10 Signs of a Hidden Plumbing Leak (And What to Do)

Hidden leaks are Los Angeles's most expensive plumbing problem — they cause mold, rot, and foundation damage before you ever see a drop of water. Catch them early with these warning signs.

The 10 Warning Signs

1. Water Bill Suddenly Higher Than Normal

A household of 4 in Los Angeles uses roughly 8,000–12,000 gallons per month. If your bill jumps 20%+ without an obvious reason (guests, hot summer), suspect a leak. A 1/8-inch crack in a pipe wastes 250 gallons per day.

2. Damp, Warm, or Soft Spots on Floors

Warm spots on a concrete or tile floor — particularly in the kitchen, bathroom, or hallway — are a classic sign of a slab leak: a leak in the pipes running beneath your foundation. This is common in LA homes built before 1990 with original galvanized or copper plumbing. Don't ignore soft or spongy hardwood floors either — water has been pooling under them.

3. Mold or Mildew Smell Without Visible Moisture

If you smell that musty, earthy odor inside a wall or cabinet but can't see any water, moisture is building up behind the surface. Black mold in LA's climate can establish itself in as little as 24–48 hours of sustained moisture.

4. Discolored Walls or Ceilings

Brown or yellow staining on drywall — especially in rings or patterns — is dried water damage. The moisture source may no longer be active, but the damage is done. Active leaks will feel damp or soft to the touch.

5. Sound of Running Water When Nothing Is On

Stand in a quiet room and listen. Do you hear hissing, dripping, or running water when every faucet and appliance is off? That's water moving through your pipes without a fixture demanding it.

6. Reduced Water Pressure (One or More Fixtures)

If your shower or a specific faucet has recently lost pressure, and the rest of the house is normal, a localized leak near that fixture is the likely cause. Whole-house pressure drop may indicate a main line issue.

7. Cracks in Foundation or Walls

New cracks in your foundation, floor tiles, or interior walls can indicate that a slab leak has been saturating the soil under your home. Once the ground shifts, structural damage follows quickly.

8. Green Patches in Your Yard

Unusually lush, green, or wet patches of grass — especially in a dry LA summer when you're not irrigating that area — can indicate a leaking underground supply line or sewer line.

9. Meter Keeps Running After Everything Is Off

This is the definitive DIY test. Shut off every faucet, appliance, and irrigation system. Go to your water meter and watch the leak indicator (a small triangle, star, or dial). If it's moving, water is flowing somewhere. The meter reading will also increase over the next hour if you suspect a slow leak.

10. Water Heater Running More Than Usual

If your water heater cycles on constantly even when nobody is using hot water, you may have a hot water line leak — a particularly expensive type because it wastes both water and gas or electricity 24 hours a day.

What to Do if You Suspect a Leak

1. Do the meter test (above) to confirm water is moving.
2. Check obvious culprits first: toilet flapper (put food dye in the tank — if color appears in the bowl without flushing, it's running), under sinks, behind the washing machine.
3. If the source isn't obvious, call a licensed plumber. Professional leak detection with electronic or acoustic equipment costs $150–$400 and pinpoints the leak non-invasively — far cheaper than tearing out walls blindly.
4. For slab leaks specifically: Don't delay. Every day a slab leak runs, it's saturating the soil and potentially damaging your foundation. LA homes with slab leaks are common due to the age of local plumbing and seismic soil shifts.

Leak Repair Costs in Los Angeles

Frequently Asked Questions