Getting your gravel base properly compacted before pouring concrete is one of the most important steps in any concreting job. Skip it or rush it, and you risk cracking, settling, and a slab that won't last. The good news is that with the right equipment and a methodical approach, this is a task many capable DIYers can handle for smaller projects.
Why a Compacted Base Matters
Concrete on its own is strong in compression but weak against movement from below. A well-compacted granular sub-base distributes the load evenly, prevents differential settlement, and reduces moisture movement from the ground. In New Zealand, where we have a wide range of soil types — from expansive clays in Auckland to loose volcanic soils in the central North Island — a solid base is non-negotiable. The NZ Building Code and good practice guidance from BRANZ both emphasise adequate ground preparation for concrete slabs.
What Material to Use
For most residential concrete slabs (driveways, paths, shed floors), the standard sub-base material in NZ is AP40 or AP65 crushed aggregate (also called compactable gravel or basecourse). These are well-graded, angular crushed stone mixes that compact tightly and drain well. Avoid rounded river gravel — it doesn't interlock and won't compact properly.
- AP40 (aggregate passing 40 mm) — ideal for most residential slabs
- AP65 (aggregate passing 65 mm) — better for thicker bases or heavier-duty applications
You can source these from local quarries, landscape suppliers, or building merchants like PlaceMakers or Mitre 10.
How Deep Should the Base Be?
As a general guide for residential work:
| Application | Recommended Base Depth |
|---|---|
| Footpath / light foot traffic | 75–100 mm compacted |
| Residential driveway | 100–150 mm compacted |
| Shed or garage floor | 100–150 mm compacted |
| Heavy vehicle access | 150–200 mm+ compacted |
Always check your local council requirements and any relevant building consent conditions, as depths can vary depending on soil type and intended use. If in doubt, a local concreter or geotechnical professional can advise.
Equipment You'll Need
- Plate compactor (wacker plate) — the right tool for the job; available to hire from most hire shops including Bunnings hire centres
- Hand tamper — suitable only for very small or tight areas where a plate compactor won't fit
- Rake and shovel — for spreading material evenly
- Straight edge or level — to check surface flatness
- Safety gear — steel-capped boots, hearing protection, safety glasses, and gloves
A plate compactor is strongly recommended over a hand tamper for anything larger than a square metre or two. Hire costs vary, so check locally for current rates.
Step-by-Step Compaction Process
1. Prepare the Sub-Grade
Excavate to the required depth, removing all topsoil, organic matter, and soft spots. The sub-grade (natural ground) should be firm. If you find soft or wet patches, these need to be dug out and filled with compactable material, or you may need professional advice.
2. Lay the Gravel in Layers
Don't dump all your gravel in at once. Spread it in layers (lifts) of no more than 100 mm loose depth. Each layer must be compacted before the next is added. Trying to compact too thick a layer in one go means the bottom won't be properly compacted, even if the surface looks fine.
3. Compact Each Layer
Run the plate compactor over each layer in overlapping passes — think of it like mowing a lawn. Make at least two to three passes in different directions (e.g., north–south, then east–west). Pay extra attention to edges and corners, which are easy to miss.
4. Check Moisture Content
Gravel compacts best when it's slightly damp, not saturated. If the material is very dry, lightly dampen it with a hose before compacting. If it's waterlogged after rain, wait for it to drain before proceeding.
5. Check for Firmness and Level
After compacting, walk over the surface — it should feel solid with no give underfoot. Use a straight edge to check for high or low spots and correct as needed. The surface should be level (or graded for drainage as required).
6. Final Check Before Pouring
Once you're happy with the compaction, lay your polythene moisture barrier (typically 0.25 mm DPM) and any reinforcing mesh before pouring. Don't drive vehicles or heavy equipment over the prepared base unnecessarily before the concrete goes in.
Common Mistakes to Avoid
- Compacting too thick a layer at once — always work in 100 mm lifts
- Using the wrong material — rounded gravel won't compact properly
- Skipping compaction near edges — this is where settlement cracks often start
- Pouring on wet or frozen ground — wait for suitable conditions
- Not checking for soft spots — these will cause problems later
When to Call a Professional
For larger slabs, driveways, or any concrete work that may require a building consent (check with your local council — Schedule 1 of the Building Act lists some exemptions, but rules vary), it's worth engaging a qualified concreter. If your soil is poor, unstable, or you're on a slope, a professional assessment is strongly recommended. You can find experienced local concreters on tradedirectory.co.nz. Note that concrete work involving structural elements, retaining walls over certain heights, or connections to drainage may also involve licensed tradespeople — always check with your council and MBIE Building Performance if you're unsure.
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