Slab Leak Signs: 7 Warning Signs of a Leak Under Your Foundation
The most reliable early signs of a slab leak are an unexplained jump in your water bill, the sound of running water when every fixture is off, and a warm spot on the floor (from a leaking hot-water line). A slab leak is a pipe leaking beneath your concrete foundation, so you rarely see the water directly — you see its symptoms. Catching it early is the difference between a targeted repair and foundation damage. Here’s what to watch for and the meter test that confirms it.
The 7 Warning Signs
- Water bill spikes with no change in usage — water is escaping continuously underground.
- Sound of running water when all faucets, toilets, and appliances are off.
- A warm or hot spot on the floor — a telltale sign of a leaking hot-water line under the slab.
- Low water pressure throughout the house.
- Cracks in the floor, walls, or foundation as water erodes soil and shifts the slab.
- Damp, warped, or discolored flooring, or a musty/mildew smell from moisture wicking up.
- Mold or mildew under carpet or in lower walls without an obvious source.
The Meter Test That Confirms a Hidden Leak
You can confirm an unseen leak yourself in about 15 minutes:
- Turn off every water fixture and appliance in the house.
- Find your water meter and note the reading (or watch the leak-indicator dial).
- Wait 1–2 hours with no water use.
- If the meter moved, water is escaping somewhere. Combined with a warm floor spot or the sound of running water, that points to a slab leak.
This won’t tell you the exact location — a plumber uses acoustic and pressure equipment for that — but it confirms you’re not imagining it before you pay for leak detection.
Why Slab Leaks Get Worse Fast
Under-slab water erodes the soil supporting your foundation, which can cause foundation cracks and movement. Hot-water slab leaks also waste energy continuously. The longer it runs, the more you risk turning a plumbing repair into a foundation repair — which is why these symptoms warrant a quick call, not a “watch it for a month.”
What It Costs and How It’s Fixed
Detection and repair depend on access and method (spot repair, reroute, or repipe):
| Step | Typical cost |
|---|---|
| Leak detection | $150 – $500 |
| Spot repair (access the slab) | $630 – $4,400 |
| Reroute / repipe (severe cases) | $2,000 – $15,000+ |
Full breakdown and methods: slab leak repair cost. For whether your policy pays, see does insurance cover a burst pipe — the sudden-vs-gradual logic and the often-excluded cost of accessing the slab apply directly.
Frequently Asked Questions
What are the first signs of a slab leak? The earliest signs are usually an unexplained spike in your water bill, the sound of running water when everything is off, and a warm spot on the floor from a leaking hot-water line. Low pressure, cracks, and damp or musty flooring tend to follow as the leak continues.
How can I confirm a slab leak myself? Do the meter test: shut off all water in the house, note the meter reading, wait 1–2 hours, and check if it moved. Movement with no water use confirms a hidden leak. Pair that with a warm floor spot or running-water sound to suspect the slab, then call a plumber for exact location.
Is a slab leak an emergency? It’s urgent even if it’s slow. Under-slab water erodes the soil supporting your foundation and can cause cracks and movement, turning a plumbing repair into a costly foundation repair. Don’t wait — get leak detection promptly once symptoms appear.
What causes slab leaks? Common causes are pipe corrosion (especially older copper in certain soils), abrasion where pipes rub against concrete or rebar, poor installation, high water pressure, and shifting soil stressing the lines. Hot-water lines tend to fail first, which is why a warm floor spot is such a strong clue.
Does homeowners insurance cover a slab leak? Policies often cover the resulting sudden water damage and the cost to access the slab (tear-out and replacement) but exclude the failed pipe itself and gradual/long-term leaks. Coverage varies a lot, so document everything and read your policy’s water-damage and “tear-out” provisions carefully.
Last updated: June 14, 2026. Sources: EPA WaterSense leak-detection guidance (meter test); Insurance Information Institute (water damage coverage and tear-out); 2026 repair ranges per our slab-leak cost guide.