A water leak under a concrete floor is one of the most stressful problems a property manager, facility engineer, or building owner can face. The source is hidden, the damage is often invisible until it’s severe, and the cost of getting it wrong — either missing the leak location or making unnecessary cuts — can escalate quickly.

This guide walks through the detection process systematically: how to recognize the signs, which detection methods work for which scenarios, and what to do once you’ve located the leak.

Signs You Have a Water Leak Under a Concrete Floor

A subslab leak often announces itself indirectly, long before water is visible. Watch for:

• Unexplained increase in water bills — A sustained jump in water consumption with no change in usage patterns is one of the earliest indicators of a subslab plumbing leak.
• Warm or hot spots on the floor — If a hot water supply line is leaking under the slab, the heat from that water warms the concrete above it. In some cases you can feel this with your feet, or detect it with a surface thermometer.
• Damp or wet flooring — Carpet that stays damp, hardwood that buckles, vinyl that bubbles — moisture migrating up through a concrete slab from a subslab leak can damage every flooring type sitting on top of it.
• Efflorescence on the slab surface — The white powdery deposit that appears on concrete surfaces is mineral salts carried to the surface by migrating water. New or worsening efflorescence can indicate increased moisture flow from below.
• Mold or mildew odor with no visible source — Subslab moisture creates conditions for mold growth in the materials above the slab even when the concrete surface appears dry.
• Slab heaving or cracking — Water eroding the soil beneath a slab creates voids; saturated soil expands. Both can cause movement, cracking, and settlement in the slab above.
• Sound of running water with all fixtures off — Listening for water movement (especially in a quiet building at night with all fixtures shut off) can identify an active leak.

Confirm It’s a Plumbing Leak, Not Moisture Vapor

Before investing in leak detection, rule out moisture vapor transmission — the natural movement of water vapor up through a concrete slab from the soil below. Moisture vapor is ubiquitous in concrete floors and can cause many of the same symptoms (damp flooring, efflorescence, mold odor) as a plumbing leak.

The key distinguishing test is a simple shut-off check for water supply systems: shut off the main water supply and check your water meter. If the meter continues to move with the supply off, you have an active plumbing leak. If the meter stops, moisture vapor is a more likely explanation for your symptoms.

For drain lines and sewer laterals, there’s no simple shut-off test — those require professional inspection.

Detection Methods

Electronic Leak Detection (Acoustic/Correlation)

Electronic leak detection uses sensitive microphones or accelerometers placed on the floor or pipe access points to “listen” for the sound of water escaping from a pressurized pipe. Acoustic correlation technology compares the sound signatures at two points along the pipe run and uses the time difference to calculate the leak location mathematically.

This method is effective for pressurized water supply lines (both hot and cold) and works through concrete without requiring any cutting or excavation. Detection accuracy is typically within 1–3 feet of the actual leak location.

Limitations: Acoustic methods don’t work well on gravity drain lines (which aren’t pressurized) or on pipes with very low flow rates. Background noise from HVAC, traffic, or adjacent equipment can interfere with detection.

Thermal Imaging (Infrared)

Infrared cameras detect temperature differences on the concrete surface. A hot water leak under the slab creates a thermal anomaly — a warm zone — that an IR camera can map even through several inches of concrete. Cold water leaks can sometimes be detected as a cooling anomaly, though this is less reliable.

Thermal imaging is best done when there’s a temperature differential between the leaking water and the ambient floor temperature — early morning (before the floor heats up from foot traffic) often gives the clearest results. It’s non-destructive and provides a visual map of suspect areas before any cutting begins.

Pressure Testing

Isolating sections of the plumbing system and testing them under pressure is a reliable method for confirming a leak and narrowing down which line or section is affected. Plumbers use pressure gauges to check whether a pressurized segment holds pressure over time — a drop indicates a leak somewhere in that segment.

This method identifies which pipe run is leaking, but doesn’t pinpoint the location without being combined with acoustic detection or physical investigation.

Tracer Gas Detection

Tracer gas (typically a nitrogen/hydrogen mix or helium) is introduced into the pipe system under pressure, and a sensitive detector is used to identify where the gas is escaping through the slab. This method is highly accurate and works for both supply and drain systems, and in noisy environments where acoustic detection would be difficult.

Tracer gas detection is commonly used when acoustic methods are inconclusive or when the leak is in a location with significant background noise.

Drain Camera Inspection (CCTV)

For drain lines, sewer laterals, and non-pressurized pipes, a CCTV camera inserted into the pipe provides direct visual inspection of the interior. A camera will show cracks, root intrusion, joint failures, and corrosion, and the camera’s locator transmitter allows the plumber to pinpoint the problem location on the surface above.

This is the standard diagnostic method for any problem with gravity drain lines under a slab.

Before Any Concrete Cutting

Once a suspected leak location is identified, confirm it as precisely as possible before cutting the slab. Every unnecessary cut in a concrete floor is a cost — in concrete replacement, flooring reinstallation, structural repair if the slab is reinforced or post-tensioned, and business disruption from the work.

Post-tensioned slabs require particular care — cutting a concrete slab without knowing the tendon layout can cause catastrophic structural failure. Ground-penetrating radar (GPR) scanning should be performed before any core drilling or saw cutting in a post-tensioned slab. GPR also locates rebar, conduit, and embedded piping that could be damaged during cutting.

After the Leak Is Found: What Comes Next

Once the leak is repaired, the work isn’t over. Any flooring that was installed over a slab with an active leak will have been exposed to elevated moisture — often well above the manufacturer’s warranty thresholds. Before any flooring reinstallation:

• Allow the slab adequate time to dry toward ambient equilibrium
• Test the concrete moisture level using ASTM F2170 in-situ relative humidity testing
• Confirm RH levels are within the flooring manufacturer’s acceptable range before reinstalling any moisture-sensitive materials
• Document all test results for warranty and insurance purposes

Skipping the post-repair moisture verification is one of the most common mistakes in leak remediation projects. Installing new flooring over a slab that hasn’t reached acceptable moisture levels just recreates the failure.

When to Call a Professional

Leak detection under concrete is almost always a job for professionals with specialized equipment. If you’ve confirmed the symptoms and isolated the problem to a subslab leak, the next call should be to a plumber experienced in slab leak detection combined with, where needed, concrete moisture testing professionals to assess the slab condition after repair.

IFTI provides concrete moisture testing services for post-leak assessment and pre-flooring reinstallation verification. If you’ve had a subslab leak repaired and need to confirm your slab is ready for flooring, contact our team to schedule testing and get documented results that protect your flooring warranty.