You walk across the hallway barefoot in August and one patch of tile is warm. Not sunny-window warm — warm from underneath, in a spot with no window near it. Maybe you hear water running when every faucet in the house is off, a faint hiss behind the baseboard. Maybe the carpet in the corner of a bedroom is damp and smells like a wet towel that never dried.

That combination points to a slab leak: a break in a water line running under the concrete your house sits on. In Turlock, Modesto, Ceres and Merced, those calls spike between June and September, and the reason is in the dirt.

Central Valley soil carries a lot of clay. Clay swells when it’s wet and shrinks when it dries. We go five months with almost no rain and a run of 100-degree days, the ground under the slab pulls back, and the slab flexes just enough to put stress on the copper lines cast into it. On homes built slab-on-grade in the ’60s through the ’80s — which is most of the older tracts around here — those lines are soft copper, sometimes rubbing against rebar or a rough pour for fifty years. Add hard water scouring the inside of the pipe and a pinhole shows up. Below, the checks that cost you nothing, then the ones that don’t.

1. Check the obvious water sources before you blame the slab

Warm floors and damp carpet have cheaper explanations, and they’re worth ruling out first because they’re free. Look at the toilet closest to the wet spot: pull the tank lid and see if water is trickling over the overflow tube. A bad flapper or a float set too high can wick water along the flooring for days. Check under the sink cabinet next to the wet area, run your hand along the angle stops and the P-trap, and look at the base of the water heater — a failing tank drips down the side and travels under vinyl or laminate before it ever shows in the open.

An HVAC condensate line is the other common decoy in summer. Air conditioners pull a lot of moisture out of Valley air, and if the condensate drain is plugged, the pan overflows and soaks drywall or subfloor near the air handler. Clear that line, wait a day, and see if the spot dries. Cheap fix, and it works often enough to check.

2. Run the water meter test

This is the single most useful thing you can do, and it takes twenty minutes. Shut off every fixture in the house — no ice makers cycling, no irrigation timer about to fire, no one taking a shower. Find the meter box in the parkway strip near the curb, lift the lid, and look at the leak indicator: a small triangle, star, or gear that spins with even a trickle of flow.

If that indicator is turning with everything off, water is going somewhere. Write down the meter reading, wait thirty minutes, read it again. Then close the main shutoff valve at the house — usually on the exterior wall near the hose bib on the street-facing side — and repeat. Still moving with the house valve closed? The leak is in the service line between the meter and the house, out in the yard. Stops moving? It’s inside, and if you can’t find it at a fixture, it’s under the slab.

3. Look for the heat signature and the pressure drop

Most slab leaks in Central Valley homes are on the hot side. Hot water lines run hotter, expand and contract more, and the copper fails first. That’s why the warm-floor symptom is such a reliable tell.

Walk the whole floor barefoot early in the morning, before the sun has loaded the house up. Note any spot that’s noticeably warmer than the tile three feet away. Then check hot water pressure at a couple of fixtures — a slab leak on the hot line often shows up as weak hot water at the far end of the house while cold pressure stays normal. If you have a hose-bib pressure gauge, a reading that drops steadily with all fixtures closed is another confirmation. Static pressure in most Valley neighborhoods runs 50 to 80 psi; a slow bleed-down means something is open somewhere.

4. Rule out the irrigation system and the hose bibs

Summer is when sprinkler systems run hardest, and a cracked lateral or a valve that doesn’t seat fully will keep feeding water into the ground next to the foundation. That’s a slab-leak look-alike: damp along one side of the house, a meter that creeps, sometimes a low spot in the lawn that’s greener than everything around it.

Shut the irrigation main — most systems have a dedicated ball valve near the backflow device — and re-run the meter test. If the indicator stops, the problem is in the sprinkler system, which is a landscape repair, not a plumbing one. While you’re out there, check the hose bibs. An old compression bib with a worn washer can weep behind the siding where you’ll never see it.

