The water bill jumped forty dollars and nothing in the house changed. There’s a warm spot on the hallway tile you can feel through a sock. At night, with the TV off, you hear water running somewhere behind the wall — a faint hiss that stops when you stop listening for it. Maybe there’s a musty smell in one corner of a closet that never used to smell like anything.

That combination almost always means water is going somewhere it shouldn’t. The good news is that several of the most common causes cost nothing to check and take about ten minutes. Work through them in order before anyone cuts into drywall or breaks concrete.

1. Start with the water meter — it’s the cheapest leak test there is

Turn off every faucet, the ice maker, the irrigation timer, and the washing machine. Then find your meter at the curb, usually under a concrete or plastic lid near the sidewalk. Pop the lid, wipe the glass, and look for the small triangular or star-shaped low-flow indicator. If it’s spinning with everything off, water is moving through your service line.

Write down the meter reading, wait an hour with no water use, and read it again. Any change confirms it. This one test separates “we have a leak” from “we have a billing question,” and it’s the first thing a plumber will do anyway. In much of Merced and Madera County, older meter boxes sit full of dirt or ants — bring a screwdriver and a flashlight and clear the glass before you decide the dial isn’t moving.

2. Check the toilets, because a silent flapper leak beats every other cause

A worn toilet flapper is the single most common cause of a mystery water bill, and it makes almost no sound. The rubber flapper at the bottom of the tank hardens with age and with the mineral content of Central Valley water, then stops sealing against the flush valve seat. Water trickles from tank to bowl around the clock.

Test it by dropping a few drops of food coloring into the tank and leaving it alone for twenty minutes. If color shows up in the bowl without a flush, the flapper is done. A replacement flapper runs about $8 to $15 at any hardware store and takes ten minutes with the supply valve closed. While you’re in there, check that the fill valve shuts off cleanly and the water level sits about an inch below the top of the overflow tube. A running toilet can waste 200 gallons a day and never leave a mark on the floor.

3. Look under sinks, behind the washer, and at the water heater

Open every cabinet and put a hand on the shutoff valves, the supply lines, and the P-trap under each sink. You’re feeling for damp, not looking for a puddle — slow leaks usually evaporate before they pool. Swollen particleboard, white mineral crust on a compression nut, or a rust ring on the cabinet floor all say the same thing.

Then check the water heater. Look at the base of the tank, the drain valve, and the temperature and pressure relief valve discharge pipe. If that pipe is dripping, the tank is either overheating or seeing pressure spikes, and that’s a real problem, not a nuisance. Braided stainless washing machine hoses should be replaced every five to seven years; the rubber ones that came with the machine are the ones that fail catastrophically while you’re at work.

4. Rule out the irrigation system and the hose bibbs

Valley yards run a lot of water, and a leak in a lawn zone can hide in plain sight. Shut the valve feeding the irrigation manifold, then repeat the meter test. If the low-flow indicator stops, the leak is on the sprinkler side of the system, which is a different repair — usually a cracked lateral, a stuck valve diaphragm, or a broken riser under a shrub.

Also check outdoor hose bibbs and any anti-siphon valve on the wall. Frost damage isn’t much of a factor here, but a bibb that’s been dripping for two seasons can run through real water. Fixing a hose bibb washer or replacing the whole bibb typically lands somewhere between $125 and $300 nationally, depending on whether it’s threaded, soldered, or buried behind stucco.

5. Watch for the signs of a slab leak — and stop DIY here

A lot of Merced, Atwater, Madera, and Chowchilla housing stock is slab-on-grade built between the 1960s and the 1980s, with copper supply lines run in or under the concrete. When those lines fail, you get a specific set of symptoms: a warm or hot spot on the floor, hot water that never quite runs out at one fixture, a hairline crack in tile that got wider this year, or the sound of running water with every fixture closed.

