How to Spot a Slab Leak Before It Wrecks Your Floor

If your house was built between about 1955 and 1985 in Almaden, Blossom Valley, Cambrian or Santa Teresa, it is almost certainly slab-on-grade with the copper supply lines buried in the concrete. That was a perfectly sensible way to build. It becomes a nuisance fifty years later, when one of those lines finally gives up and there is a slab sitting on top of it.

What catches people off guard is that a slab leak almost never announces itself as water. You do not usually get a puddle. You get symptoms that are easy to explain away for months.

A warm patch on the floor. This is the classic one, and it only happens when the leak is on the hot side. You walk across the same bit of tile every day and one day it is noticeably warmer than the floor around it. People assume it is sun through a window. If it is warm at night too, it is not the sun.

The water bill climbing with no explanation. It does not double overnight. It creeps. Two or three bills in a row, each a bit higher, with nothing different about how the house is being used. A slab leak runs twenty-four hours a day and nobody sees it.

The water heater running more than it used to. If the leak is on the hot line, the heater is reheating water that is going straight into the ground. You may notice the burner firing at odd times, or hot water running out sooner than it once did.

A hiss with the house shut off. Late at night, everything off, stand still in a quiet room and listen. A pressurized line losing water under concrete sometimes makes a faint continuous sound. It is subtle, and once you have heard it you cannot un-hear it.

Cracks that are new, or a floor that has started to lift. Water under a slab moves soil around. This is a later-stage sign and it is the expensive one, which is why the earlier symptoms are worth taking seriously.

Here is the simplest test you can do yourself in about twenty minutes. Turn off every fixture and appliance in the house that uses water. Go look at the meter, note the reading, and leave it a couple of hours without anybody flushing or washing anything. If the meter has moved, water is going somewhere it should not be. That does not prove it is under the slab, but it proves you have a leak, and it is the single most useful piece of information you can give us over the phone.

What happens next is the part people worry about, and it is usually better than they expect. We pressure-test the system and isolate the hot and cold sides to work out which one is leaking, then locate the spot acoustically before anybody touches concrete. Nine times out of ten we leave the slab alone. We abandon the failed run and reroute a new line overhead through the attic or the walls, which is faster and cheaper and keeps the jackhammer out of your living room. Sometimes the slab genuinely is the only route, and if that is your situation we will tell you and explain why.

If any of this sounds familiar, read more about how we find leaks, or have a look at the Blossom Valley and Almaden Valley pages, where this is most of what we do.