How to Tell If You Have a Slab Leak

Published by Water Damage Restoration Specialists of San Antonio · Last updated 2026-07-25

A slab leak is a failure in the water lines running beneath or within a concrete foundation. Because you cannot see it, it is usually diagnosed late — often after months of water saturating the ground under the house.

The warning signs

A water bill that jumps with no change in how much you are using
A warm patch on the floor — typically means a hot water line
The sound of running water when everything is switched off
Flooring lifting, buckling or cupping in one specific area
Cracks appearing in floors, walls or door frames as soil moves
Doors that suddenly stick as the foundation shifts
Persistently damp carpet or a musty smell with no visible source
Unusually green grass in one patch beside the foundation

The two-minute test you can do yourself

Turn off every water fixtureInside and outside. No taps, no irrigation, no ice maker, no washing machine.
Find your water meterUsually in a box near the curb.
Watch the leak indicatorMost meters have a small triangular or star-shaped dial that spins with even tiny flow. Note the reading.
Wait fifteen minutes and check againIf the dial has moved or the reading has changed with everything off, water is escaping between the meter and the house.

That does not prove it is under the slab — it could be an irrigation line or a service line in the yard — but it does prove you have a leak worth locating properly.

Why San Antonio gets so many

The soil. Much of Bexar County, particularly the south, west and far west, sits on expansive clay that swells dramatically when saturated and shrinks in drought. San Antonio delivers both, often in the same year. That cyclical movement stresses foundations and the plumbing cast within and beneath them until something cracks.

It also works in reverse, which is what makes it insidious: a leaking line saturates the clay around it, that clay swells and moves more, and the movement worsens the leak. Foundation problems and plumbing leaks in this city are frequently the same problem viewed from two directions.

Do they have to break the concrete?

Not always, and it is worth asking. Depending on where the leak is, re-routing the line through the attic or walls is often less destructive and less expensive than opening the slab. That is a decision for you and your plumber — our part is locating the moisture accurately with acoustic and thermal equipment first, so the least invasive option can actually be chosen, and drying the slab and flooring afterwards.

Reducing the risk

Keep soil moisture consistent — soaker hoses around the foundation during drought months genuinely reduce movement
Make sure gutters and downspouts discharge well clear of the foundation, not against it
Grade soil so water falls away from the slab on all sides
Check your water bill monthly for unexplained increases
If your house is 40-plus years old, consider a plumber's camera survey of the drain lines

See our ZIP code pages for the specific soil and plumbing conditions in your part of San Antonio.

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