A slab leak is a break or pinhole in the water lines running beneath your home’s concrete foundation. Left alone, even a slow one can erode soil, crack your slab, spike your water bill, and feed mold inside walls before you ever see a drop of standing water. The seven signs below are what that leak looks like from inside the house — and after each one, there’s a practical note on what to do right now.
The 7 Warning Signs
1. Your water bill jumps without explanation
A sudden increase of 20–30 percent or more on your bill — with no change in household habits — is one of the earliest flags. Slab leaks run continuously because they’re pressurized supply lines. Pull your last three bills and compare them. If the spike is consistent and you haven’t filled a pool, irrigated more, or had houseguests, the water is going somewhere you can’t see.
What to do: Turn off every fixture and appliance in the house. Find your water meter (typically near the curb in a covered box) and watch the dial or digital readout for one to two minutes. If it moves, water is flowing somewhere in the system. That’s enough reason to schedule a leak detection inspection.
2. You can hear water running when everything is off
A faint hissing, rushing, or dripping sound coming from the floor — especially in a hallway, bathroom, or near an exterior wall — is the pressurized leak announcing itself. In a quiet house at night, some homeowners describe it as a faint trickle that seems to move as they walk across the room.
What to do: Don’t dismiss it as the refrigerator or HVAC. Shut off the water heater and any appliances with water connections, then listen again. If the sound persists, mark the spot on the floor with a piece of tape and note the room. That location is useful data for a technician using acoustic or electronic leak detection equipment.
3. Hot spots on the floor
If a hot-water supply line is leaking beneath the slab, the heat transfers up through the concrete. You’ll notice it as a warm patch — sometimes uncomfortably hot — on a tile or hardwood floor, often in a room with no heating vent nearby. Socks and bare feet catch this one faster than shoes.
What to do: Walk your floors slowly, especially in bathrooms, kitchens, and laundry areas. If you find a warm zone, note its size and location. A hot spot that spans more than a few square feet suggests the leak has been running long enough to saturate the subbase.
4. Cracks in the floor or baseboards
Water migrating under a slab softens the soil beneath it. As the soil shifts, the concrete can crack — first in hairline fractures, then wider. You may also see baseboards pulling away from the wall, door frames that suddenly stick or won’t latch, or tile grout cracking in lines that follow the room’s perimeter.
What to do: Photograph every crack and note the date. Structural movement from a slab leak tends to progress, so a dated photo record helps both a plumber and a structural engineer understand the timeline. Do not fill or patch cracks before a plumber has assessed the cause — you’ll obscure evidence.
5. Damp or discolored flooring
Water wicking up through the slab will first show as a darker patch on concrete, a soft spot in hardwood, or a bubbling seam in vinyl. Carpet may feel wet or spongy in one area with no obvious spill. In some cases the surface looks dry but the padding underneath is saturated — you’ll smell it before you see it.
What to do: Press a dry cloth firmly against the suspect area for 30 seconds. If it picks up moisture, the source is below the surface, not a spill. Resist the urge to run a fan and dry it out — you need a plumber to confirm the source before remediation begins, or you risk trapping moisture under a new floor finish.
6. Mold or mildew smell with no visible source
Chronic moisture beneath a slab creates ideal conditions for mold to colonize the underside of flooring, the bottom of baseboards, and the lower courses of drywall — all out of sight. The smell is earthy, musty, and tends to be strongest in the morning when the house has been closed overnight.
What to do: Don’t mask it with air fresheners. If you can’t find a visible mold source (under the sink, around the toilet base, in the shower grout), the moisture is likely coming from below. A musty smell paired with any other sign on this list is a strong indicator that a slab leak has been running long enough to create a secondary problem.
7. Low water pressure at fixtures
A supply-line leak beneath the slab bleeds pressure before the water reaches your faucets and showers. If pressure has dropped gradually over weeks — especially if it’s worse at fixtures on one side of the house — the pipe routing under that area is worth investigating. A sudden pressure drop is more likely a different failure, but a slow decline fits the slab-leak pattern.
What to do: Test pressure at multiple fixtures. If the drop is isolated to one zone of the house rather than everywhere at once, that narrows the probable location of the leak. Note which fixtures are affected — your plumber will use that to map the supply lines before opening anything up.
What NOT to Do
- Don’t ignore a single sign because it seems minor. Slab leaks are progressive. A pinhole that costs a few hundred dollars to repair today can become a full slab repipe or foundation repair if it runs for months.
- Don’t jackhammer or cut the floor yourself. Even if you’re certain of the location, breaking concrete without first confirming the pipe route with electronic detection often means cutting in the wrong place — and making access harder for the repair.
- Don’t turn off the water and assume the problem is paused. If water has already migrated into the subbase or wall cavities, it will continue to cause damage and feed mold even after the supply is shut off.
When to Call a Professional
Call a plumber if you see two or more of the signs above, or even one sign combined with a water bill that doesn’t add up. Slab leak diagnosis requires acoustic listening equipment, electronic line tracing, or thermal imaging — none of which is a DIY tool. The faster a leak is located precisely, the smaller the access point needed to repair it.
RT Olson Plumbing, Heating and Air Conditioning offers leak detection and slab leak repair for homes in Corona and the surrounding Inland Empire communities. If you’re seeing signs and want a professional assessment, call (951) 344-5596.
What Happens During a Slab Leak Repair
Once the leak is located, repair options typically include direct access (breaking a small section of concrete directly above the pipe), rerouting (running a new line through the walls or attic to bypass the damaged section entirely), or — for older homes with widespread corrosion — a full repipe. The right choice depends on the pipe material, the leak’s location relative to the slab edge, and whether the pipe has shown signs of failure in other areas. Your plumber should walk you through the tradeoffs before any concrete is touched.
Most homeowners’ insurance policies cover sudden and accidental slab leaks but exclude slow leaks that developed over time. Document everything — photos, meter readings, your water bills — before the repair, and ask your plumber for a written scope of work you can submit to your carrier.