Slab Leaks & Concealed Hot Water Leaks: Signs, Detection and What to Do

Updated May 2026 — Around The Clock Plumbing Pty Ltd, Oakleigh South.

A slab leak or concealed hot water leak is one of the most damaging — and most commonly missed — plumbing failures in Melbourne homes. Water escaping inside a concrete slab or within a wall cavity can run undetected for months, lifting tiles, rotting framing, driving up water bills and causing mould before a single visible wet patch appears. This guide covers the warning signs and explains how acoustic leak detection pinpoints the source without unnecessary excavation.

Warning Signs of a Slab Leak or Concealed Hot Water Leak

Slab leaks are difficult to detect precisely because the leak is inside concrete — hidden from direct view. The signs are indirect, and easy to dismiss as other problems:

• Unexplained spike in your water bill — with no visible leak, dripping tap or running toilet to account for it.

• Warm or hot spots on the floor — particularly on tiles or polished concrete, often the first sign of a hot water slab leak.

• Tiles lifting or cracking with no obvious cause — water pressure from below breaks adhesion.

• Damp or mouldy smell — particularly in rooms without obvious moisture sources, or along a wall cavity.

• Water meter moving when every tap is off — a definitive sign of an active leak somewhere in the system.

Hot water slab leaks are particularly common in Melbourne homes where copper hot water lines were laid in or under the slab during construction. As copper ages and the concrete moves with seasonal temperature changes, pinhole leaks develop at stress points. Because the water is under pressure and hot, it spreads further and faster than a cold-water leak before it's detected.

How Acoustic Leak Detection Finds a Slab Leak Without Breaking Concrete

Acoustic leak detection uses a highly sensitive ground-contact microphone to listen for the sound signature of pressurised water escaping a pipe — even through a concrete slab. We use the Sewerin Aquaphon A200, a professional-grade instrument that filters background noise and isolates the specific acoustic frequency of a leak. The technician walks a systematic grid across the slab surface, listening at each point until the signal peaks — indicating the leak's location directly below.

The result is a precise location — typically within 200–300mm — which means any excavation needed is targeted and minimal. This is a fundamentally different outcome from the old approach of breaking up a large section of slab, hoping to find the leak, then restoring the whole area.

Thermal Imaging as a Second Layer of Confirmation

For hot water slab leaks, thermal imaging provides a second, non-invasive confirmation. A thermal camera shows the temperature differential between the warm wet area spreading from the leak point and the surrounding cooler slab — often making the leak's position visible as a distinctive heat plume on the camera image before any concrete is touched. We use thermal imaging in combination with acoustic detection on concealed hot water leak investigations.

A Real Example: Acoustic Detection at an Oakleigh Commercial Site

In September 2025 we were called to a commercial property in Oakleigh where the owner had reported a rising water bill with no visible source. Using the Sewerin A200, we located a pressurised leak in a cold-water main running beneath the concrete floor — in a position that would have required significant slab removal to find by inspection alone. The targeted repair took half a day. Fast arrival → accurate diagnosis → permanent fix, with minimal disruption to the site.

What Happens After the Leak Is Found?

Once the leak is located, your options depend on the pipe type, depth and condition. For an isolated pinhole in otherwise sound copper, targeted repair with minimal excavation is usually the right call. For aged copper that's already failed at one point and is likely to fail at others, rerouting the hot water line above the slab — rather than repairing in-slab — is often the better long-term decision. We'll give you an honest assessment of both options and their respective costs before any work begins, with a fixed-price quote.

Common Questions About Slab Leaks and Concealed Leak Detection

How do I know if I have a slab leak?

The most reliable early indicator is an unexplained rise in your water bill. Combine this with any warm floor spots, lifting tiles or damp smells, and a slab leak is the likely cause. Turning off all taps and checking whether your water meter is still moving is a useful first test.

Does detecting a slab leak require breaking the concrete?

Detection does not require breaking concrete. Acoustic detection and thermal imaging locate the leak from the surface. Concrete is only opened at the confirmed location — and only for the repair, not the investigation.

What is the Sewerin A200?

The Sewerin Aquaphon A200 is a professional acoustic leak detection instrument used by water utilities and specialist leak detection plumbers. It uses a ground-contact microphone to pick up the sound of escaping pressurised water through solid surfaces, including concrete slabs, asphalt and compacted ground.

Can a concealed leak affect a building's structure?

Yes. Long-running concealed leaks can saturate the ground beneath a slab, cause subsidence, rot timber framing within wall cavities, promote significant mould growth and — in heritage brick buildings — progressively damage mortar. Early detection prevents structural damage that is far more expensive than the leak repair itself.

Written and reviewed by Christopher Unwin — founder, Around The Clock Plumbing Pty Ltd, Oakleigh South. BPC Licence #50694, Type A gas, 22 years' experience. National Council member, Master Plumbers Australia & New Zealand.