Getting the thickness of a concrete slab right is one of the most important decisions you'll make when laying a driveway or path. Too thin and it'll crack under load; too thick and you're wasting money on unnecessary concrete. Here's what you need to know before you pour.
Why Thickness Matters
Concrete slabs work by spreading load across the subbase and into the ground below. If the slab is too thin for the loads it carries, it will flex, crack, and eventually break up — often within just a few years. The right thickness depends on what will travel over it, the quality of the subbase, and the soil conditions underneath. New Zealand's variable ground conditions — from Auckland's clay soils to Canterbury's gravelly subsoils — mean there's no single answer that fits every situation.
Recommended Thickness for Pedestrian Paths
For a standard footpath or garden path that will only carry foot traffic, a slab thickness of around 75–100 mm is generally sufficient, provided you have a well-compacted subbase beneath it. Most concreters in NZ will recommend 100 mm as a practical minimum because it gives a little extra margin for ground movement and the occasional wheelbarrow or ride-on mower.
Subbase matters too
A compacted 75–100 mm layer of AP20 or AP40 crushed aggregate beneath the slab is standard practice. This improves drainage, reduces the risk of the slab settling unevenly, and helps prevent cracking caused by moisture movement in the soil below.
Recommended Thickness for Residential Driveways
For a standard residential driveway carrying everyday passenger vehicles (cars, SUVs, light vans), the widely accepted minimum in New Zealand is 100 mm, though many concreters recommend 125 mm as a more robust option — especially on softer or clay-heavy soils.
Key considerations:
- 100 mm is suitable for good ground conditions with a well-compacted subbase
- 125 mm is a safer choice on softer soils or where occasional heavier vehicles (like a small truck or campervan) may use the driveway
- Steel reinforcing mesh (REF 193 or similar) or polypropylene fibres added to the mix significantly improve crack resistance and are strongly recommended for driveways
- Concrete strength of at least 25 MPa is typical for residential driveways in NZ
Thickness for Heavy Vehicle Access
If your driveway needs to handle heavy vehicles — delivery trucks, concrete trucks, large motorhomes, or similar — you're looking at a minimum of 150 mm, and often 175–200 mm with steel reinforcing bar (rebar) rather than just mesh. This kind of work should always be designed and poured by an experienced concreter, as the subbase preparation and reinforcing layout become critical.
Comparison Table: Slab Thickness at a Glance
| Application | Recommended Thickness | Reinforcing | Min. Concrete Strength |
|---|---|---|---|
| Garden / pedestrian path | 75–100 mm | Optional (fibres helpful) | 20–25 MPa |
| Residential driveway (cars) | 100–125 mm | Mesh or fibres recommended | 25 MPa |
| Driveway with occasional heavy vehicles | 125–150 mm | Mesh required | 25–32 MPa |
| Heavy vehicle / commercial access | 150–200 mm+ | Rebar required | 32 MPa+ |
The Role of the Subbase and Soil
Thickness alone won't save a slab laid on poor ground. Before any concrete is poured, the subgrade (natural soil) should be firm, well-drained, and evenly compacted. Soft spots, tree roots, or poorly compacted fill are common causes of cracking and slab failure in NZ driveways. BRANZ provides useful guidance on ground preparation and slab construction for residential projects.
If you're on expansive clay soils (common in many parts of Auckland, Waikato, and Hawke's Bay), your concreter may recommend additional subbase depth or even a thicker slab to compensate for seasonal ground movement.
Control Joints and Curing
Even a correctly thick slab will crack if control joints aren't cut or formed at regular intervals — typically every 3–4 metres in each direction for a residential driveway. These joints give the concrete a predetermined place to crack as it shrinks during curing, keeping those cracks neat and hidden.
Curing the concrete properly after pouring (keeping it moist and protected from direct sun and wind for at least three days) is just as important as getting the thickness right. Rushing this step is a common DIY mistake.
Do You Need a Building Consent?
In most cases, a standard residential driveway or path does not require a building consent in New Zealand. However, if your driveway involves significant earthworks, retaining walls, or changes to stormwater drainage, you may need to check with your local council. The MBIE Building Performance website is a good starting point for understanding what work requires consent.
When to Call a Professional
While an experienced DIYer can lay a simple garden path, a concrete driveway is a job where professional help pays for itself. Mistakes in thickness, subbase preparation, reinforcing, or finishing are expensive to fix once the concrete has set. A qualified concreter will:
- Assess your soil conditions and recommend the right thickness and subbase depth
- Specify the correct concrete mix and reinforcing
- Place and finish the slab to a professional standard
- Cut control joints at the right time and spacing
You can find experienced, local concreters in your area on tradedirectory.co.nz.