A slab leak is a break or pinhole in the water lines running beneath your home’s concrete foundation. Left alone, it can silently erode the soil under your slab, spike your water bill by hundreds of dollars a month, and invite mold into places you can’t see. The tricky part is that the leak itself is buried — so your first clues are almost always indirect. Here are seven warning signs to watch for, what each one means, and the steps to take before a plumber arrives.
The 7 Warning Signs
1. Your Water Bill Jumps Without Explanation
A sudden increase in your monthly water bill — especially during a stretch where your habits haven’t changed — is often the first measurable signal. A pressurized pipe leaking beneath the slab can lose dozens of gallons per hour without ever producing a puddle you can see. Pull up two or three months of bills and compare them. If usage climbed by 20 percent or more and you haven’t added irrigation or a new appliance, treat it as a red flag.
2. You Hear Running Water When Everything Is Off
Stand near your water meter or in a quiet hallway and listen. A faint rushing or hissing sound — particularly at night when the house is still — can indicate water moving through a damaged line under the slab. You can do a quick confirmation: turn off every faucet, appliance, and valve in the house, then watch your water meter. If the dial or digital readout keeps moving, water is escaping somewhere.
3. There Are Hot Spots on Your Floor
This one is specific to hot-water line leaks. When a hot-water supply pipe cracks beneath the slab, the heated water seeps into the concrete and radiates upward. Walk barefoot across your tile or hardwood on a cool morning. A patch of floor that feels noticeably warmer than the surrounding area — not near a heating vent — is worth taking seriously. In Southern California homes built on post-tension slabs, this symptom can appear quickly once a leak starts.
4. Cracks Appear in Your Flooring or Baseboards
Concrete is rigid, but sustained moisture underneath it causes the soil to shift. That movement telegraphs upward as hairline cracks in tile grout, warped wood planks, or separating baseboards. A single crack near a doorway or expansion joint isn’t necessarily alarming, but multiple new cracks appearing over a few weeks — especially paired with any other sign on this list — point toward foundation movement from water intrusion.
5. Damp or Warm Carpet With No Obvious Source
If a section of carpet feels damp or spongy underfoot and there’s no plumbing fixture directly above it, the moisture is likely wicking up through the slab. In Corona’s dry climate, carpets don’t stay damp from humidity alone. Press a dry paper towel firmly into the carpet pile. If it comes up with visible moisture, the source is below, not above.
6. Mold or Mildew Smell at Floor Level
Mold needs three things: moisture, warmth, and an organic surface. A slab leak delivers the first two directly to the subfloor, carpet backing, or drywall base — all of which qualify as the third. If you notice a musty odor concentrated near the floor rather than near a bathroom or laundry area, and you can’t find a visible source, a slab leak may already be feeding mold growth you can’t see yet.
7. Low Water Pressure Throughout the House
A significant pipe breach under the slab bleeds pressure from the entire system. If multiple fixtures — not just one faucet — are delivering noticeably weaker flow than usual, and your neighbors aren’t reporting a main-line issue, the pressure loss may be happening below your foundation. This symptom tends to appear later in the leak’s progression, once the break has widened.
What To Do Right Now
If you recognize two or more of these signs, take these steps before calling for help:
- Locate your main shutoff valve. In most Corona homes it’s near the front of the property, close to the street-side meter box, or on an exterior wall near the garage. Turn it clockwise until it stops.
- Do the meter test. Before shutting off, note your meter reading. After shutting off all interior valves, check again in 15 minutes. Movement confirms an active leak somewhere in the system.
- Document what you’re seeing. Take photos or short videos of damp carpet, warm floor spots, cracks, or stains. This record is useful for both the plumber and your homeowner’s insurance claim.
- Don’t run water unnecessarily. Every gallon you push through the system while the line is compromised adds pressure to the break and more water to the soil beneath your slab.
- Move valuables off affected flooring. Moisture wicking through a slab can spread laterally. Rugs, furniture, and stored items near the damp zone are at risk.
What NOT To Do
- Don’t attempt to jackhammer or chip the slab yourself. Post-tension slabs — common in Riverside County construction from the 1980s onward — contain steel cables under tension. Cutting into one without knowing its location can be dangerous and cause structural damage that dwarfs the original leak repair.
- Don’t ignore it hoping it resolves. Slab leaks don’t seal themselves. The longer pressurized water contacts soil, the more the ground shifts — and foundation repairs cost significantly more than pipe repairs.
- Don’t assume the meter test rules it out. A very slow leak may not move the meter noticeably in 15 minutes. A clean meter test is reassuring but not conclusive if you still have multiple symptoms.
When To Call a Professional
Call a licensed plumber who offers electronic leak detection as soon as you’ve confirmed or strongly suspect a slab leak. Modern leak detection uses acoustic listening equipment and pressure testing to pinpoint the break within a few inches — without opening the slab unnecessarily. This matters because repair options vary widely depending on location:
- Direct access (spot repair): The plumber opens the slab directly above the break. Best when the leak is isolated and the pipe is otherwise in good condition.
- Rerouting: A new line is run through the walls or ceiling, bypassing the damaged section entirely. Often the right call when the original copper piping is aging and prone to future leaks.
- Epoxy pipe lining: In some cases, a resin liner can be applied from the inside of the pipe, sealing the break without excavation.
The right method depends on your pipe material, slab type, and how much of the system is affected. That assessment requires someone on-site with the right equipment — not a phone estimate.
RT Olson Plumbing, Heating and Air Conditioning offers slab leak detection and repair for Corona-area homeowners. If you’re seeing these signs, call (951) 344-5596 to schedule a diagnostic before the situation worsens.