For a typical residential concrete path in New Zealand, a compacted gravel sub-base of 100mm is the standard starting point. However, the right depth for your project depends on your soil type, expected load, and local ground conditions — so it's worth understanding the reasoning before you order your materials.
Why a Gravel Base Matters
A well-prepared gravel sub-base does two critical jobs: it provides a stable, load-spreading platform for the concrete slab, and it assists with drainage, reducing the risk of water pooling beneath the path and causing heave or cracking over time. Skimping on the base is one of the most common reasons DIY concrete paths crack or sink prematurely — especially in New Zealand's variable climate, where wet winters and clay-heavy soils are common.
Standard Depth Recommendations
Here's a general guide for residential concrete paths:
| Application | Concrete Thickness | Gravel Base Depth |
|---|---|---|
| Pedestrian path (light foot traffic) | 75–100mm | 100mm compacted |
| Path with occasional vehicle overhang | 100mm | 100–150mm compacted |
| Driveway or heavy-use area | 100–125mm | 150mm compacted |
For a standard garden or pedestrian path, 100mm of compacted AP20 or AP40 crushed aggregate (also called road base or hardfill) is widely accepted practice across New Zealand. If you're in doubt about what applies to your specific situation, BRANZ publishes guidance on concrete slab construction that is well worth consulting.
How Soil Type Affects Your Decision
The condition of your existing ground — called the subgrade — is just as important as the base depth itself.
- Clay soils: Clay is expansive and poorly draining. In many parts of NZ (Auckland, Waikato, parts of Wellington), clay subgrades are common. On clay, you may need to increase your base to 150mm or more, and ensure the clay is well-compacted and not waterlogged before you lay the gravel.
- Sandy or free-draining soils: These are more stable and may be adequate with 100mm of base, but still require thorough compaction.
- Soft or fill ground: If the ground has been recently disturbed, filled, or feels spongy underfoot, you should get a professional assessment before proceeding. Poorly consolidated fill can cause significant settlement.
- Reactive or expansive soils: Some NZ soils (particularly volcanic or highly plastic clays) can move significantly with moisture changes. In these cases, engineering advice is recommended.
Choosing the Right Gravel
Not all gravel is equal. For a concrete path sub-base, you want compactable crushed aggregate, not round river stones (which don't lock together and won't compact properly).
- AP20 (all-passing 20mm) is ideal for paths — it compacts tightly and provides a firm, even surface.
- AP40 is suitable for thicker base layers or where you need to build up significant depth.
- Avoid using builders' sand or topsoil as a sub-base — neither will compact adequately.
You can source suitable aggregate from most landscape supply yards or building merchants such as PlaceMakers or Mitre 10.
Compaction: The Step Most DIYers Skip
Simply tipping gravel into the excavation isn't enough. Proper compaction is essential — loose gravel will settle unevenly under the weight of concrete and foot traffic, leading to cracking.
- Compact in layers no deeper than 100mm at a time using a plate compactor (available to hire from most tool hire shops).
- Check the surface is firm and doesn't deflect when you walk on it.
- Ensure the compacted base is level or graded to direct drainage away from buildings — a fall of around 1:100 (10mm per metre) is a common minimum.
Excavation Depth to Allow For
When planning your excavation, add up all your layers:
- Gravel base: 100mm (minimum)
- Concrete slab: 75–100mm for a pedestrian path
- Any surface finish allowance (e.g., if you want the path flush with lawn level)
So for a basic path, you're typically excavating 200–220mm below your finished surface level. Mark this out carefully before you start digging.
Do You Need a Building Consent?
In most cases, a residential concrete path does not require a building consent under Schedule 1 of the Building Act 2004 — but this depends on factors like proximity to boundaries, drainage implications, and whether it's associated with a retaining structure. Always confirm with your local council before starting work. You can find general guidance at MBIE Building Performance.
When to Call a Professional
While laying a concrete path is within reach of a capable DIYer, there are situations where you should bring in a qualified concreter:
- Soft, unstable, or fill ground that needs assessment or remediation
- Sloping sites where drainage or retaining is involved
- Large areas where consistent compaction and finishing is critical
- Proximity to the house foundation, where drainage must be carefully managed to comply with the NZ Building Code (particularly Clause E1 – Surface Water)
- Any uncertainty about soil conditions or load requirements
A skilled local concreter will assess your subgrade, recommend the right base depth, and ensure the finished path is built to last. You can find qualified concreters in your area on tradedirectory.co.nz.
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