Finding hidden leaks
Under-floor leak detection in Maple Valley, WA
The water bill doubled, a patch of floor feels wrong, and there’s nothing to see. Most hidden leaks here are under the floor, in the crawl space. This is how they’re found.
A higher bill with no visible leak is one of the most common calls we get.
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The problem
Why leaks under the floor go unnoticed
In a house over a crawl space, most of the plumbing runs below the floor: supply lines to each fixture, drain lines back to the main, and sometimes a hot water recirculation loop. A small leak down there drips onto the vapor barrier, out of sight and out of earshot.
Unlike a leak in an upstairs bathroom, it won’t stain a ceiling. Unlike a leak in a wall, it won’t bubble paint. It announces itself indirectly, if at all, and often weeks after it started.
Leak detection is the work of turning those indirect clues into a precise location, so the repair is small and the drying is targeted rather than guessing and opening up floors.
What you notice
The clues that point to a hidden leak
- A water bill noticeably higher than the same period last year.
- The meter’s leak indicator turning with every fixture off.
- A well pump or pressure tank that cycles when nothing is running.
- The sound of running water in a quiet house, often at night.
- A warm spot on the floor, which can mean a hot water line leak below.
- A soft or spongy patch near a bathroom, kitchen or laundry.
- A musty smell in one room or from floor registers.
- Hardwood or laminate lifting in one area.
Check it yourself
Two tests you can do before anyone arrives
The supply-side test
Close every tap, turn off the dishwasher, washer, ice maker and irrigation, and make sure no toilet is refilling. Read the meter, or watch the small triangle or wheel-shaped leak dial, then wait fifteen minutes without using water. Any movement means water is leaving the pressurized system somewhere. To narrow it down, close the shutoff to the water heater and repeat: if the meter stops, the leak is likely on the hot side.
The drain-side test
Drain leaks only happen when water drains, so the meter won’t show them. Run one fixture at a time, such as the tub, then the shower, then the kitchen sink, for a few minutes each, and look or listen through the crawl space hatch afterward. Water or dripping that appears after one particular fixture runs points to that drain or its connection.
Tell us what the meter did and we’ll help you work out the next step.
Call and We’ll Walk You Through ItLocal causes
What typically leaks under houses around Maple Valley
Most of the housing here was built from the 1990s onward, which shapes what fails.
Fittings and connections
Crimp rings, compression fittings and threaded adapters at fixtures and water heaters are the most common points of failure in houses of this age.
Polybutylene pipe
Some homes built from the late 1980s to the mid-1990s have gray polybutylene supply pipe, which has a history of failing at fittings. If you see gray flexible pipe in the crawl space, mention it.
Tub and shower drains
The drain assembly under a tub or shower, and the seal around it, can leak slowly for months. It often shows as damp subfloor directly under the bathroom.
Toilet flange and wax ring
A failed seal under a toilet leaks a little every flush, soaking the subfloor around the flange.
Recirculation lines
Some houses have a hot water loop so hot water arrives faster. It runs through the crawl space, and a leak on it shows as a warm floor and a busy water heater.
Lines near vents
Pipes running close to foundation vents are the ones most exposed during an occasional hard freeze, and can split where they’re coldest.
In the first hour
What to do once you suspect a leak
Confirm with the meter
Run the supply-side test. If the meter moves, close the main shutoff between uses until the leak is found.
Stay clear of wet electrics
If water is near outlets or panels, do not enter. Call your utility and emergency services.
Look through the hatch
With a flashlight, check for water on the vapor barrier, insulation hanging down, or anything dripping. Photograph it.
Map the clues
Write down where the floor feels soft or warm, where the smell is strongest and which fixtures seem to make things worse.
Hold off on opening the floor
Cutting into flooring to look is rarely necessary. The crawl space almost always gives better access.
Keep records
Save recent water bills and note dates. They help show when the leak likely started.
Step by step
How under-floor leaks are traced
Listening to your description
The clues you’ve noticed, together with the meter test, narrow down whether it’s supply or drain, hot or cold.
Moisture mapping from above
Non-invasive meters on the floor surface show where the subfloor below is wetter than surrounding areas.
Thermal imaging
A thermal camera can show temperature differences on the floor or in the crawl space, which helps trace hot water leaks and wet areas.
Crawl space inspection
The crew goes under the floor and follows the water: wet barrier, damp insulation and staining on the subfloor lead toward the source.
Isolating lines
Closing individual valves, or pressure-testing a section with a plumber, confirms which line is leaking.
Drain testing
Running fixtures one at a time while someone watches below confirms drain leaks.
Repair and drying plan
Once the source is confirmed, a plumber repairs it, and the crew dries the affected area. See crawl space water damage.
Pricing for detection and drying is explained before work begins.
Ask About Leak Detection PricingTime and cost
What affects how long it takes and what it costs
- How clear the clues are, and whether the leak is constant or intermittent.
- Supply leaks are usually faster to find than drain leaks.
- Crawl space height, access and insulation that has to be moved.
- Whether a plumber is needed to pressure-test or repair.
- How much water has soaked into framing and insulation.
- Whether drying and insulation replacement follow.
Afterward
What happens after the leak is found
Once the leak is repaired, the next job is drying the area it soaked. That may be a small patch of subfloor and a few batts of insulation, or much more if it ran for months. Your water bill is the easiest way to confirm the repair held.
If your house has polybutylene pipe or several failures at fittings, it may be worth asking a plumber about the rest of the system. The crew can provide documentation of what they found.
A quick call is often enough to narrow down where the leak is.
Soft Spot in the Floor? Call UsRelated services
Questions
Leak detection questions
The clearest sign is the water meter moving with every fixture off. A higher-than-usual bill, a well pump cycling on its own, a soft or warm patch of floor or a musty smell are other common clues.
Yes. In a crawl space house, water from a small leak drips onto the vapor barrier where nobody sees it. It’s common for the first sign to be a bill or a smell, weeks after it started.
If the leak is in a drain line, it only leaks when water drains, so the pressurized supply side stays tight. Running fixtures one at a time and looking below is the way to find drain leaks.
Rarely. In a crawl space house, the leak can almost always be reached from below. Detection from above is used to narrow down where to look.
It can mean a hot water line is leaking below that spot. A thermal camera and a look in the crawl space usually confirm it.
The restoration crew finds the leak and handles drying. The pipe repair is plumbing work, done by a plumber. The crew will tell you what’s needed.
It’s a gray, flexible plastic supply pipe that was widely used from the late 1980s until the mid-1990s and has a history of failing at fittings. Some houses in Maple Valley from that era have it.
Yes, in the sense that ruling out a leak is half of the question. If the meter is still and the water arrives with the rain, the problem is more likely groundwater. See the groundwater guide.
Supply leaks with clear clues can often be found in a single visit. Intermittent drain leaks sometimes take longer, because the right fixture has to be running while someone watches.
Not usually. Meters, thermal imaging and pressure testing narrow the leak down first, so any opening is small and in the right place. Where there’s crawl space access, the finished floor may not need to be touched.
Paying for water you can’t see?
Call and we’ll help you narrow it down, then connect you with a local crew to find it and dry what it soaked.
A straightforward conversation about the clues and what they point to.
Find the Leak: Call (425) 380-0163