Slab leak detection: finding it before anyone breaks concrete

The signs, in the order people usually notice them

Almost nobody discovers a slab leak by seeing water. They discover it by noticing something slightly wrong for weeks and finally connecting the dots. The sequence is remarkably consistent.

The bill goes up without explanation. Not double, usually. Twenty or thirty percent, quietly, over a couple of billing cycles. A pinhole in a hot water line under a slab loses a surprisingly modest volume per hour, but it never stops, and the arithmetic of a leak running continuously for sixty days is what shows up on the statement.

A warm patch appears on the floor. Roughly seventy percent of residential slab leaks are on the hot side, because hot water lines run hotter, expand and contract more, and corrode faster. A section of tile or laminate that is noticeably warm underfoot, in a spot with no reason to be, is one of the more reliable indicators there is.

You hear water with everything off. Late at night, with the house quiet, a faint continuous hiss or rushing sound near a floor or a wall base. This is the sign people most often dismiss and most often turn out to be right about.

Flooring starts to fail. Wood cupping or lifting. Grout darkening in a line. Carpet damp in a patch that dries and returns. Baseboards separating slightly from the wall. By this stage the leak has been running long enough to saturate the soil beneath the slab.

Water pressure drops, or the heater runs constantly. A hot-side leak makes a water heater cycle far more than the household's actual usage justifies.

The meter test you can do yourself in fifteen minutes

Before calling anyone, you can establish with reasonable confidence whether water is escaping. It costs nothing and it makes the conversation far more productive.

Turn off every water-using fixture and appliance in the house. Toilets, ice makers, irrigation controllers, water softeners in regeneration, humidifiers, all of them. Go out to the meter box at the front of the property and lift the lid. Most Texas meters have a small triangular or star-shaped low-flow indicator that spins at even a trickle. Note the exact reading.

Leave everything off for two hours and read it again. If the number has moved, water is leaving the system somewhere on your side of the meter. If the low-flow indicator is turning while you watch, the loss is significant enough to act on today.

That test tells you there is a leak. It does not tell you whether it is under the slab, in a wall, or out in the yard between the meter and the house. Narrowing that down is the next step, and it needs equipment.

How the leak is actually located

Detection is a process of elimination, and it runs in a deliberate order because each step makes the next one cheaper.

Isolation first. The water heater's outlet is closed, separating hot from cold. If the meter stops, the leak is on the hot side; if it keeps running, cold. That single valve turn halves the search area before any equipment comes out of the vehicle.

Pressure testing. The suspect side is isolated and pressurised with air or water, then watched on a gauge. A line that holds is ruled out. A line that drops confirms the loss and gives a rough sense of size. A fast drop is a substantial break, a slow bleed is a pinhole.

Acoustic listening. This is the core of it. Water escaping under pressure through a small opening makes a distinctive high-frequency noise that travels through concrete. Ground microphones and amplified listening discs are moved across the floor in a grid; the operator works towards the loudest point. On a quiet house with a decent-sized leak, this can put the location within a foot or so.

Thermal imaging. Particularly effective on hot-side leaks, where escaping water warms the slab above it into a visible plume on an infrared camera. It confirms and refines what the acoustics suggested rather than replacing it.

Tracer gas, when the others disagree. A safe hydrogen-nitrogen mix is introduced into the drained line; it escapes at the breach, migrates up through the concrete, and is picked up by a sensitive detector at the surface. It is the fallback for quiet leaks, deep lines and noisy environments, slower and more involved, but conclusive.

Why Texas produces so many of these

This is not bad luck and it is not bad plumbing. It is geology and construction practice meeting each other.

Large parts of the state sit on expansive clay soils that swell dramatically when wet and shrink when dry. A Texas summer bakes moisture out of the ground; the ground contracts; the slab settles unevenly. Autumn rain arrives and the clay swells again, lifting sections of the same slab. A copper line cast into that concrete is dragged through those cycles year after year until a joint fatigues or a section abrades against the aggregate around it.

Then there is the water itself. Much of Texas runs hard, mineral-heavy water, and a good deal of it is chemically aggressive to copper. Interior pitting works from the inside out and produces the classic pinhole. A leak with no impact damage and no obvious cause, in a pipe that looks fine everywhere else.

Age matters too. Homes built from the 1960s through the early 1990s commonly have soft copper under the slab with no protective sleeving. Those systems are now thirty to sixty years old, which is squarely within the window where under-slab copper starts to fail. Newer construction typically routes supply lines through the attic instead, precisely to avoid this.

What happens once it is found

Detection ends with a marked spot on your floor and a straight conversation about options. There are three, and which one is right depends on the pipe's condition rather than the leak's location.

Spot repair. Open the slab at the marked point, expose the pipe, cut out the failed section, replace it. Right when the rest of the system is sound and this is a first, isolated failure. Smallest job, smallest cost, and it leaves the remaining under-slab pipe in service.

Reroute. Abandon the failed run entirely and route a new line overhead through the attic and down interior walls. Right when the pipe has already failed once before, when the leak sits under something you do not want to open, a kitchen island, a tiled entry, a staircase, or when the system is old enough that a spot repair simply buys time. No concrete gets cut.

Tunnelling. Access the pipe from outside by tunnelling beneath the foundation rather than through it. More involved, but it leaves your floor untouched, which matters a great deal on stained concrete, hardwood or expensive tile.

You will be told which of the three the plumber would choose for their own house, and why. Detection is worth doing properly precisely because it turns that decision into an informed one instead of a guess.