Slab Leak Detection Guide — Finding Leaks in Concrete Foundations

A slab leak is a water leak in a pipe that runs beneath or within a concrete foundation slab. In British Columbia, slab leaks are most common in multi-family buildings with concrete podium slabs — the typical Vancouver townhouse or low-rise condo construction where wood-frame residential units sit above a concrete parkade or ground floor. However, slab leaks can occur in any building with concrete slab-on-grade construction. The pipes may be water supply lines, in-floor radiant heating loops, or even drains that pass through the slab.

How do you know if you have a slab leak? The signs can be subtle and slow-developing. Common indicators include: a hot or warm spot on the floor (from a hot water supply line or radiant heating leak), the sound of running water when no fixtures are in use, unexplained increases in your water bill, low water pressure throughout the building, water stains or dampness on the floor near walls or at slab cracks, and in some cases, water pooling in the basement or parkade below the slab. In multi-unit buildings, the leak may first appear as moisture or water staining in the unit below the affected slab.

What causes slab leaks in BC buildings? Several factors contribute. Copper pipe pinhole corrosion is one of the most common — Metro Vancouver's water chemistry, with its low pH and high dissolved oxygen content in some municipalities, can cause pitting corrosion in copper pipes over time. PEX and Uponor fitting failures have been documented in some manufacturing batches. Improper installation where pipes are placed too close to rebar or at sharp bends creates stress points that fail years later. Chemical attack from some floor leveling compounds can degrade certain pipe materials. And building settlement over time can stress pipe joints and connections.

Finding a slab leak without unnecessary demolition requires specialized equipment and expertise. At LeakInspection.ca, our slab leak detection protocol begins with a visual inspection of all accessible plumbing and pressure testing to confirm there is an active leak. We then use acoustic listening directly on the slab surface — a ground microphone or acoustic sensor placed at grid points on the concrete floor detects the sound of water escaping from the pipe within the slab. This sound is distinctive: a hissing or rushing noise that is different from normal building background sounds.

For precise location, we use leak noise correlation. Two sensors are placed at accessible points along the suspect pipe — typically where the pipe enters and exits the slab, or at access points on floors above and below. The correlator measures the time delay of the leak sound reaching each sensor and calculates the exact position along the pipe. This mathematical approach is accurate to within a few centimetres, allowing the repair contractor to break out a single small section of slab rather than trenching the entire floor.

Thermal imaging is used as a confirmatory tool for slab leaks involving hot water or radiant heating. The warm water escaping into the cooler concrete creates a thermal hotspot on the slab surface that our high-resolution cameras can detect. Even cold water leaks can sometimes be detected by thermal imaging — the escaping water causes evaporative cooling of the slab surface around the leak, creating a cool thermal pattern. This is less reliable than acoustic methods for cold water but provides valuable visual evidence when present.

What happens after the slab leak is located? The typical repair involves breaking out a small section of concrete (approximately 30x30 cm) at the leak point, repairing or replacing the damaged pipe section, pressure testing to confirm the repair, and then patching the concrete. In some cases, particularly with radiant heating loops, the affected loop may be abandoned and a replacement loop routed above the slab or through baseboard heating. The slab removal should be done with a concrete saw after GPR scanning to confirm the pipe location and avoid cutting reinforcing steel or other embedded elements.

Slab leak prevention: if you live in a building with in-slab plumbing, consider installing a leak detection system with automatic shut-off valves at the main water supply. Replace washing machine hoses every 5 years before they fail. Know the age and material of your plumbing — copper pipes in concrete over 20 years old in areas with aggressive water chemistry are at elevated risk. Maintain consistent building temperature to reduce thermal stress on pipes. And schedule a baseline slab leak assessment as part of your preventative maintenance program.