How a Slab Leak Is Actually Located — and Why We Open the Concrete

Locating a slab leak does not produce an X on the floor. It produces a small area — often a foot or two across — where the evidence converges, and then somebody cuts the concrete to see what is actually down there. Every honest version of this work ends with that sentence, and most plumbing websites in this county stop one sentence earlier, which is how homeowners end up surprised by a saw.

Here is the whole sequence, including the parts that are unglamorous.

First: confirm there is a leak at all

More calls about suspected slab leaks turn out to be something else than turn out to be slab leaks, and the cheap tests come first for that reason.

With a meter: shut every fixture in the house, including the ice maker and any irrigation, and watch the meter. Movement with nothing running means water is leaving the system somewhere. It does not yet mean under the slab.

Without a meter: a pressure-decay test. Isolate a section, bring it to a known pressure, shut it in, watch the gauge. Ten of the 22 communities we cover have no registered community water system, so a great many Baldwin County properties never had a meter to watch in the first place. On those, this is not a fallback — it is the opening move.

Then rule out the cheap explanations. A running toilet flapper will move a meter all night. So will an irrigation zone with a stuck valve, a leaking hose bib on the far side of the house, or a water heater discharging through its relief line. Working through those first is not stalling; it is the difference between a service call and a saw.

Second: isolate which side of the system it is on

If the leak is real, the next question is hot or cold, because it halves the search area immediately.

Shutting the cold-water inlet at the water heater separates the two. If the system keeps losing pressure with the heater isolated, the failure is on the cold side. If it stops, it is on the hot side — and on the hot side you also get the warm-floor symptom, because a hot line leaking under concrete transfers heat up through the slab.

This step is also what distinguishes a supply-side slab leak from a drain problem under the same floor. They present similarly and they are located completely differently, since a drain line is not pressurised and therefore makes almost no noise.

Third: listen

A pressurised line escaping under concrete makes a sound — a specific, persistent hiss at the point where water is forcing through a small opening. Ground microphones and listening discs pick it up through the slab, and the location is worked out by comparing intensity as the equipment is moved across the floor.

Two things affect how well this works, and it is worth knowing them so the result makes sense.

Pressure helps. A line at full system pressure is loud. A line that has already lost most of its pressure through a large failure can be quieter than a pinhole, which is counterintuitive and occasionally sends a search in the wrong direction.

Floor covering matters. Tile and bare concrete transmit well. Thick carpet with pad, engineered flooring over foam underlayment and glued-down hardwood all attenuate it. That is not a reason to pull up flooring in advance — it is a reason not to expect the same precision in every room of the same house.

Fourth: mark, open, verify

The concrete gets cut over the marked area — a targeted opening, not a floor. Below it is soil, below that the line. The opening is made big enough to expose enough pipe on both sides of the failure to work on it and to actually see the condition of the run.

Then the repair is made and the section is re-pressurised and re-tested before anything goes back. That last step is not a formality. Re-testing is how a second failure on the same line gets discovered while the floor is still open rather than three months later — and it is the moment when this stops being a locating job and starts being a decision. What that decision looks like is set out in when chasing a slab leak stops paying.

Why is so much of this county built on slab?

The Insurance Institute for Business and Home Safety puts it plainly: houses built on slab foundations are common in the South, and in those houses water pipes frequently run through the attic — a consequence of having no basement and no crawlspace to put them in. The same absence is why supply lines end up under the concrete instead.

Foley shows the pattern at scale. Its median year built is 2004 across 11,569 housing units, which puts it among the three highest for median year built of the 22 communities Streamlined Plumbing serves — newer stock, built the way the region has built for decades. Drive Glenlakes, Liveoak Village, Cypress Gates or the subdivisions off Hwy 59 toward OWA and you are looking at slab-on-grade construction almost without exception.

The elevation is the other half of it. Foley's coordinate reads 80.7 feet, against a 100-foot median across the 22 communities, a minimum of 2.9 feet at Fort Morgan and a maximum of 271 feet at Bay Minette. Ground that sits mid-range does not tell you anything about the pipe, but it does tell you about the soil around it — and moisture that cannot drain away from a slab keeps working on whatever is buried under it.

One number is worth reading correctly while we are here. Foley's registered water system reports 50,919 people served against a resident population of 22,330. Those measure different things — a system's registered footprint versus a headcount inside a municipal boundary — and neither should be read off the other.

Before you call, and while you wait

Find your main shutoff valve and confirm it closes. A valve that has not moved in fifteen years in this humidity frequently will not, and discovering that during an active leak is the worst possible time.

Photograph everything before anything is opened — the wet floor, the buckled boards, the stain, the meter reading. Keep the failed pipe once it is cut out.

If you want a rough sense of what a continuous leak is costing while you decide, the water waste calculator will put a figure on a given flow rate. A water stain or hidden leak with no obvious source is the same investigation from the other end — leak detection work either way, and what it involves here is on the Foley leak detection page.

A single located failure is a repair, and that is straightforward water line repair. Repeated failures in the same line are a different conversation and I would rather have it before the first cut than after the third — that is where a whole-home repipe enters, and you can size one against your own house with the repipe cost estimator before anybody comes out.

(251) 423-7654, any hour. Brandon Byrd, Alabama Master Plumber MP-4462.

Sources. Insurance Institute for Business and Home Safety, "Freezing and Bursting Pipes," on slab foundations and pipe routing in southern construction. Median year built and housing-unit counts from Census ACS 2023 5-year estimates at place level. Ground elevations from the USGS 3DEP point query service (epqs.nationalmap.gov) at each community coordinate. Water system identifiers and population served from EPA SDWIS (data.epa.gov); ten of the 22 communities return no registered community water system in that record. Baldwin County Commission Resolution No. 2025-067, March 5, 2025, adopting the 2024 International Plumbing Code and 2024 International Residential Code.