5. Cracks, doors that stick, and what soil movement looks like inside

Ground movement leaves fingerprints. Hairline cracks in drywall above door frames, interior doors that latched fine in March and drag in September, gaps opening between baseboard and floor, tile grout lines cracking in a straight run. None of that is proof of a leak by itself — Valley clay moves seasonally in plenty of sound houses.

What matters is when those signs show up alongside a moving meter or a warm floor. A leak under the slab saturates the soil in one area, that soil swells while the rest of the yard is bone dry, and the differential movement is worse than either condition alone. That’s the sequence that turns a $1,200 pipe repair into a foundation conversation. Which is why you don’t wait it out until the rains come.

6. Electronic leak location — and why guessing is the expensive option

Here’s the line between homeowner work and pro work. Finding the exact spot under concrete takes acoustic listening equipment, line tracing, sometimes a thermal camera, and often pressure isolation of the hot and cold loops separately. It is not something you can eyeball, and it is absolutely not something to solve with a hammer drill and a hunch.

Cutting concrete on a guess is how a small repair becomes a big one. A proper leak detection and repair process (live Oct 4) narrows the location to within a foot or so before anyone touches the floor, so you open one hole instead of three. Typical national cost for professional slab leak detection runs about $150 to $500. Spot repair — breaking the slab, cutting out the failed section, sweating in new copper or a PEX splice, then patching the concrete — commonly lands between $1,500 and $4,500. What pushes it high: flooring that has to be removed and replaced, a leak under a kitchen island or interior bearing wall, and asbestos-containing tile or mastic in older homes.

7. Rerouting or repiping when the slab has more than one failure

One pinhole in an otherwise healthy line is a spot repair. Two leaks in two years in the same house usually means the copper under that slab is at the end of its service life, and chasing individual holes gets expensive fast.

The usual fix then is a reroute: abandon the under-slab line and run new pipe overhead through the attic and down inside walls, or repipe the house entirely. Rerouting a single hot line typically runs $2,000 to $5,000. A whole-house repipe generally falls between $6,000 and $15,000 depending on square footage, number of bathrooms, and whether the attic is accessible in a single-story ranch or a two-story with limited crawl space. Repiping needs a permit and inspection, and drywall patch and texture is often quoted separately — ask. If you’re weighing that decision, a whole-home repipe (live Sep 25) ends the cycle of slab repairs for good, and it also solves the pressure and pinhole problems hard water causes in aging copper.

What can wait a week, and what can’t

A slow slab leak that shows up only as a slightly high water bill and a barely moving meter dial is not a tonight problem. Get it located within a week or two. It’s still costing you water and still wetting soil, but you have time to get quotes.

What can’t wait: standing water on the floor, a hot water line leak scalding a floor surface, a sewage smell with damp flooring, sagging or soft spots in the subfloor, or any wet area near an electrical outlet or panel. In those cases, close the main shutoff valve at the house — turn it clockwise until it stops — and leave it closed until a plumber gets there. If the valve is corroded stuck, which is common on gate valves that haven’t been turned in twenty years, use the curb stop at the meter with a meter key. Also kill the breaker to the water heater, or set a gas unit to “pilot,” so it isn’t heating an empty tank.

One more: if the wet spot is anywhere near a gas line running through the slab or along the foundation, and you smell anything like rotten eggs, leave the house first and call from outside. Water and gas failures don’t always travel separately in an old slab, and gas line work (live Sep 19) is never a DIY job.

When to make the call

Call a licensed plumber if the leak indicator on your meter is still spinning after you’ve shut off every fixture, closed the irrigation valve, and confirmed no toilet is running. Call if a section of floor stays warm with the water heater off overnight. Call if the hot side has lost pressure everywhere while the cold side is fine. And call the same day if flooring is buckling, baseboards are wet, or you can hear water running inside a wall.

Those four conditions are past the point where anything you do without equipment helps. Everything before them, you can check yourself in an afternoon — and often that’s where the answer is.

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