How Slab Leak Detection Works

Finding a leak under a concrete slab is not guesswork — it is a sequence of tests, each one narrowing down where the water is escaping. No single tool does the whole job. A pressure test confirms that a leak exists and tells us whether it is on the hot or cold side. Listening equipment, thermal imaging, tracer gas and cameras then narrow the location to a small area, so any concrete that has to be opened is opened in the right place. This guide explains each method, what happens during a visit, and what detection costs in Olathe.

Good detection matters because it controls the cost of everything that follows. A leak located to within a foot or two means a small opening and a small floor repair. A leak guessed at can mean opening concrete in the wrong place and starting again. That is why our slab leak detection and repair in Olathe work always starts with detection, before anyone talks about breaking a slab.

Pressure Testing: Confirming the Leak

Pressure testing is the first step, and it answers the most basic question: is there a leak at all, and which side is it on? The technician isolates sections of the water system and pressurises them, then watches whether the pressure holds. If the hot side loses pressure while the cold side holds, the leak is on a hot-water line — which also explains a warm spot on the floor. If both sides hold, the problem may be elsewhere, such as a drain line or a fixture. Pressure testing does not find the exact spot, but it confirms the diagnosis and tells the technician which tools to use next. It is also the test that separates a slab leak from look-alikes like a running toilet or an irrigation leak.

Acoustic Listening: Hearing the Leak

Water escaping from a pressurised pipe makes a sound — a hiss, a whoosh, or a faint rushing noise — and acoustic listening equipment picks it up through the slab. The technician places a sensitive microphone on the floor and moves it in a grid, listening for where the sound is loudest. The leak is usually at or near the loudest point. Acoustic listening works on both hot and cold lines, and it is often the main tool for pinpointing a leak once pressure testing has confirmed which side it is on. Background noise matters: the house needs to be quiet during this part of the visit, so the technician may ask you to turn off appliances and keep the rooms still.

Thermal Imaging: Seeing Warm Spots

A thermal camera shows temperature differences across the floor, and it is at its best on hot-water leaks. Warm water escaping under the slab heats the concrete above it, and the camera shows that warmth as a bright patch against the cooler floor around it. The shape and size of the warm area can show which direction the pipe runs and roughly where the leak sits. Thermal imaging is less useful on cold-water leaks, which do not warm the floor, and it can be fooled by sun-warmed floors or heating vents — so it is used together with acoustic listening, not instead of it. If you have already noticed a warm spot by walking barefoot, tell the technician where it is; that is a useful starting point for the camera.

Tracer Gas: For Leaks That Are Hard to Hear

When a leak is small, deep, or masked by other noise, tracer gas can help. The line is drained and filled with a safe mixture of nitrogen and a small amount of hydrogen, which escapes through the leak and rises through the concrete. A sensitive gas detector at floor level picks up the gas and the technician follows the strongest reading to the leak. Tracer gas is especially useful for small leaks that are hard to hear, and for confirming a location that acoustic listening has only narrowed down. It is a slower method than listening, so it is usually used when the first methods have not given a clear answer.

Video Camera Inspection: Inside Drain and Sewer Lines

Not every under-slab leak is in a water supply line. Drain and sewer lines also run under the slab, and a camera inspection looks inside them directly. A small camera on a flexible cable is fed into the line, and the technician watches for cracks, breaks, bellies where water pools, or joints that have separated. Camera inspection is the right tool when the signs point to a drain problem — dampness and odour without a high water bill, for example — and it can also show whether a supply-line leak has been washing soil away around a drain line.

What Happens During a Detection Visit

A detection visit usually starts with a conversation: when did you first notice the signs, where are the warm or damp areas, and what do your recent water bills show? Have your bills and any photos of warm or damp spots ready — they help the technician start in the right place. Then comes the pressure test, followed by the locating methods that fit your situation: acoustic listening and thermal imaging first, tracer gas or camera inspection if needed. Most visits take one to a few hours, depending on the home and how clear the signs are. At the end you should have a confirmed answer — whether there is a leak, which line it is on, and where it is — and a repair recommendation with the options that fit. Detection in Olathe has a typical range of $150–400, averaging about $280, and it is usually billed separately from the repair. For the full cost picture, see our Olathe slab leak repair cost guide, and if you are still deciding whether you have a leak at all, start with our 9 signs of a slab leak checklist and the free meter test.

What Detection Cannot Do

Detection can confirm a leak and locate it, but it cannot tell you the condition of the rest of your pipes, or whether another leak will appear next year. That judgement comes from the age of the system, the pipe material, and whether this is your first leak or your third. It is also why a detection visit sometimes ends with a recommendation for rerouting or repiping rather than a spot repair — if the pipe is at the end of its service life, fixing one pinhole may only buy a short time before the next one. Our spot repair vs reroute vs repipe comparison explains how that decision is made. If you are ready to book a visit, call (913) 369-6679. Our Olathe slab leak repair team serves Olathe only, ZIP codes 66051, 66061, 66062, 66063 and 66215.

Frequently Asked Questions

How do plumbers find a water leak under a slab?

Plumbers find slab leaks with pressure testing to confirm a leak and isolate the hot or cold side, acoustic listening equipment to hear water escaping, thermal imaging to see warm spots on hot lines, tracer gas, and video camera inspection inside drain and sewer lines.

How much does it cost to detect a slab leak?

Slab leak detection in Olathe has a typical range of $150–400, averaging about $280. Detection is usually billed separately from the repair.

How long does slab leak detection take?

Most detection visits take one to a few hours, depending on the home and how clear the signs are. A straightforward hot-line leak with a clear warm spot can be located quickly; a small leak with no visible signs takes longer, because several methods may be needed to narrow it down.