The 75mm Inspection Zone — Why It Matters

Most homeowners who've had a termite inspection have heard the phrase "75mm inspection zone." Most don't know what it means. It sounds like a measurement — which it is — but the significance of it is about visibility, not distance.

The 75mm inspection zone is a foundational requirement in Australian termite management — easy to overlook, hard to recover when it goes wrong. Here's what it is, why it has to be visible, and what kills it.

What the 75mm zone is

AS 3660.1:2014 — the Australian Standard for termite management in new building work — requires a minimum 75mm vertical separation between the soil or mulch surface and any timber in the building structure. In practice, for a concrete slab construction, this means the slab edge must be exposed and visible to at least 75mm above the finished ground level. One standard brick height. That's the benchmark.

For pier-and-bearer constructions, the equivalent requirement is that the inspection zone around the perimeter is clear — the inspection path between the soil and the timber structure of the house must be unobstructed and visible.

The 75mm figure is the minimum. It's not a target to just clear. Exposed slab edge above that height is better than the minimum; what matters is that the zone is intact and visible.

Why it has to be visible

The logic of the inspection zone isn't about keeping termites out. No termite management system stops termite activity on a site entirely — AS 3660.1 is explicit about that. What the inspection zone does is force termites to cross an exposed surface if they want to reach the timber structure.

Subterranean termites travel in mud shelter tubes when they move above ground. They build these tubes to maintain humidity and avoid exposure. A tube running up a concrete slab face or across a clear inspection gap is visible to an inspector — and to a homeowner checking their own foundation perimeter. That's the mechanism: forced exposure, routine detection.

If the slab edge is buried under soil or covered by a garden bed, termites can build their mud tubes on the back of the slab face, hidden from view, and access the wall framing without any tube ever being visible at the surface. The inspection zone has been defeated — not by termites, but by landscaping.

What kills the inspection zone

This is where the advice becomes practical, because the most common ways the inspection zone gets compromised are all ordinary domestic activities.

Mulch piled against the external walls. Organic mulch holds moisture, provides food-source material, and creates an ideal foraging environment directly against the structure. When mulch is in contact with weatherboards or sitting above the slab edge, it buries the inspection zone and creates a conducive condition at the same time. AS 3660.1 requires a 75mm minimum — mulch in contact with the base of the wall means the zone doesn't exist.

Garden beds raised against the foundation. Garden beds that have been built up over years against the external wall face can completely bury the slab edge. From the outside, the brick base of the house is invisible — lost in a raised garden. Termites can be active on the back of that buried concrete face for years before anything shows on the surface.

Pavers laid flush with the original slab level. Pavers extend over the original ground level and against the side of the house, effectively raising the finished surface to slab height or above. Same result as a raised garden bed — buried slab edge, no inspection zone.

New concrete poured against an existing slab. Additional paving or paths poured flush to an existing slab face fills in the inspection gap permanently. Common on older properties where driveways or entertainment areas have been extended over the decades.

Buried slab edges turn up on most inspections of older Sydney properties. The homeowner often has no idea — the garden or paving has been that way since they moved in, or since a landscaping job years ago that nobody flagged as a risk.

How to fix it

The good news is that most inspection-zone problems are physically correctable without significant cost.

Pull mulch back. The immediate fix. Keep organic mulch at least 30–45cm back from external walls. In the immediate 30cm against the foundation, use pebbles, gravel, or lava rock instead — materials that don't hold moisture the same way and leave the slab edge visible.

Lower the soil level. If a garden bed has been raised against the foundation, excavating back to expose the slab edge restores the inspection zone. The depth varies by property — the goal is visible slab face to at least 75mm above the finished soil level.

Expose buried slab edges. Digging out soil that's built up against a slab face — even a weekend afternoon's work — can restore what was never visible in the first place. Once you can see it, you can check it.

Keep mulch depth itself below 75mm in any area adjacent to the house and keep it from contacting the wall face. These two rules together address the majority of mulch-related inspection-zone problems.

Book an inspection if you're not sure

If your home has garden beds against the foundation, mulch that's been there for years, or paving that's level with the slab, the inspection zone is worth checking before anything else. I find a buried slab edge on most inspections of older Sydney properties — it's the rule, not the exception. Most of the time, the fix is straightforward.

Book a termite inspection (see termite inspections) at activetermitecontrol.com.au or call 0405 790 927. I'll tell you what the inspection zone looks like and what — if anything — needs to happen.