What It Is
A slab leak is a leak in a water supply line or drain pipe that runs beneath a home's concrete foundation. Water escapes at the failure point and travels through the concrete, typically surfacing as a wet or damp patch on the floor, warm flooring over a hot-water line break, or unexplained mold or musty odor. Slab leaks are most common in homes built on concrete slab foundations, and in older copper pipes that have corroded over time.
What It Does
A slab leak allows water to escape continuously from pressurized supply lines or to back up from drain lines beneath the floor. Supply line slab leaks raise the water bill and can saturate the soil under the foundation, potentially causing settling or cracking over a long period. The water that migrates up through concrete can damage flooring, subfloor materials, and promote mold growth in places that are difficult to dry out.
Hot-water slab leaks are often detectable by feel - a section of floor that is noticeably warmer than the surrounding area. Cold-water slab leaks are harder to sense and may only reveal themselves through a rising water bill or visible moisture.
Where It Is Located
By definition, a slab leak is in the piping beneath the concrete floor of the lowest level of the home. The pipes run in a layer of gravel or soil directly below the poured slab. They cannot be seen or accessed without cutting through the concrete - which is why professional leak detection equipment is essential for locating the leak precisely before any repair work begins.
How It Works
Supply line slab leaks are most commonly caused by corrosion in copper pipe. Copper reacts with acidic or hard water over time, developing pinhole leaks - small perforations that start slow and gradually widen. The pipe is also in contact with soil and sometimes aggregate, and slight movement from soil settling or temperature cycling causes abrasion at contact points.
Drain line slab leaks are different - drain pipes typically run without pressure, so a crack or joint failure causes standing water to accumulate beneath the slab rather than spraying under pressure. These can be harder to detect and are often found incidentally during other work.
Common Problems or Signs of Failure
- Warm or hot spot on the floor with no external heat source nearby
- Damp or wet area on the floor or carpet with no obvious cause
- Water bill rising significantly month over month
- Sound of running water when no fixtures are on
- Musty or mildew smell at floor level
- Cracks appearing in flooring or baseboards near the affected area
- Water meter low-flow indicator spinning with all fixtures off
Can a Homeowner Check It?
A homeowner can gather evidence that a slab leak may be present, but locating and confirming it requires professional equipment.
- Turn off all fixtures and check the water meter's low-flow indicator - if it spins, water is escaping somewhere
- Walk the floors barefoot and feel for unusual warmth, particularly along routes where hot water lines would run
- Look for damp, soft, or discolored flooring
- Listen for a faint hissing or dripping sound near walls or floor when the house is quiet
- Check whether your water bill has climbed without explanation
When to Call a Plumber
If you notice any of the signs above - particularly a warm floor section, unexplained water usage, or sounds of running water with fixtures off - call a plumber. A suspected slab leak should be investigated before doing anything else, since additional water use accelerates the damage. The plumber will use acoustic listening equipment or thermal imaging to pinpoint the leak and can then advise on repair options, which range from direct-access concrete cutting to pipe rerouting through the walls if the pipe condition warrants it.
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