Garden Mulch, Pebbles, and the 75mm Rule — Landscaping That Doesn't Invite Termites

Landscaping choices affect termite risk more than most homeowners realise, and the central issue is mulch. Not because termites eat mulch — they don't — but because organic mulch piled against the building's perimeter creates exactly the moist, warm, cellulose-adjacent conditions that attract subterranean foragers. Once they're at the structure, they go straight to the joists and bearers.

There's a practical rule that governs how this should work, and it comes from AS 3660.1.

The 75mm rule — what it actually means

AS 3660.1:2014 requires a minimum 75mm vertical separation between the soil or mulch surface and the timber of the house. One standard brick height. The separation has to be visible.

The purpose: termites travelling from soil toward the building have to use mud tubes to cross exposed surfaces — they can't survive open-air exposure without the protection the tube provides. If the 75mm zone is visible, an inspector walking the perimeter can see a mud tube before it reaches the timber frame. If soil or mulch fills that zone, the tube is hidden — and the termite can reach the timber without detection.

The 75mm zone isn't a suggestion. It's a minimum requirement under the pre-construction standard, and it's the baseline that post-construction inspections reference when assessing whether the inspection zone is functional.

The zone has to remain clear through all landscaping changes. Adding a new garden bed, raising the ground level, or laying mulch against the wall can reduce the visible gap below 75mm — or eliminate it entirely.

What kills the inspection zone

In practice, the inspection zone is eliminated more by gradual drift than dramatic events. The common scenarios:

Organic mulch piled against the wall. The single most common finding on suburban residential inspections. The gardener tops up mulch in spring, adds it freely along the house perimeter, and ends up with 100–150mm of mulch covering the slab edge entirely. The 75mm zone is gone. Any termite activity that develops in that section is invisible until it's already in the wall.

Garden beds raised against the weatherboards. Timber-framed homes with weatherboard cladding close to ground level are particularly vulnerable. A raised garden bed that brings soil to within 50mm of the bottom weatherboard has essentially provided direct soil-to-timber contact around the inspection zone.

Pavers or concrete laid over original ground level. Where new paving has been laid at a higher level than the original ground, the net effect is a raised surface that can sit level with or above the slab edge. The inspection zone is effectively buried even though it's covered in hard material rather than soil.

New soil brought in for planting. Raised planter boxes, new garden bed soil, or fill brought in for landscaping changes the effective soil height around the perimeter.

Alternatives for the immediate zone

Hard against the building — in the zone where the slab edge and the first course of brickwork need to be visible — organic mulch simply shouldn't be. The material in that zone needs to:

• Not create a termite foraging corridor
• Allow the slab edge to be seen
• Allow drainage away from the building
• Look reasonable

The options that work: pebbles, gravel, crushed rock, and lava rock. All of these allow drainage, don't provide food sources for termites, and can be installed without burying the slab edge. Pebbles and gravel beds along the building perimeter — with the slab edge clearly visible above them — meet the practical and inspection requirements simultaneously.

The threshold: the slab edge above the pebble line must be visible. If the pebbles come to the top of the slab, add drainage rather than raise the pebble layer.

Where organic mulch can still go

Organic mulch has practical value in garden beds, and the goal isn't to eliminate it — it's to keep it back from the structure.

The practical guideline: keep organic mulch at least 30–45cm back from the building's foundation line. Keep mulch depth under 75mm. At that depth and setback, mulch isn't a direct pathway to the structure, and moisture from the mulch bed isn't accumulating against the slab edge.

If using timber edging for garden beds near the house, choose naturally durable species: hardwood, cedar, or treated pine at minimum. Note that even naturally durable timbers lose their repellent properties over time — a decade-old cypress sleeper is not termite-resistant in any meaningful sense. The best approach for edging immediately near the house is non-timber material: steel or concrete edging profiles.

Other landscaping decisions that matter

Mulch is the main story, but a few other landscaping choices affect the termite risk picture:

Ground grading. Finished ground levels should always slope away from the building. Water that drains toward the house after rain accumulates in the subfloor soil and against the slab — the foundation of termite risk.

Garden sleepers. Timber sleepers on the soil surface near the house are a high-risk item. If used, keep them away from the external walls. Never place them against the building's perimeter, and inspect them regularly for termite activity.

Compost bins. Organic material, warm temperatures, and soil moisture in one container — a compost bin against the external wall is directly attractive to foraging workers. Keep compost bins well away from the structure.

Tree stumps. Any stump left in ground adjacent to the building is a termite incubator. If a tree near the house has been removed, destump it.

Next step

If your garden is overdue for a review of what's sitting against the building's perimeter, the best time to address it is before the next inspection.

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