How to Detect a Slab Leak in Your Home

What Is a Slab Leak?

A slab leak is a leak in the water supply or drain lines that run beneath your home's concrete foundation. These pipes were installed before the slab was poured and are now encased in or beneath several inches of concrete. When they develop pinhole leaks, crack, or separate at joints, the water has nowhere to go but into the soil under and around your foundation — causing damage you can't see until it becomes severe.

Warning Signs of a Slab Leak

• Unexplained water bill increase: A supply-side slab leak runs 24/7. Even a small leak — 1 gallon per hour — adds 720 gallons per month to your bill. A sudden $30–$100 jump with no change in usage is the most common first sign.
• Sound of running water: With all fixtures off, put your ear to the floor in quiet rooms. A supply-side slab leak creates a hissing or rushing sound audible through the slab, especially on hard flooring.
• Warm or hot spots on the floor: If the leak is in a hot water line, the constant flow of hot water heats the slab above it. Walk barefoot on tile or hard floors — a warm patch that doesn't correspond to sun exposure or heating vents suggests a hot-water slab leak.
• Damp or buckled flooring: Carpet that's persistently damp in one area, hardwood that warps or cups, or tile that loosens without explanation points to moisture migrating up through the slab.
• Foundation cracks: Prolonged water erosion under the slab can wash away supporting soil, causing the foundation to settle unevenly. New cracks in walls, floors, or the exterior foundation — especially if accompanied by other signs — suggest a slab leak.
• Mold or mildew smell: Persistent moisture under flooring creates ideal conditions for mold growth. A musty odor near floor level in rooms without obvious moisture sources warrants investigation.
• Water heater running constantly: If your hot water line is leaking under the slab, the water heater refills and reheats continuously. An always-running water heater with no visible hot water use is a strong indicator.

The Water Meter Test

1. Shut off every faucet, toilet, appliance, irrigation system, and ice maker in the house.
2. Locate your water meter (usually at the street or property line).
3. Note the meter reading or the position of the flow indicator dial.
4. Wait 15–30 minutes without using any water.
5. Check the meter again. If the reading changed or the flow indicator moved, you have a leak somewhere in your system.
6. To isolate it to the slab: shut off the main valve at the house. If the meter still moves, the leak is between the meter and the house (yard line). If the meter stops, the leak is inside the house — likely under the slab.

Professional Leak Detection

Licensed leak detection specialists use non-invasive tools to pinpoint slab leaks without destroying flooring:

• Acoustic listening devices: Amplify the sound of water escaping from pressurized pipes through the slab. Can pinpoint leaks within 1–2 feet.
• Thermal imaging cameras: Detect temperature differences in the slab surface caused by hot or cold water flowing beneath it.
• Pressure testing: Isolate and pressurize individual sections of pipe to identify which line is leaking.

Professional detection typically costs $150–$400 and saves thousands by avoiding unnecessary demolition. Most plumbers who repair slab leaks offer detection as part of the service or credit the detection fee toward the repair.