Do not go looking for it with a hammer. Guessing at a slab leak and opening concrete on a hunch is how a $1,500 repair becomes a $6,000 one, plus flooring. This is the point where professional leak detection and repair (live Oct 4) earns its keep: acoustic listening equipment amplifies the hiss of pressurized water escaping a pinhole, thermal imaging finds the temperature signature of a hot line, and tracer gas — a harmless helium-nitrogen mix pushed into an isolated line — lets a technician pinpoint the exit point within inches. The line gets isolated, the leak gets located, then one hole gets opened.

Nationally, slab leak detection typically runs $200 to $600 as a standalone service, and the repair that follows ranges from about $1,200 for a single spot repair to $4,000 or more if the line has to be rerouted overhead. What pushes it toward the high end: tile or hardwood over the break, multiple pinholes on the same run, and a home with aggressive water chemistry where fixing one hole just moves the problem down the pipe. When a house starts producing repeat pinholes, a whole-home repipe in PEX or copper usually costs less over five years than chasing them one at a time.

6. Sewer and drain leaks: different symptoms, different tools

A supply leak is under pressure and runs all the time. A drain leak only shows up when something upstream is used, so the symptoms come and go — a smell after someone showers, a soft spot in the yard over the lateral, gnats in a bathroom, a section of lawn that stays green in August when everything else is brown.

Older laterals here are clay or cast iron, and mature shade trees do exactly what you’d expect to a clay joint. Roots get in, the joint separates, and sewage starts wetting the soil. Locating that takes a camera down the cleanout and a locator to mark the depth and position from the surface. It isn’t a DIY job for an obvious reason: you’re dealing with raw sewage and, in cast iron, pipe that can collapse when it’s disturbed. If the camera finds root intrusion rather than a break, hydro jetting the line (live Sep 10) — high-pressure water that scours the pipe wall instead of just punching a hole through the blockage — often buys years before anything gets replaced. Sewer camera inspection typically runs $250 to $500; spot repairs start around $1,500, and trenchless replacement of a full lateral usually falls between $6,000 and $15,000 depending on length, depth, and whether the run crosses a city sidewalk.

7. Gas line leaks: the one you don’t investigate at all

If you smell rotten egg or sulfur near the water heater, furnace, range, or the meter, you’re done troubleshooting. Get everyone out, don’t flip switches, don’t use a phone inside, and call the gas utility from outside. Once the utility has shut it off, a licensed plumber can pressure-test the system with a gauge, isolate the section that’s dropping, and replace the failed run of black iron or CSST.

Underground gas lines to pool heaters, shop buildings, and outdoor kitchens are a common failure point after twenty or thirty years in Valley soil. That’s gas line work that requires a permit and a licensed contractor (live Sep 19) in every jurisdiction in Merced and Madera County, and it is never a homeowner project.

What can wait, and what can’t

A dripping faucet, a running toilet, a weeping angle stop under a sink with a bowl under it — those can wait a week. They cost money in water, not in restoration.

These can’t wait: water coming up through the slab or wicking up baseboards, a ceiling stain that’s growing, a water heater leaking from the tank body rather than a fitting, any sewage backing up into a tub or floor drain, and any smell of gas. In those cases, shut off the fixture supply valve if there is one. If there isn’t, close the main shutoff — usually a ball valve or gate valve where the service line enters the house, near the front hose bibb, or inside the meter box at the curb. Turn it now, before you need it. A valve nobody has touched in twenty years has a habit of seizing or snapping at the worst possible moment, and knowing which way it turns in daylight is worth more than any tool in the garage.

For a water heater specifically, close the cold inlet valve on top of the tank and, on a gas unit, turn the control knob to “off” or “pilot.” On electric, kill the breaker first.

When to make the call

Call a plumber if the meter’s low-flow indicator is still moving after you’ve dye-tested every toilet, shut the irrigation valve, and checked under every sink. Call if you have a warm spot on the floor, hot water pressure that dropped at one fixture and nowhere else, or the sound of running water with the house quiet. Call if a stain on a ceiling or wall is bigger today than it was yesterday. And call the gas utility, not a plumber, the moment you smell gas.

What you shouldn’t do is start opening walls or floors to find it yourself. Detection equipment exists so that exactly one hole gets cut, in the right place, on the first try.