Slab Leak Warning Signs (And What to Do Before It Gets Expensive)

A slab leak is a leak in a water supply line beneath your home's concrete foundation. They're sneaky — by the time you see visible damage, the leak has often been running for months. Here's what to watch for.

7 Warning Signs

1. Unexplained spike in water bill — the single most reliable early indicator.
2. Warm spots on the floor from a leaking hot-water line.
3. The sound of running water when no fixture is on.
4. Cracks in the foundation or floor tile appearing in new patterns.
5. Reduced water pressure across the whole house.
6. Mildew smell in carpeting or low cabinets without visible water.
7. Water meter spinning with every fixture off.

What Causes Them

• Abrasion from poorly bedded pipes rubbing against concrete or rebar.
• Corrosion in old copper from acidic water or galvanic reactions.
• Poor installation — kinks, sharp bends, or insufficient padding.
• Soil shifting or foundation settling.

Repair Options

1. Spot repair: jackhammer through the slab, cut out and replace the failed section. Cheapest if there's only one leak.
2. Reroute: abandon the failed line in the slab and run new pipe through the attic or walls. Common for single-line failures.
3. Whole-house repipe: for older homes with multiple failures, replace the entire supply network with PEX overhead. Most disruptive but most permanent.

Get a Second Opinion

Slab repairs range from $1,500 spot fixes to $15,000 repipes. Always get two licensed plumber estimates and ask each contractor to mark the suspected leak location with electronic detection — not just a guess.