You walk across the hallway barefoot in October and one stretch of tile is warm. Not sunny-window warm — warm in a spot the sun never reaches. Later that week the house is quiet at 11 p.m. and you hear water moving somewhere, faint, with every faucet off. Then the water bill shows up forty dollars higher than last October and nothing about your habits changed.
That combination has a short list of explanations. Most Inland Empire homes sit on a concrete slab with copper or PEX water lines running through or under it, and when one of those lines develops a pinhole, the evidence surfaces slowly. Fall is when people finally connect the dots. The ground under the slab cools off after summer, the contrast between a hot-water line leaking under the floor and the concrete around it gets sharper, and the warm patch that was invisible in August is obvious in October. The leak didn't start in fall. You just started noticing.
Work down this list in order. The first four checks cost nothing and rule out the boring explanations, which are more common than a slab leak. The last three are the ones that need a licensed plumber and equipment.
1. Check the toilets before you check anything else
A silently running toilet is the single most common cause of a mystery water bill increase, and it can waste 200 gallons a day without making a sound you'd notice from another room. The usual culprit is a flapper — the rubber seal at the bottom of the tank — that has gone stiff or warped, letting tank water dribble into the bowl.
The test takes ten minutes. Drop a few drops of food coloring into the tank, don't flush, and come back in fifteen minutes. Color in the bowl means the flapper is leaking. A replacement flapper runs $8 to $15 at any hardware store and takes about fifteen minutes with the supply valve shut off. If the fill valve is hissing and refilling every few minutes, that's the same problem from the other direction; a full fill-valve replacement is typically $10 to $25 in parts, or $100 to $200 if you have a plumber do it as part of another visit.
2. Read the water meter with everything off
This is the check that separates "something is leaking" from "something is warm." Find the meter box near the curb, lift the lid, and look for the small triangular or star-shaped low-flow indicator on the dial face. Turn off every fixture in the house, including the ice maker and any irrigation timer.
If that indicator is spinning, water is moving somewhere. Note the meter reading, wait thirty minutes without using water, and read it again. Any change confirms a leak on your side of the meter. Now shut the main house valve — usually on the front hose-bib side of the house or in a box near the meter — and repeat. If the meter still moves with the house valve closed, the leak is between the meter and the house. If it stops, the leak is inside, and that includes under the slab.
Everyone in the house should know where that main shutoff is and whether it actually turns. Old gate valves in Colton and Rialto homes seize up. Finding out at 2 a.m. is the wrong time.
3. Rule out irrigation and the hose bibs
Fall is also when people finally shut off summer irrigation, and a stuck valve or a cracked lateral will keep drinking water quietly all season. A wet or unusually green patch of lawn during a dry October week is the tell. So is a sprinkler zone that weeps at the lowest head after the timer stops — that's usually a failed diaphragm in the zone valve, a $15 to $40 part.
Check the hose bibs too. A dripping bib with the hose off means the packing nut or the stem washer is worn. Snug the packing nut a quarter turn with a wrench first; if it still drips, a washer replacement or a new frost-free bib runs $75 to $200 installed. None of this explains a warm floor, but it can absolutely explain the bill, and it's free to look.
4. Test the water heater's relief valve and pan
Before you blame the slab, look at the water heater. In a lot of Inland Empire homes it lives in the garage, where summer heat pushes the tank and the temperature-and-pressure relief valve harder than the manufacturer ever planned for. A T&P valve that has started weeping will drip into the drain pan and run out to the garage floor or the side yard, and homeowners often mistake that stain for something coming up from below.
Look under and behind the tank for rust streaks, a crusted mineral trail down the side, or standing water in the pan. Hard water is aggressive here, and it does two things: it packs the bottom of the tank with sediment and it eats the anode rod, the sacrificial metal rod that corrodes so the tank lining doesn't. A neglected tank in this water often gives out closer to 8 years than the 12 you'd get with annual flushing. Tankless units run around 20 years but need descaling on a similar schedule.
If the tank itself is leaking from the shell, it's done. Replacement typically runs $1,400 to $2,600 for a standard tank and $3,000 to $5,500 for tankless. Expect a permit on a water heater swap in San Bernardino County, and expect code to add seismic strapping, a drain pan with a routed discharge line, and often a thermal expansion tank to the invoice.
