Four causes account for most slab leaks: corrosion, abrasion, ground movement, and poor original installation. On the Gulf Coast, ground movement does more damage than the other three combined, because the clay soil here expands when it takes on water and shrinks when it dries out, and everything buried in it moves with it.
Which one you had matters more than it sounds, because it decides whether you have a repair or a pattern.
What does the soil actually do to a pipe?
Clay soil is not a stable thing to build on. It swells after rain and shrinks in a dry spell, and around here it does both every year. That movement gets transferred into the slab and into everything running under it.
A pipe does not fail the first time it is moved. It fails after being moved a fraction of an inch a few thousand times, and it fails at the weakest point in the run — almost always a joint, because a joint is where two things are held together rather than one continuous piece.
That is the same mechanism we find on under-slab drain lines. On a Clear Lake job the camera showed the line sitting full of water. When we tunneled fifteen feet in, the line had shifted underground and pulled apart at a joint. Nothing had been hit and nothing had corroded through. The ground had simply moved. See tunneling and under-slab drain repair.
A pipe does not fail the first time it is moved. It fails after a few thousand times.
Why is it usually the hot water line?
Three reasons stack up on the hot side.
Heat accelerates corrosion. Hot pipe expands and contracts more than cold pipe over a day, so it moves more against whatever it is touching. And that movement is exactly what wears a line through where it passes over concrete, gravel or a rock left in the trench.
It is why a warm patch on a floor is such a common first symptom — the hot line is statistically the one that goes. See how to tell if you have a slab leak.
What is abrasion, and why does it matter here?
A pipe under a slab is not floating in space. It rests on something, passes through something, or is sleeved through the concrete. Every time it heats up it grows slightly and rubs. Do that for twenty years and you get a wear point — a shiny worn patch, then a pinhole.
Abrasion is the most local of the four causes, meaning it usually produces one leak in one spot rather than a failing system. That is good news, because it means a repair is a repair.
What counts as poor installation?
- A line laid straight onto rock or rubble instead of on sand or a proper bed.
- A bend made too tight, thinning the wall on the outside of the curve.
- A section nicked during the pour and buried in that condition.
- A joint made badly and then covered with sixty tons of concrete before anyone could see it.
When a young house leaks, this is usually why. There has not been time for corrosion or for enough movement cycles, so the cause is more often something that was wrong on day one.
The cause tells you what to do next. One abrasion point or one bad fitting is a repair and you move on. Corrosion is different: if the pipe is failing from the inside, the rest of the run is the same age, in the same soil, carrying the same water. That is the point where a repipe becomes the cheaper answer rather than the bigger one.
Does water chemistry play a part?
It contributes. Water chemistry affects how fast copper corrodes from the inside, and scale affects what happens at fittings and at the water heater. It is a slower mechanism than ground movement, but it is part of why two identical houses on the same street can have completely different plumbing histories.
We would rather test your water than quote you a hardness figure that came from somewhere else. See water filtration and treatment.
Can a slab leak be prevented?
Not entirely, and anyone who tells you otherwise is selling something. What you can do is reduce the two things that accelerate it: keep water moving away from the foundation so the soil around it stays as consistent as possible, and deal with a small leak while it is still small.
The second one matters most. Water running under a slab washes out the soil that supports it, and undermined soil is what turns a pipe repair into a foundation problem.
What happens after it is found?
Depends on what caused it and where it is. A single accessible failure is a repair. A failing run is a reroute or a repipe. A failed drain line under the slab is a tunneling job, so the work happens under the house rather than through your floor. See leak detection.
Common questions
What causes a slab leak?
Four things account for most of them: corrosion of the pipe from the inside or outside, abrasion where a pipe rubs against concrete or gravel as it expands and contracts, ground movement that puts stress on a joint until it separates, and poor original installation such as a line laid on rock or bent too tight. On the Gulf Coast, ground movement does more damage than anything else.
Why are slab leaks so common in this area?
Because of the soil. Clay soils expand when they take on water and shrink when they dry out, and that movement is transferred to everything sitting in and under the concrete. A pipe that has been moved a fraction of an inch a few thousand times eventually finds a weak point - usually a joint.
Does water quality cause slab leaks?
It contributes. Water chemistry affects how fast copper corrodes from the inside, and scale affects what happens at fittings. It is a slower mechanism than ground movement, but it is why two identical houses on the same street can have different histories.
Are slab leaks more common in older houses?
Generally yes, because there has been more time for corrosion and more cycles of ground movement. But new houses are not immune, and when a young house leaks the cause is more often installation - a line resting on a rock, a bend made too tight, or a section that was nicked during the pour.
Is it the hot line or the cold line that usually fails?
The hot line fails more often. Heat accelerates corrosion, hot pipe expands and contracts more, and that movement is what abrades a line against concrete. A warm patch on the floor is the common symptom for exactly that reason.
If I have one slab leak, will I get another?
Not necessarily, but it depends which cause you had. A single abrasion point or one bad fitting is a one-off. Corrosion is not - if the pipe is failing from the inside, one leak means the rest of the run is the same age, in the same soil, carrying the same water. That is the point at which a repipe becomes the cheaper answer.
Last updated · written by a licensed master plumber, MPL 38224