Yes, you absolutely should use a plate compactor on gravel before pouring concrete — in fact, skipping this step is one of the most common reasons concrete slabs crack, sink, or fail prematurely. Proper compaction creates a stable, load-bearing base that supports your slab for decades. Here's what you need to know to do it correctly in a New Zealand context.
Why Compaction Matters
Concrete itself is strong in compression but weak in tension. If the ground beneath it shifts or settles unevenly — even slightly — the slab can crack. A well-compacted gravel base (often called AP20 or AP40 crushed aggregate in NZ) distributes the load evenly, improves drainage, and prevents frost heave in colder South Island or high-altitude locations.
The BRANZ Good Practice Guide for concrete slabs recommends a compacted granular sub-base as standard practice, and this aligns with the requirements of the NZ Building Code for ground-bearing slabs covered under Clause B1 (Structure).
What Equipment Do You Need?
A plate compactor (also called a wacker plate or vibratory plate) is the right tool for compacting granular material like gravel. It uses a vibrating steel plate to densify the material beneath it.
- Forward plate compactors suit most residential jobs — driveways, shed bases, paths.
- Reversible plate compactors are heavier and better for larger areas or thicker lifts.
- Jumping jack compactors are better suited to cohesive soils (clay) rather than gravel, so stick with a plate compactor for aggregate.
Plate compactors are widely available for hire from most tool hire shops around NZ, and from larger stores like Bunnings or Mitre 10.
How to Compact Gravel Correctly
1. Prepare the Sub-Grade First
Before adding gravel, compact the natural ground (sub-grade) itself. Remove any topsoil, organic matter, or soft spots. Soft or wet ground should be assessed — if you hit clay or very soft material, you may need professional advice.
2. Lay Gravel in Layers (Lifts)
This is the most important rule: never try to compact a thick layer all at once. The plate compactor can only effectively compact material to a certain depth. As a general guide:
| Gravel Type | Maximum Lift Thickness (loose) |
|---|---|
| AP20 (20 mm crushed aggregate) | 100–150 mm |
| AP40 (40 mm crushed aggregate) | 150–200 mm |
Always check the compactor manufacturer's specifications, as machine weight and frequency affect penetration depth.
3. Make Multiple Passes
Make at least two to four overlapping passes over each lift. Work in a consistent pattern — rows or a grid — so no area is missed. You're aiming for the gravel to stop visibly moving or "dancing" under the plate, which indicates it's approaching maximum density.
4. Check Moisture Content
Gravel compacts best when it has a small amount of moisture — completely dry or saturated gravel won't compact as effectively. If it's been very dry, lightly dampen the gravel with a hose before compacting. If it's been raining heavily, wait for the material to drain slightly.
5. Achieve Your Target Depth
For a typical residential concrete slab in NZ, a compacted gravel base of 100–150 mm is common, though this can vary depending on soil conditions, slab use (foot traffic vs. vehicle loads), and local council or engineer requirements. A driveway or garage slab carrying vehicle loads will generally need a deeper, more carefully engineered base than a garden path.
Do You Need a Building Consent?
This depends on what you're building. Many small concrete slabs (such as garden paths or small shed bases) may fall under Schedule 1 exemptions of the Building Act and not require a consent. However, larger slabs — particularly those attached to a dwelling, supporting a structure, or in certain locations — may require consent. Always check with your local council before starting work, as exemptions vary and getting this wrong can cause problems when you sell your property. You can find general guidance at MBIE Building Performance.
Safety When Using a Plate Compactor
Plate compactors are powerful machines — treat them with respect.
- Always wear steel-capped boots, hearing protection, and gloves.
- Keep bystanders and children well clear of the operating area.
- Be aware of underground services (gas, water, electrical, telecoms) before you start. Call Before U Dig (0800 248 344) to locate buried services.
- On slopes, take extra care — compactors can be difficult to control on grades.
- Review the hire company's safety instructions and refer to WorkSafe NZ for general plant safety guidance.
When to Call a Professional
While homeowners can handle straightforward compaction work, there are situations where you should bring in a qualified concreter or geotechnical professional:
- Poor or unstable sub-grade — soft, wet, or organic soils that won't compact properly.
- Large or structurally significant slabs — garage floors, house foundations, or anything requiring an engineer's sign-off.
- Sloping sites — drainage and compaction become more complex.
- Reinforced concrete — placing and tying rebar correctly is a skilled job.
- Any slab requiring a building consent — a Licensed Building Practitioner (LBP) may need to be involved. Check the LBP register to find qualified tradespeople.
For peace of mind and a quality result, consider hiring an experienced local concreter — you can find qualified professionals in your area on tradedirectory.co.nz.
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