A slab leak is one of those problems that can run for weeks before you notice anything’s wrong. By the time it’s obvious, water’s already been working away under your floor. The good news is the early signs are real and learnable, and catching them early makes a big difference.
Here’s what a slab leak actually is, what to watch for, and how to tell when it’s time to get a professional involved.
What a slab leak is
Most homes built on a concrete foundation have water lines running under or through that slab. Some are pressurized supply lines carrying clean water to your fixtures. Others are drain lines carrying water away. When one of those pipes develops a leak beneath the concrete, that’s a slab leak.
Because the pipe is buried under several inches of concrete, you can’t see the leak directly. The water has to go somewhere, so it works through the soil, wicks up through the slab, or finds a path along the foundation. That’s why the warning signs tend to be indirect. You’re reading the symptoms, not looking at the leak itself.
The signs worth knowing
No single clue proves a slab leak. But when a few of these show up together, it’s worth paying attention.
Your water bill jumps for no reason. This is one of the most reliable early flags. If your usage looks normal but the bill climbs, water is escaping somewhere. A pressurized slab leak can waste a surprising amount of water while you go about your day none the wiser.
You hear running water when everything’s off. Turn off every faucet, the dishwasher, the washing machine, and any ice maker. Then stand still and listen. A faint hiss or trickle in the walls or floor, with nothing actually running, points to water moving where it shouldn’t.
Warm or hot spots on the floor. If the leak is on a hot water line, the heat transfers up through the concrete. You might feel a patch of floor that’s noticeably warmer than the area around it. People often notice this walking barefoot across tile or vinyl.
Damp, musty, or moldy areas. Carpet that stays damp in one spot, a musty smell that won’t quit, or mold showing up low on a wall can all mean moisture is coming up from below. Flooring that warps, buckles, or lifts is another tell.
New cracks in floors or walls. A leak under the slab saturates the soil unevenly. As that ground shifts and settles, it can stress the foundation and show up as cracks in tile, hardwood, or drywall. Hairline cracks happen in any home, but a sudden cluster of new ones is worth noting.
Low water pressure. If a supply line is leaking, less water reaches your fixtures. Pressure that drops across the whole house, with no other explanation, can be part of the picture.
The water meter test you can do yourself
This is the simplest check, and it costs nothing.
- Turn off every water-using fixture and appliance in the house. Faucets, toilets, the washer, the dishwasher, the ice maker, irrigation, all of it.
- Find your water meter. It’s usually near the street or sidewalk in a box at ground level.
- Look at the meter. Many have a small leak indicator, a little triangle or dial that spins when water is flowing. Note the reading or the position of the indicator.
- Wait. Don’t use any water for a couple of hours, then check again.
If the meter has moved or the leak indicator is still turning with everything off, water is leaving the system somewhere. That doesn’t prove the leak is under the slab. It could be a toilet flapper, an irrigation line, or another hidden spot. But it tells you a leak exists, and that’s the signal to dig deeper.
Why Bay Area homes are prone to them
A few local factors stack up here.
Hard water is the big one. Much of the East Bay and Tri-Valley has hard water, whether you’re on EBMUD in Oakland and Berkeley, Cal Water, or a Zone 7 or city system in Dublin, Pleasanton, and Livermore. Hard water carries minerals that, over years, corrode pipes from the inside. Copper and galvanized lines are especially vulnerable.
Then there’s the age of the housing. Plenty of homes around Walnut Creek, Concord, Lafayette, and the older Oakland and Berkeley neighborhoods were built with copper or galvanized steel pipe. Galvanized in particular rusts and narrows with age. Decades of hard water moving through aging metal is exactly the recipe for a pinhole leak under the slab.
Ground movement plays a part too. We’re earthquake country, and even routine soil shifting and seasonal expansion can put stress on pipes locked in concrete. Over a long enough span, that stress adds up.
Worth a side note: if you’re buying or selling in EBMUD territory, the Private Sewer Lateral program may require testing or repairing your sewer lateral at point of sale. That’s a drain-side issue rather than a pressurized slab leak, but it’s part of the same habit of knowing what’s happening with the pipes you can’t see.
When to call a licensed plumber
If you’ve run the meter test and water keeps flowing with everything off, or you’re seeing several of the signs above together, it’s time to bring in a professional. Confirming a slab leak takes specialized leak detection equipment, acoustic listening tools, and experience reading a foundation. It’s not something to guess at, and chipping into concrete blindly does more harm than good.
A licensed plumber can pinpoint the leak’s location and lay out your options before anything gets opened up. Slab leaks don’t resolve on their own and usually worsen, so confirming the source early tends to save both water and repair costs down the line.
Before you hire anyone, verify their license. In California, plumbing contractors must be licensed through the Contractors State License Board. You can look up any contractor’s license status, classification, and standing for free at cslb.ca.gov. It takes two minutes and tells you whether the person you’re about to trust with your foundation is properly credentialed.
The takeaway: learn the signs, run the meter test if something feels off, and when the evidence points to a hidden leak, get a licensed pro to confirm it before it grows.