Getting the gravel base right under a concrete path is one of the most important steps in the whole job. Skip it or rush it, and you risk cracking, sinking, or an uneven surface within a few years. The good news is that with the right equipment and a methodical approach, this is a task most capable DIYers can tackle themselves.
Why a Compacted Gravel Base Matters
Concrete on its own is strong in compression but brittle when it flexes. If the ground beneath it shifts or settles unevenly, the slab cracks. A well-compacted gravel sub-base spreads the load, improves drainage, and gives the concrete a stable, uniform platform to sit on. Under the NZ Building Code, concrete paths and driveways must be constructed to avoid water ponding and structural failure — a proper sub-base is fundamental to meeting those requirements.
What You'll Need
- Plate compactor (also called a wacker plate) — hire one from your local hire shop or Bunnings or Mitre 10
- Hand tamper — useful for edges and tight corners
- Compactable gravel (AP20 or AP40 aggregate — ask your supplier)
- Wheelbarrow and shovel
- Straight-edged screed board or rake
- String lines and pegs for level reference
- Safety boots, gloves, eye protection, and hearing protection
How Deep Should the Gravel Base Be?
For a standard residential concrete path (typically 75–100 mm thick concrete), you'll generally want a compacted gravel sub-base of at least 100 mm. On softer or clay-heavy soils — common in many parts of New Zealand — you may need 150 mm or more. If you're unsure about your soil type, it's worth getting advice from a local earthmover or geotechnical professional. BRANZ has useful guidance on ground conditions and slab construction.
Step-by-Step: Compacting Gravel Correctly
1. Prepare the Ground
Excavate the area to the required depth, accounting for both the gravel sub-base and the concrete slab thickness. Remove all topsoil, organic material, and loose fill — these will compress and cause settlement. The subgrade (the natural ground beneath) should be firm. If it's soft or spongy, compact it first with your plate compactor before adding any gravel.
2. Add Gravel in Layers (Lifts)
This is the most important rule: never try to compact more than 100 mm of gravel at a time. Add your gravel in layers — called lifts — of no more than 100 mm loose depth. Attempting to compact a thick layer all at once means the bottom stays loose, no matter how many passes you make on top.
3. Spread Each Lift Evenly
Use a rake or screed board to spread each lift as evenly as possible before compacting. Aim for a consistent depth across the whole area.
4. Compact Each Lift Thoroughly
Set your plate compactor running and make overlapping passes across the entire area. A good rule of thumb is three to four passes in one direction, then rotate 90 degrees and repeat. This ensures even compaction across the full surface. Pay extra attention to edges — use a hand tamper here, as the plate compactor can't always reach right to the formwork.
5. Check for Firmness
After compacting, walk across the surface. It should feel solid underfoot with no give or movement. If you notice soft spots, add a little more gravel and compact again.
6. Repeat for Each Lift
Add your next layer of gravel and repeat the process until you reach your target sub-base depth. Once the final lift is compacted, check your levels with a string line to make sure the surface is even and has the correct fall for drainage (typically a minimum 1:100 fall away from buildings, as required under the Building Code).
7. Final Check Before Concreting
Before you pour, the compacted gravel surface should be:
- Firm and stable with no movement
- Even and at the correct level
- Free of loose material on top
- Slightly damp (not wet) — this stops the dry gravel from sucking moisture out of the fresh concrete
Choosing the Right Gravel
Use compactable aggregate — in New Zealand this is commonly referred to as AP20 (20 mm all-passing) or AP40 (40 mm all-passing). These are graded mixes that include fines, which bind together under compaction. Avoid using single-sized decorative gravels or river pebbles — they won't compact properly and will shift under load. Your local PlaceMakers or building supplies yard can advise on the right product for your area.
A Note on Building Consents
Most residential garden paths don't require a building consent under Schedule 1 of the Building Act — but this depends on the size, location, and intended use. A path close to a boundary, retaining structure, or with drainage implications may be a different story. Always check with your local council before starting to confirm whether consent is needed for your specific project.
Safety Notes
Plate compactors are heavy, vibrating machines. Always wear steel-capped boots, gloves, eye protection, and hearing protection when operating one. Keep bystanders — especially children — well clear of the work area. Be aware of any underground services (pipes, cables) before excavating; contact Dial Before You Dig or your local council to locate them.
When to Call a Professional
If your ground is particularly soft, waterlogged, or made up of fill material, a DIY approach may not be sufficient. Poor subgrade conditions can lead to significant settlement and cracking regardless of how well you compact the gravel. In these situations, it's worth calling a qualified earthmover or geotechnical engineer to assess the site. Similarly, if your path is large, close to a structure, or part of a more complex project, a professional will ensure the work meets the NZ Building Code requirements. You can find experienced local earthmovers and concreters on tradedirectory.co.nz.
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