Slab Leak Detection: How Leaks Are Found Before the Jackhammer

The most important thing about slab-leak detection is that it happens before the jackhammer. A few hundred dollars of listening, camera, and pressure work pinpoints the leak so a plumber breaks one square of concrete directly over it — not your whole floor searching for it. Detection runs roughly $150–$600 all-in, the slab-leak-specific average is about $280, and every method below is non-destructive or works from an existing access point like a cleanout. This page is the detection deep dive: how each method works, what it finds, what it costs, and how a good specialist combines them. For why a leak moves a foundation in the first place we hand off to our plumbing-leaks cause page; for fixing the leak and the bill, see repair and cost.

First: confirm you have a leak

Before anyone reaches for a microphone, the question is whether water is actually escaping. The cheapest test is one you can run yourself: the water-meter test. Shut off every fixture and water-using appliance in the house, then watch the meter — or the small leak-indicator dial or triangle many meters carry. If it keeps creeping with everything off, water is going somewhere it shouldn't, and on a slab home that "somewhere" is often under the concrete.

The meter test pairs with the warning signs that point to a slab leak rather than ordinary wear: an unexplained spike in the water bill with no change in use, the sound of running water when every fixture is off, warm or hot spots on the floor (heat from a hot-water-line leak transferring through the flooring), damp, warped, or buckling flooring, musty odors or mold, a sudden drop in water pressure, and pooling or soggy patches around the perimeter. None of these alone proves a slab leak — but together with a moving meter they are enough to call a leak-detection specialist rather than guess.

One reason to move quickly: per US EPA mold and moisture guidance, mold can develop within 24–48 hours of water intrusion, so a confirmed under-slab leak is not a problem to sit on. For the full picture of how a leak versus the soil moves a foundation — and the leak-or-soil decision table — see our plumbing-leaks page; this page assumes the leak is plausible and gets on with finding it.

The detection methods, compared

Once a leak is suspected, a specialist chooses among a defined toolkit. Each method answers a different question — some only confirm a leak exists, others pinpoint exactly where — and each suits a particular pipe type. This table is the heart of the page.

The slab-leak detection toolkit: what each method finds and roughly what it costs. Pressure and hydrostatic tests confirm a leak; acoustic, camera, infrared, and tracer gas pinpoint it.

Two of these — the pressure test and the hydrostatic test — only tell you a leak exists, on the supply side and the drain side respectively. The rest are about location. Knowing which is which is what keeps a homeowner from paying for a camera run on a supply-side problem, or an acoustic survey on a drain that only leaks when water is flowing.

Pressure-side vs drain-side: different tools

The single most useful thing to understand about detection is that your plumbing has two halves, and they fail — and are found — differently.

The pressure side is your supply: the hot and cold water lines, under constant pressure. A pressurized leak pushes water out continuously, so it tends to announce itself faster (a warm spot, a bill spike, a pressure drop) and it responds to the pressure-based toolkit. A pressure test confirms the supply side is losing water; acoustic listening then pinpoints the escaping hiss in metallic pipe; infrared catches the warm signature of a hot-water-line leak. Acoustic is the workhorse here, with two honest limits worth knowing: it struggles on plastic pipe and in background noise, and it reads best within roughly 5–8 feet of depth.

The drain side is your gravity sewer and waste lines, which only carry water when something is draining. That changes everything: there is no constant pressure to test and often no continuous sound to listen for, so pressure and acoustic methods don't apply. Instead a hydrostatic (static isolation) test confirms a drain leak by filling the lines and watching the level hold or drop, and a sewer camera then travels the line to show the exact cracked joint, broken section, or root intrusion. When the failing pipe is plastic — common in newer drain runs — tracer gas steps in where acoustic can't help.

So the first real diagnostic decision isn't which device to buy time on; it's which side is leaking. Get that wrong and you're using the right tool on the wrong half of the system.

How pros combine methods

No single method does the whole job, and a good specialist doesn't pretend otherwise. The reliable workflow is two stages: confirm presence, then pinpoint location.

