Why slab leaks hide so well
Much of Fullerton's housing was built in the postwar decades as single-story tract homes poured on a concrete slab. In many of them the copper water lines were laid in the ground beneath the concrete before it was poured. When one of those lines develops a pinhole, the water spreads into the soil and the gravel under the slab instead of dripping somewhere you can see.
That means the first evidence is often something unrelated to water at first glance: a bill, a sound, a warm spot under bare feet. Taken one at a time each sign can have another explanation. Taken together they point clearly to a leak below the floor.
Common signs and how to check them
| Sign | Quick check | Other possible cause |
|---|---|---|
| Water bill jumps | Run the meter test with everything off | Irrigation schedule, a toilet, billing period length |
| Warm patch on the floor | Close the water heater cold inlet and watch the meter | Radiant heat, a duct under the floor, sun through a window |
| Sound of running water | Listen near the floor at night with all fixtures off | Toilet refilling, neighbor's plumbing, water heater noises |
| Damp carpet or lifting wood floor | Feel the pad and check nearby walls for stains | Leaking appliance, shower pan, condensation |
| New cracks in tile or slab | Note location and whether it is damp or warm | Normal settling and shrinkage cracks |
| Water heater runs constantly | Listen for the burner or element cycling often | Thermostat setting, sediment, an aging heater |
| Low pressure at fixtures | Compare several fixtures and both hot and cold | Clogged aerators, scale in old galvanized pipe |
Walk-through to do this evening
- Turn off every fixture and appliance, then read the water meter and note the time.
- Walk the floors barefoot, especially between the water heater, kitchen and bathrooms.
- Lift a corner of carpet near exterior walls and feel the pad for dampness.
- Look at baseboards for swelling, peeling paint or dark lines at the bottom.
- Stand quietly in each room and listen for hissing near the floor.
- Check the outside foundation edge for wet soil or water seeping from the slab line.
- Read the meter again after 30 minutes.
What not to do
Resist the urge to start breaking tile or drilling into the slab where you think the leak is. A warm spot can sit several feet away from the actual leak, and opening the wrong area adds a repair without solving anything. Let pressure testing and acoustic listening find the spot first.
Also avoid ignoring a confirmed leak in hopes it stays small. Water moving under a slab can wash out soil, soak flooring and drywall, and raise the bill month after month.
Questions to ask a plumber about a suspected slab leak
- How will you confirm the leak is under the slab rather than in a wall?
- Will you test the hot and cold sides separately?
- What equipment will you use to pinpoint the location?
- Is the detection visit priced separately from any repair?
- What repair options would you consider, and why?
- What is your CSLB license number?
Common questions
Can a slab leak happen in a newer home?
Yes, though it is more often reported in older homes with original copper or galvanized lines. Any house with water lines under the slab can develop one.
Do slab cracks mean I have a leak?
Not necessarily. Most concrete develops shrinkage cracks. A crack is worth noting if it is new, damp or warm, or appears alongside other signs.
How quickly should a slab leak be repaired?
Once a leak is confirmed, have it located and repaired promptly. If water is spreading into the house, close the main shutoff until then.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.