leak is active or you see water surfacing. Breaking concrete near buried pipes, gas lines and electrical conduit is hazardous, and most jurisdictions require a plumbing permit and inspection for repairs, reroutes or repipes that alter supply lines. Check with your local building department about permit requirements.

Call a licensed plumber right away for any gas line near the work area, sewer or main drain line problems, the main water service line, or any repair that must meet code. If you see widening foundation cracks, doors that suddenly stick, or a sinking floor, bring in a structural engineer or foundation contractor as well, since eroded soil under the slab may need attention beyond the plumbing fix.

Get at least two written quotes that spell out the detection method, repair approach, who handles concrete and flooring restoration, and the warranty. A good plumber will explain why a spot repair, reroute or lining makes the most sense for your specific pipe and house rather than defaulting to the most expensive option.