What Reinforcing Mesh Size Should I Use in a DIY Driveway Slab?

For most standard residential driveways in New Zealand, 665 mesh (6mm diameter bar at 150mm x 150mm centres) is the commonly used reinforcing mesh — but the right choice depends on your slab thickness, expected loads, and ground conditions. Getting this right from the start saves you from costly cracking and subsidence down the track.

Understanding Mesh Designations

Reinforcing mesh in NZ is labelled with a number that reflects its wire diameter and spacing. The most common options you'll encounter at your local supplier are:

For a standard 100mm thick residential driveway carrying normal passenger vehicles, 665 mesh is generally considered the minimum. If you're expecting heavier vehicles — like a motorhome, trailer, or regular delivery trucks — step up to 768 or 817 mesh, and consider increasing your slab thickness to 125mm or more.

Slab Thickness and Mesh Go Hand in Hand

Mesh alone won't save a thin slab. The two work together:

• 100mm slab — suitable for cars and light SUVs; use 665 mesh as a minimum
• 125mm slab — better for heavier vehicles or softer ground; use 768 or 817 mesh
• 150mm slab — for very heavy loads or poor subgrade; consult a structural engineer

Always check with BRANZ or your local council if you're unsure whether your project needs engineering input. Ground conditions in NZ vary enormously — from stable volcanic soils in Auckland to expansive clays in Wellington and Christchurch's post-earthquake TC2/TC3 land zones.

Correct Mesh Placement Is Critical

One of the most common DIY mistakes is placing mesh flat on the ground. Reinforcing only works when it's embedded in the concrete at the right depth — typically in the lower third of the slab, around 30–40mm up from the bottom.

Use bar chairs (plastic or wire supports) to hold the mesh at the correct height before the pour. These are inexpensive and available at Bunnings or Mitre 10. Don't use rocks, timber offcuts, or anything that can shift during the pour.

Lapping Sheets Correctly

When joining sheets of mesh, overlap by at least one full grid square (150mm minimum for 665 mesh). Tie overlaps with wire ties to stop sheets shifting during the pour.

Subgrade Preparation Matters Just as Much

Even the best mesh won't prevent cracking if your base isn't solid. Before laying mesh:

• Excavate to a consistent depth (typically 150–200mm below finished surface level to allow for base and slab)
• Compact the subgrade thoroughly with a plate compactor
• Lay a 75–100mm compacted AP20 or AP40 gravel base for drainage and stability
• Consider a polythene vapour barrier (0.2mm minimum) over the gravel to reduce moisture movement

Good subgrade prep is arguably more important than mesh selection for preventing long-term cracking.

Control Joints — Don't Skip These

Control joints (saw cuts or formed grooves) allow the concrete to crack in a controlled, straight line rather than randomly. For a driveway, space them at roughly every 3–4 metres, and no panel should be more than 1.5 times longer than it is wide. Cut to a depth of at least one-quarter of the slab thickness (25mm for a 100mm slab) within 4–12 hours of the pour.

Concrete Mix for Driveways

For a driveway, specify a minimum 25 MPa concrete (often called a 25/20 mix — 25 MPa strength, 20mm aggregate). If you're in a frost-prone area (Central Otago, Manawatū highlands, etc.), ask your ready-mix supplier about air-entrained concrete for freeze-thaw resistance. MBIE Building Performance has guidance on NZ Building Code compliance for concrete work.

Do You Need a Building Consent?

In many cases, a residential driveway slab is exempt from building consent under Schedule 1 of the Building Act — but this depends on your council, the size of the slab, and whether it's attached to a building. Always confirm with your local council before starting work. Some councils have specific requirements around impermeable surfaces and stormwater runoff.

When to Call a Professional

While laying a driveway slab is within reach of a capable DIYer, there are situations where you should bring in a qualified driveway layer or concretor:

• Your ground is soft, waterlogged, or has been recently filled
• You're on TC2 or TC3 land in Canterbury
• The driveway is on a slope greater than about 1:10
• You need a slab thicker than 125mm or are expecting heavy vehicle loads
• You're unsure about drainage and stormwater compliance
• The pour is large and you need a team to finish it before it sets

A licensed professional will also know local council requirements and can ensure the job meets the NZ Building Code. You can find experienced local driveway layers and concretors on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Concrete Slabs and Driveways
• MBIE Building Performance – Building Code Compliance
• Bunnings – Reinforcing Mesh and Concrete Supplies
• Mitre 10 – Concrete and Masonry Products
• PlaceMakers – Concrete and Reinforcing
• WorkSafe NZ – Safe Work on Construction Sites