5. The warm floor itself: a hot-water line under the slab
Now the part you came for. A slab leak is a break in a pressurized water line running under or inside the concrete foundation. About two-thirds of the ones we see reported are on the hot side, which is why the first symptom is usually a warm patch of floor rather than a wet one — hot water heats the concrete before enough volume escapes to reach the surface.
Recognize it by the cluster, not any one sign: a persistently warm spot on tile or laminate, the sound of running water with all fixtures off, hot water pressure that drops noticeably while cold stays fine, a musty smell at the edge of a room, baseboard or carpet tack strip that's damp near a wall, and a meter that keeps ticking with the house valve open but every fixture closed.
Why it happens here specifically: copper lines poured directly into concrete without a sleeve rub against the slab as the house moves, and the expansive clay soils common across the valley move plenty between wet winters and dry summers. Add hard, mineral-heavy water and you get pinhole corrosion from the inside out. Mid-century San Bernardino and Highland homes with original galvanized or thin-wall copper are the usual candidates.
This is not a DIY diagnosis. Locating a slab leak means isolating the hot and cold systems, pressure testing, and then pinpointing with acoustic listening gear, thermal imaging, or tracer gas before anyone touches concrete. Guessing is expensive; looking is cheap. Professional leak detection typically runs $150 to $600 as a standalone service. Repair depends entirely on the approach: a spot repair through the slab commonly falls between $1,500 and $4,000, while rerouting the affected line overhead through the attic and walls — often the smarter call on an aging copper system — runs $2,000 to $6,000. A full repipe of the house is typically $4,000 to $15,000 depending on fixture count and drywall access.
6. Sewer and drain lines under the slab
Not every under-slab problem is pressurized. Drain lines under the foundation don't hold pressure, so they don't show up on a water meter test and they don't make a warm spot. What they produce instead is a sewer smell that comes and goes, repeated backups in the lowest fixture in the house, and sometimes a soft or settling area of floor.
Cast iron drain lines were standard in homes built here through the 1960s and they corrode from the inside, scaling until the pipe diameter shrinks and eventually cracking at the bottom. The diagnostic is a camera inspection through the cleanout — the capped access point on the drain line, usually outside near the foundation. A camera run typically costs $200 to $600 and it tells you whether you're looking at a root intrusion that hydro jetting will clear or a collapsed section that has to be dug or lined.
Skip the chemical drain cleaner entirely. It rarely clears the actual blockage, it sits in the trap eating at old cast iron and gaskets, and it turns a routine cable job into a hazard for whoever opens that line next. Mechanical clearing removes the clog. Chemicals just soak it.
7. Gas lines and anything behind a wall
If the smell isn't sewage but sulfur or rotten eggs, stop reading and get out of the house, then call the gas utility from outside. Underground gas piping to a pool heater, a fire pit, or a detached garage corrodes the same way water lines do. Gas work is licensed work in California, full stop, and there's no homeowner version of a pressure test on a fuel line.
The same rule applies to leaks inside walls. Opening drywall on a wet wall without knowing what's behind it puts you in the neighborhood of electrical runs and load-bearing framing, and a wall that's been damp for weeks may already have mold that needs containment rather than a utility knife.
What can wait and what can't
A running toilet, a dripping hose bib, or a slow drain can wait until Monday. Annoying, wasteful, not damaging.
A warm floor with no visible water can usually wait a few days for a scheduled appointment, but not a few months. The pinhole gets bigger, and the water going under your foundation is quietly saturating soil that supports the house.
These cannot wait: water coming up through flooring or wicking into baseboards, a sudden drop in overall water pressure paired with the meter spinning, sewage backing up into a tub or shower, or any gas smell. In the meantime, shut the main water valve to the house. If the water heater is the source, close the cold-water inlet valve on top of the tank and kill power at the breaker for an electric unit or turn the gas control to "off" for a gas one. Don't leave a tank powered with the supply closed.
When to stop checking and pick up the phone
Call a licensed plumber when the meter's low-flow indicator is still moving after you've shut every fixture, dye-tested every toilet, and confirmed the irrigation is off. That single result means water is escaping inside the pressurized system, and there's no free check left that will tell you where.
Call when a warm spot on the floor persists for more than a couple of days, when hot-water pressure drops while cold-water pressure stays normal, or when the same drain backs up twice in three months. That last one isn't a clog problem. It's a pipe problem, and it's visible on a camera screen before anyone quotes a repair. Ask for CSLB license verification before work begins — ours is 1111400 — and ask that the leak be located and shown to you before anybody opens concrete.