What Is a Slab Leak and Why Rerouting Beats Breaking Concrete
A slab leak is a failure in a pressurized water supply line that runs inside or directly beneath the concrete pad your house sits on. Every other leak in a home is reachable: you open a cabinet, cut a square of drywall, crawl under a floor, or pull a ceiling panel, and there it is. This one sits inside the structure, which means the first question a plumber has to settle is not how to fix the pipe but how to reach it, and what reaching it will cost the house. That question has a standard answer in the trade: leave the failed line exactly where it is.
Quick Answer: A slab leak is a break in a water supply line under or inside a home's concrete floor. Rather than opening the concrete, plumbers usually abandon the failed run and reroute a new line overhead through the attic or through walls.
How Water Lines Ended Up Beneath a Concrete Slab
Slab-on-grade construction sets the finished floor of a house directly on a poured concrete pad instead of over a crawl space or basement. It spread through residential buildings in the postwar decades because it was fast and removed an entire level of framing. The plumbing still had to go somewhere, and the somewhere was down. Supply lines were laid in the compacted sub-base or tied in place before the pour, then buried when the concrete was poured.
Soft copper tubing was the usual choice, run in long coiled lengths that could be bent around corners rather than joined at fittings. In plenty of houses, those runs went in with little or no protective sleeving, in direct contact with gravel, sand, and concrete.
What followed was slow. A hot line lengthens slightly every time it heats and shortens when it cools, and a tube pressed against aggregate wears at the contact points over thousands of those cycles. Soil moisture acts on the copper from the outside, thinning the wall until it opens into a pinhole. Neither mechanism is dramatic. Both take decades, which is why the failure usually arrives long after anyone has thought about that pipe.
The water service line running from the meter at the street to the house is a separate pipe with its own failure patterns, and a leak in it surfaces in the yard or the parking strip rather than through your floor.
How a Slab Leak Announces Itself
Because the pipe is sealed inside the structure, the ordinary evidence never appears. Nothing drips into view. Every sign you get is second-hand, and each one points down rather than at a wall or a fixture.
Damp that surfaces well away from the break: Water under a slab moves wherever the sub-base allows, then rises at a carpet edge, a baseboard, or a joint in the flooring, which can be on the far side of the room from the failure.
A water heater cycling more than it used to: A hot-side loss keeps pulling heated water out of the tank around the clock, so the heater fires on a rhythm unrelated to anyone drawing hot water.
Keep a written record from the day you first notice any of this: dates, photographs of the damp area, and meter readings taken with the house shut off. A written record of the leak and the work that follows is what a contractor works from, and it is far easier to build as you go than to reconstruct later.
Why the Pipe Is Rarely Repaired Where It Lies
Reaching a line under a slab is mostly a demolition task with a plumbing repair buried in the middle of it. The floor covering over the area comes up. A section of concrete is cut and removed. The sub-base underneath is excavated until the line is exposed and the failure point is confirmed. Only then does the actual pipework happen, after which the trench is backfilled and compacted, the concrete is repoured, the patch is left to cure, and the floor covering is reinstated. Cutting concrete inside a living space also means dust containment throughout.
Tunneling is the other access route. Instead of opening the floor, a crew excavates a horizontal shaft under the slab from outside the foundation and works back to the failure point. It keeps the mess outdoors and protects the flooring, but it is still excavation and leaves the rest of the buried run exactly as it was.
There is also what else lives in that concrete. Some slabs are post-tensioned, meaning steel tendons run through them under enormous stored tension. Electrical conduit is frequently cast in as well, and some floors carry radiant heating tubing. This is why the area is scanned before anything is opened, and why the scan happens no matter how confident anyone is about the line's path.
Never cut, core, or drill into a slab yourself to chase a leak. A post-tension tendon nicked under that stored stress can release violently, and a cast-in conduit may be live, so the scan comes first.
Set against all of that is the return. A spot repair restores one point on a run whose remaining length is the same age, the same material, and sitting in the same soil carrying the same water. Nothing about it improves the pipe on either side.
What a Bypass Reroute Involves
A bypass, or reroute, resolves the problem by changing the route rather than the repair. The failed under-slab run is taken out of service and left in place. A new line is built to serve the same fixtures, and it runs above the floor rather than below it.
