Slab leaks: 7 warning signs

A slab leak is a water line leaking underneath your home's concrete foundation — where you can't see it, can't reach it, and often can't hear it until the damage is well underway. Caught early, a slab leak is a manageable repair. Caught late, it's foundation damage, mold, and a water bill that makes you flinch. Here are the seven signs we tell every homeowner to watch for.

In most Southern California homes built on a slab foundation, the hot and cold water lines run through or under the concrete. Over decades, those lines corrode, shift with the soil, or get nicked by the original construction — and eventually they leak. Because the leak is under concrete, the water goes into the soil and up through the slab instead of showing itself as an obvious drip.

Hot-water-line leaks are the most common, and they're the ones that cause the fastest damage: constant hot water under the slab erodes soil, undermines the foundation, and feeds mold inside the flooring assembly. This is a 'call a professional' situation — not a wait-and-see one.

• Hot spots on the floor: A warm patch on tile or concrete — especially with no heating vents nearby — is the classic sign of a hot-water slab leak. Walk barefoot; your feet will find it before your eyes do.
• The sound of running water: Water running when every fixture is off. Stand quietly near the water heater or in the bathroom at night — a faint hiss or rush with nothing running means water is going somewhere it shouldn't.
• A water bill that jumps for no reason: A sudden, unexplained spike in your water bill is one of the most common ways slab leaks get discovered. If your usage hasn't changed, the water is leaking somewhere.
• Low water pressure: A leak under the slab bleeds pressure from the whole system. If pressure drops house-wide and there's no visible leak, the slab is a prime suspect.
• Damp or warped flooring: Water wicking up through the slab shows as damp carpet, cupping wood floors, or tile grout that stays dark. By the time flooring shows it, the leak has been active a while.
• Mold or mildew smells: Persistent musty odors near the floor — especially in rooms over the slab — point to moisture feeding mold under the flooring. Don't just mask the smell; find the water.
• Foundation cracks or shifting: In advanced cases, eroded soil lets the slab settle or heave, cracking the foundation or interior walls. This is the expensive end of ignoring the first six signs.