First, confirm a leak exists and on which side — a pressure test for the supply lines, a hydrostatic test for the drains. This is the cheap, low-risk step that prevents chasing a leak that isn't there, and it sorts the problem into pressure-side or drain-side so the right pinpointing tool comes next.

Then pinpoint the exact spot with acoustic listening, infrared, a sewer camera, or tracer gas — chosen for the pipe material and the leak type the confirmation step revealed. A metallic supply leak goes to acoustic or infrared; a drain leak goes to the camera; a plastic-pipe leak goes to tracer gas. Line tracing maps the pipe route so the eventual opening lands in the right place.

The payoff of doing both stages is the thing this whole page is built around: a confirmed, pinpointed leak means the plumber opens a small, marked section of concrete over the leak, instead of breaking up a floor to hunt for it. Confirmation without location leaves you breaking concrete to search; location without confirmation risks opening the slab for a leak that was never there. The combination is what makes the find trustworthy.

What detection costs

Detection is inexpensive relative to what it protects. The slab-leak-specific figure is an average of about $280, with a typical range of $150–$400 (HomeAdvisor / Angi / Bob Vila, 2025–2026); the full confirm-plus-pinpoint job usually lands in the $150–$600 band, and genuinely hard-to-reach or deep slab leaks can run past $1,000.

Detection costs by method. Figures are industry-estimate ranges that vary by region, access, and how many methods a job needs.

These are detection figures only. The repair that follows — spot repair, rerouting, trenchless lining, tunneling, or breaking the slab — is a separate and much larger number, and it has its own page: see slab-leak repair for the methods and slab-leak cost for the full bill. The point of spending on detection is precisely to keep that larger number as small as possible.

Detect before you demolish

Here is the money rule, stated plainly: detect before you demolish. Spending roughly $150–$600 on detection routinely prevents thousands in unnecessary slab-breaking, because a pinpointed leak lets a plumber open one square of concrete instead of searching with a jackhammer. Insist the pro pinpoint the leak and demonstrate its location before any concrete is broken. A contractor who wants to start breaking concrete "to find it" has the order backwards — detection finds the leak; demolition only reaches it.

Two add-ons belong in this conversation. First, if your slab is post-tension — reinforced with tensioned steel cables, as many slab homes are — the leak location must be scanned or X-rayed before any cutting. A severed post-tension cable can cause structural failure, so ground-penetrating scanning is not optional on those slabs.

Second, if the leak may have already moved the foundation — a dome you can feel, doors that have started to stick, floors gone out of level — pair the leak detection with an elevation / manometer survey so you measure the structural picture, not just the plumbing. The two readings together tell you whether you have a plumbing problem, a foundation problem, or both. See our manometer-survey page for how that measurement works and our engineer-report coverage for the sealed document an insurer will want.

FAQ Note

The FAQ below answers what San Antonio homeowners ask most once a slab leak is suspected — how plumbers actually find it, what detection costs, whether it can be found without breaking concrete, what a hydrostatic test is, how drain-side and supply-side detection differ, what you can do yourself, the most accurate method, and how long it takes. For the cause behind the leak see our plumbing-leaks page, and for the steps after the find, the repair and cost pages.

Get Matched With a Vetted San Antonio Slab-Leak Specialist

If your meter is moving, a room has a warm spot, or a bill jumped with no explanation, the right next step is a leak-detection visit — not a sales pitch to start breaking concrete. We'll match you with a vetted San Antonio specialist who detects before they demolish, and point you to an independent engineer if the foundation may have moved. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for confirm-then-pinpoint detection (a hydrostatic or pressure test before any cutting), a demonstrated leak location, post-tension scanning where the slab calls for it, and a sealed engineer's survey paired with the leak test when movement is in play. If a quote skips detection and reaches for the jackhammer, we'll tell you — because on a slab home, finding the leak is the job, and breaking concrete is only how you reach it.