The new run typically begins near the point of supply, at the main entry or at the water heater, and rises into the attic. From there, it crosses over the ceiling framing, following the shortest sensible path to the group of fixtures it feeds. Where it needs to come down, it drops inside an interior wall cavity and lands at the fixture stub-outs already in that wall: behind a vanity, inside a closet wall, in the back of a kitchen cabinet, or in the wall behind a tub valve.
Houses without usable attic space get the same idea by a different path: wall cavities, a cabinet chase, or a boxed soffit built along a ceiling line, coming down at the same points.
Flexible PEX tubing does most of this work because it bends through framing in continuous lengths, which keeps the number of concealed joints low. Fittings that do exist are placed so a future plumber can access them without cutting into finished surfaces. That is the underlying logic of the whole approach: everything in the new system stays reachable.
What the House Looks Like During the Work
The visible footprint is smaller than most people expect. Access openings are cut in drywall at each drop point and near the top of the walls the line crosses. Furniture moves away from those walls, floors get covered, and plastic goes up around the work areas. The attic hatch becomes the staging point for the overhead run. Water is off for part of the day while the new line is tied in and pressure tested.
When the plumbing is finished, the openings are patched and the wall surfaces restored. The old line under the slab stays where it is, carrying nothing. Ceilings usually go untouched except where a run has to pass through one. A whole-house version of the same work uses the same overhead path at a larger scale, and most of those are finished inside a single working day.
Before the new path is designed, ask which fixtures the failed run feeds. Nothing under a slab is visible, so that grouping comes from locating and isolating the failed line, which sets the attic path and the number of wall drops.
Deciding Between One Reroute and the Whole House
A single reroute treats a single run. That is the right scope when the failed line is well isolated from the rest of the system, when the other supply lines in the house are a different material or a different vintage, or when the rest of the plumbing was already moved out of the slab during an earlier remodel.
The question turns the other way when the failed run is representative. Under-slab lines in one house generally went in on the same day, from the same stock, into the same soil, so a pinhole in one of them is information about the others rather than about itself alone.
There is a practical argument as well. A reroute already opens the attic, cuts the wall drops, stages material through the same hatch, and shuts down the same main. Carrying the rest of the lines over while all of that is set up is a smaller job than mobilizing for it again in two years and cutting a fresh set of openings in walls that were just repaired. When several under-slab runs are the same age as the one that failed, moving all of them at once is usually the more sensible plan, and it retires the under-slab system rather than shrinking it one leak at a time.
Frequently Asked Questions
Yes, and it behaves differently. Drain and sewer piping under a slab, often cast iron in older houses, is unpressurized, so it does not lose system pressure and does not register at the water meter. It shows up as sewer odor, slow drains, or settling in the floor, and locating it means a camera inspection of the line, sometimes with a smoke or dye test, rather than the pressure work used on supply lines.
An isolation test is the common confirmation. The house is closed off at the main shutoff and pressurized, and a static gauge is monitored with every fixture shut off. A gauge that drops steadily places the loss on the supply side. Closing the outlet valve at the water heater separates the hot side from the cold side, so a technician can tell which of the two is losing pressure before any locating equipment comes out.
Both ends get cut back to an accessible point and capped, so the abandoned run cannot hold water or draw air, and the floor penetration where the line came up through the slab is sealed rather than left open. Have the dead run noted on the house record while the crew is still there, so a future plumber tracing lines does not spend a morning chasing pipe that stopped carrying water years ago.
Pressure at the fixture stays the same as long as the new line is sized correctly for the fixtures it serves. Hot water timing can shift slightly because a rerouted run holds a different volume of standing water than the buried one it replaces, and a longer path holds more. Overhead runs through the unconditioned attic space are insulated, which keeps the standing water from releasing its heat between draws.
Occasionally. The strongest case is a failure at or very near a stub-up where the line already comes out of the concrete, since the pipe is partly accessible, the opening stays small, and the concrete work sits where the line already surfaces rather than in the middle of a floor. Mid-run repairs are a much harder sell.
What the water does depends on the soil under the pad. It can wash fines out of the sub-base and leave voids under sections of the slab. Concrete also wicks moisture upward, which lifts flooring adhesive, cups wood planks, and feeds mold growth at the bottom edge of drywall. None of it is visible in real time, which is why an unexplained meter reading warrants quick action.
Have a suspected slab leak located before any concrete is opened — a repipe specialist can confirm the failed run and reroute it overhead instead. PEX Plumbing & Repiping serves Portland, Beaverton, Tigard. Call (971) 232-3079.
