Slab Leak Warning Signs
The classic signs are a warm spot on the floor, a water bill that climbs with no change in usage, and the sound of running water when everything is off. Confirm with a meter test before authorizing any concrete work. Detection runs $350 to $800; repair $1,200 to $4,500.
What a slab leak actually is
A slab leak is a failure in a water supply line running beneath a concrete foundation. Because the water has nowhere visible to go, it soaks into the soil under the slab and runs continuously, sometimes for months, without producing the puddle that would make it obvious.
The reason it matters beyond the water bill is that escaping water erodes the soil supporting the foundation, and in expansive clay it also causes localized swelling. Either way the slab loses uniform support, which is how a plumbing problem becomes a structural one.
The warning signs, roughly in order of reliability
- A warm or hot spot on the floor with no heating source beneath it, which indicates a hot water line leak and is close to diagnostic
- The water meter dial moving with every fixture in the house closed
- A water bill rising steadily with no change in household usage
- The sound of running water in a quiet house with nothing turned on
- Damp patches, buckling, or discoloration in flooring with no surface source
- A drop in water pressure throughout the house
- New cracks in flooring, walls, or the slab itself as soil support changes
- Unexplained mildew smell or mold at the base of walls
Confirm before anyone cuts concrete
The single most useful thing you can do is the meter test, and it costs nothing. Turn off every fixture and appliance that uses water, read the meter, wait an hour without using any water, and read it again. Any movement means water is leaving the system somewhere.
Then rule out the easy candidates. Dye-test every toilet, because a failed flapper is far more common than a slab leak and produces the same bill increase. Shut off the irrigation system for a day, since irrigation leaks routinely masquerade as household ones.
If the meter still moves after those, professional detection is the next step, and it should happen before any concrete is opened. Cutting a slab on a guess is how a $1,500 repair becomes a $5,000 one.
Repair or reroute
Once located, there are two main paths. A spot repair cuts through the slab, replaces the failed section, and patches the concrete. A reroute abandons the under-slab run entirely and runs a new line overhead or around.
The spot repair is cheaper today. The reroute is often cheaper over a decade, because the pipe elsewhere under that slab is the same age, same material, and same condition as the section that just failed. On an older copper system that has already leaked once, the second leak is generally a question of timing.
One caution specific to post-tension slabs, which are common in Texas, Arizona, and Nevada: they contain steel cables under tension, and cutting one is a serious structural matter. Any concrete work on a post-tension slab requires scanning first, and a plumber who proposes cutting without it should not proceed.
Frequently asked questions
How much does a slab leak cost to fix?
Detection alone typically runs $350 to $800, and detection plus repair generally lands between $1,200 and $4,500. A single accessible spot repair sits at the low end. Post-tension slabs requiring cable scanning, multiple leaks, or a decision to reroute or repipe push toward and past the high end.
Can a slab leak cause foundation damage?
Yes, and it is the main reason not to leave one running once identified. Water escaping under a slab washes out supporting soil and, in expansive clay, causes swelling in the wet area. Both remove the uniform support the slab was designed around, which shows up as cracking in floors and walls.
Is a slab leak covered by insurance?
Coverage varies genuinely by carrier and state. Many policies cover the resulting damage and the cost of accessing the pipe, meaning the concrete cutting, while excluding the pipe repair itself. Gradual-damage exclusions are commonly invoked, which is why documenting when you first noticed symptoms is worth doing.
How long can a slab leak go undetected?
Months, routinely, and sometimes longer. A cold water line leak in particular produces no warm spot and no visible symptom other than the water bill, which is why an unexplained bill increase deserves a meter test rather than a wait-and-see. That is often the only signal you get.
Related guides
- Why Your Water Bill Suddenly Went UpA jump in water usage with no change in habits almost always means a leak. How to find it yourself, in order of likelihood.
- When to Call an Emergency Plumber (and When to Wait)After-hours plumbing costs 1.5 to 2 times standard rates. Here is which problems justify it and which are cheaper to schedule.
- What Not to Flush or Pour Down the DrainThe specific items responsible for most avoidable drain and sewer emergencies, including the ones labelled flushable.
- How to Shut Off Your Water in an EmergencyFind and operate your main water shutoff before you need it. The single most valuable thing a homeowner can know during a plumbing emergency.
Related services
- Slab Leak Detection and RepairLocation and repair of water lines leaking beneath a concrete foundation, using acoustic and thermal detection to minimize concrete work.
- Emergency Leak DetectionNon-invasive location of hidden water leaks behind walls, under floors, and beneath slabs using acoustic, thermal, and pressure testing equipment.
- Whole-House RepipingReplacement of a home's supply piping where the existing system has reached end of life, using PEX or copper.
Cost guides
- Drain Cleaning CostWhat drain and sewer clearing costs by method and location, and when jetting is worth the premium over cabling.
- Sewer Line Repair and Replacement CostWhat sewer lateral repair costs by method, including trenchless lining and pipe bursting, and how to compare quotes honestly.
- Slab Leak Repair CostWhat it costs to find and fix a leak under a concrete foundation, and when rerouting beats repairing in place.
Common questions
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