5 Signs You Have a Hidden Water Leak (And What to Do About It)
Hidden water leaks can run for weeks before you see them. The five warning signs, three free tests you can do at home, why slab leaks are common in the San Fernando Valley, and when to call a professional.
Why hidden leaks are so costly
Unlike a burst pipe that demands immediate attention, a hidden leak works quietly behind walls, under the slab, or beneath flooring. Over time it feeds mold, damages framing and finishes, and pushes up the water bill. The EPA's WaterSense program estimates that the average household's leaks can account for more than 9,300 gallons of water wasted every year, and that nine percent of homes have leaks that waste 50 gallons or more per day.
The good news from the same source: fixing easily corrected household leaks can save homeowners about 10 percent on their water bills. The harder cases are the ones you cannot see, which is what this guide is about.
1. Your water bill keeps going up
If your water habits have not changed but the bill has climbed, a hidden leak is a likely cause. Compare the same billing period across a few months, and remember that irrigation season changes your normal use. WaterSense gives a simple benchmark: if a family of four uses more than 12,000 gallons a month in a period without heavy outdoor watering, there could be serious leaks.
2. You hear running water when nothing is on
Turn off every faucet, shower, appliance and irrigation valve. If you can still hear a faint hiss or trickle, water is moving somewhere it should not be. The water meter confirms it. WaterSense suggests reading the meter before and after a two-hour period when no water is being used; if the reading changes at all, you probably have a leak.
In many San Fernando Valley homes the meter sits in a box near the street. Many meters also have a small leak indicator dial that spins when even a little water is flowing.
3. Warm or wet spots on the floor
A warm spot on a concrete or tile floor often points to a slab leak, which is a leak in a supply pipe under the foundation. Escaping hot water heats the concrete from below, and you can often feel it barefoot before the house warms up in the morning. Wet or soft areas in hardwood or carpet can mean a leak under the floor. A cold-water leak will not warm the floor, so no warm spot does not rule a slab leak out.
4. Mold or a musty smell
Mold thrives in dark, damp places. A persistent musty odor in a bathroom, kitchen or laundry area, or mold along a baseboard, can mean moisture is building up inside a wall or under the floor from a slow leak.
5. Stained, bubbling or cracked walls and floors
Yellow or brown stains, bubbling paint, warped drywall or new cracks in flooring are signs of water damage. By the time you can see them, the leak has usually been running for some time, so act quickly to keep mold from spreading.
Three free tests you can do today
Before you call anyone, these checks tell you a lot:
- The two-hour meter test. Read the meter, use no water for two hours, then read it again. Any change means water is leaving the system somewhere.
- The toilet dye test. WaterSense suggests putting a few drops of food coloring in the toilet tank. If color shows up in the bowl after 10 minutes, the flapper is leaking. Flush right after the test so the tank does not stain. A worn flapper can silently leak thousands of gallons a year.
- Fixtures and irrigation. A faucet that drips once per second can waste more than 3,000 gallons a year, and an irrigation leak about the thickness of a dime can waste about 6,300 gallons a month, according to WaterSense. Walk the yard while each zone runs and look for soggy patches or water bubbling up.
If the meter moves but the toilets, faucets and irrigation are fine, the leak is most likely in a supply line inside a wall or under the slab, and that is the point where professional leak detection pays for itself.
Leaks outside the house
Not every hidden leak is under the floor. Pool and spa plumbing and in-ground irrigation run underground for long distances, and a leak there can show up as a high bill, a patch of lawn that stays green in dry weather, or a pool that needs topping off more often than evaporation explains.
These lines are tested differently from the supply lines inside the house. We isolate each zone and pressure-test it with dye or tracer gas to find the leaking section, so only that section is dug up. If the two-hour meter test shows a leak but the house checks out, ask for the outdoor lines to be tested as well.
Why slab leaks are common in the San Fernando Valley
Most homes in the Valley and across Los Angeles are slab-on-grade, with the copper supply lines run in or under one slab of concrete. Copper that is 25 to 40 years old starts to develop pinholes, and hard, mineral-heavy water speeds up the corrosion from the inside. Because the pipe is buried, a slab leak can run for weeks or months before you notice it.
Finding it precisely is what keeps the repair small. We pressure-test the supply line, listen with ground microphones and, on hot-water lines, use thermal imaging, so the floor is opened in one spot instead of several. Our slab leak cost guide covers what detection, a spot repair and a reroute typically cost.
What to do next
If you spot any of these signs, close the shutoff for the affected fixture if there is one, or the main water shutoff if water is actively spreading, and call. Our standard residential leak detection runs $295 to $595, with the upper end for slab leaks that need acoustic and thermal location, and repairs are quoted separately once the leak is found.
Last Drop Plumbing & Rooter is a licensed C-36 plumbing contractor (CSLB #1120613) based in Reseda. We answer 818-292-3330 live, 24/7, and we give a written flat-rate quote before any work starts.
Frequently asked questions
- How do I check for a hidden water leak myself?
- Read your water meter, use no water for two hours, then read it again. If the reading changed, water is leaving the system somewhere. Then rule out the easy causes: put a few drops of food coloring in each toilet tank and check the bowl after 10 minutes, look for dripping faucets, and walk the yard while each irrigation zone runs.
- How much water does a hidden leak waste?
- The EPA's WaterSense program estimates that the average household's leaks can account for more than 9,300 gallons wasted every year, and that nine percent of homes have leaks that waste 50 gallons or more per day. A slab leak runs around the clock, so it can add thousands of gallons to a bill before anyone notices.
- How much does leak detection cost?
- Our standard residential leak detection runs $295 to $595. The lower end is for above-slab, easy-access leaks and the upper end is for slab leaks that need acoustic and thermal location. Repairs are quoted separately after the leak is found.
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Sources
Sources checked October 23, 2026. Prices are typical ranges, not quotes, and program terms can change.