Most leaks announce themselves by dripping on something. A slab leak cannot, because there is nothing underneath it except more concrete.
So it shows up sideways, and the signs are easy to attribute to something else for months.
Why it happens here
Single-story properties across Kendall, Pinecrest and much of west Miami-Dade sit on a concrete slab with the supply pipework running underneath it. That is normal construction and perfectly sound — until a line under the slab develops a fault, at which point the water has nowhere obvious to go.
It goes into the ground, works sideways, and eventually finds a way up.
The signs
A warm patch on the floor. The classic one, and it only happens if it is the hot line. Walk the house barefoot; you will find it faster than any instrument.
The sound of running water with everything closed. Usually heard at night, usually dismissed.
A water meter that never rests. See the test below.
A bill that moved before anything was visible. A jump with no change in how the property is used is worth investigating that week, not that quarter.
Damp at the base of a wall, or a skirting that has gone soft, with nothing above it that could explain it.
Tile that sounds hollow or has started to lift. By this stage the bed underneath has moved and the leak has been running a long time.
Cracking in a floor or a wall that was not there before. Uncommon, and worth taking seriously when it appears alongside anything else on this list.
The test anyone can do
Fifteen minutes, no tools:
- Close every faucet in the property. Turn off the ice maker. Do not run the washing machine or the dishwasher.
- Find the water meter and note the reading, including the small sweep hand or leak indicator if it has one.
- Wait fifteen minutes without using any water at all.
- Read it again.
Any movement means water is going somewhere. That does not prove a slab leak — a running toilet is far more common and vastly cheaper — but it proves there is something to find.
If the meter is still, the problem is elsewhere.
Rule out the cheap causes first
Before anybody starts talking about concrete:
- A running toilet. By a distance the most common cause of a mystery bill. Put a few drops of food coloring in the cistern, wait twenty minutes without flushing, and look in the bowl. Color in the bowl means the flapper is passing.
- Irrigation. On larger lots this is the usual answer. Isolate the irrigation and repeat the meter test.
- A pool system, for the same reason.
- A hose bib or an outside faucet left barely open.
Working through those first is not stalling. It is the cheapest possible diagnosis, and it is right often enough to be worth the fifteen minutes.
Why breaking the floor is usually the wrong answer
When a slab leak is confirmed, the instinct is to dig down to it.
Re-routing the affected run overhead — up into the attic, across, and back down to the fixtures — is normally faster, cheaper and far less disruptive than excavating a floor in an occupied house. It also leaves the new pipework accessible for the next thirty years instead of buried in concrete again.
There are cases where a spot repair through the slab is the right call. But it should be a decision, not a default — and if somebody quotes to break your floor before they have located the leak precisely, get a second opinion.
If it has happened twice
Two slab leaks in the same property is the system telling you something. At that point the conversation is about repiping rather than about a third patch.
How a slab leak is actually located
The point of tracing is to open the smallest hole that works, and the trade has three tools for it. None of them involves guessing where the warm patch is widest.
Pressure isolation comes first and costs nothing. Hot and cold are isolated separately and watched on a gauge. A cold side that holds and a hot side that falls has told you which line is leaking before anyone has touched the floor — and on a slab it is usually the hot line, because it runs hotter, moves more, and rubs.
Acoustic listening finds it. A ground microphone picks up the hiss of water escaping under pressure, and the operator walks a grid narrowing on the loudest point. It works badly over thick tile and well over bare slab, which is one reason it is worth clearing a room before the visit rather than during it.
Tracer gas is the fallback when acoustics cannot hear through the finish. The line is drained and charged with a hydrogen-nitrogen mix, which is small enough to find its way up through concrete, and a detector walks the same grid.
Repair or reroute
Once it is found there are two answers, and the cheaper one is not always the better one.
A spot repair opens the floor, cuts out the failed section and renews it. It is the right answer for a single failure in a line that is otherwise sound — a nail strike, or a rub in one place.
A reroute abandons the run under the slab and takes a new line overhead through the attic or through walls. It costs more on the day, opens no floor, and means that run can never leak under concrete again.
The deciding question is not price, it is whether this is the first failure. Copper under a slab that has failed once from corrosion rather than damage is telling you about the whole run, not about one spot. Two spot repairs in two years usually cost more than the reroute would have.