Ordering ready-mix concrete for a small DIY job is very achievable — but getting the details right before you call the plant will save you money, stress, and a half-set truck sitting in your driveway. Here's a straightforward guide to help you order with confidence.
Work Out How Much Concrete You Need
Before you pick up the phone, calculate your volume in cubic metres (m³). The basic formula is:
Length (m) × Width (m) × Depth (m) = Volume (m³)
For example, a 4 m × 3 m driveway slab at 100 mm thick works out to: 4 × 3 × 0.1 = 1.2 m³
Always add 5–10% extra to allow for wastage, uneven subgrades, and spillage. So in this example, order around 1.3–1.35 m³.
For irregular shapes, break the area into rectangles or triangles, calculate each section separately, then add them together.
Minimum Loads and Small-Job Options
This is where many DIYers get caught out. Most ready-mix plants in New Zealand have a minimum order of around 0.5–1 m³, and some charge a short-load fee for anything under 3–4 m³. If your job is very small (under 0.5 m³), it may be more practical to use bagged concrete mix from Bunnings or Mitre 10 and mix it yourself.
For jobs between roughly 0.5 m³ and 2 m³, ask your local plant about mini-mix or agri-mix trucks — smaller vehicles that can access tighter sites and carry smaller loads. Not all regions have these, so check availability early.
Choosing the Right Concrete Mix
Ready-mix is specified by compressive strength, measured in megapascals (MPa). Here's a general guide for common residential uses:
| Application | Typical Strength | Notes |
|---|---|---|
| Footpaths, garden paths | 17.5–20 MPa | Light foot traffic |
| Driveways, garage floors | 25 MPa | Standard residential |
| Structural footings | 25–30 MPa | Check NZS 3604 or engineer's spec |
| Exposed aggregate, decorative | 25 MPa+ | Discuss finish with supplier |
For most residential slabs and driveways, 25 MPa is the standard go-to. If your project involves structural elements like foundation footings, check the requirements under NZS 3604 or consult a Licensed Building Practitioner — you can find one at lbp.govt.nz.
You may also be asked about slump (how workable or fluid the mix is). A slump of around 80–100 mm suits most flatwork. Higher slump makes pouring easier but can reduce strength if overdone — don't be tempted to ask the driver to add extra water on site.
What to Tell the Concrete Plant
When you call, have this information ready:
- Volume required (in m³, with your wastage buffer)
- Strength (e.g. 25 MPa)
- Slump (e.g. 80–100 mm)
- Delivery address and access details — truck length, overhead clearances, ground conditions
- Preferred delivery time — early morning is usually best so you have maximum working time
- Whether you need a pump — if the truck can't get close enough to pour directly, you may need a concrete pump (an additional cost and booking)
Be honest about site access. A standard ready-mix truck is large and heavy, and a boggy lawn or narrow driveway can cause real problems.
Preparing Your Site Before the Truck Arrives
Ready-mix waits for no one — once it's in the drum, you typically have 90 minutes to place and finish it. Make sure you:
- Have your formwork (boxing) set up, levelled, and firmly staked
- Lay any reinforcing mesh or rebar required (check your consent conditions or engineer's spec)
- Have enough helpers on hand — at least one or two extra people for anything over 1 m³
- Organise your tools: screed board, float, edging tool, broom (for a non-slip finish), and wheelbarrows if needed
- Arrange a washout area — the driver will need to clean the chute, and concrete washwater must not go into stormwater drains
Do You Need a Building Consent?
Some concrete work requires a building consent. Under Schedule 1 of the Building Act, certain small slabs (like garden paths or patios) may be exempt, but driveways, garages, and anything structural often require consent. Always check with your local council before you start — rules vary by region and site. Find guidance at MBIE Building Performance.
When to Call a Professional
While placing a simple slab is within reach of a capable DIYer, there are situations where you should bring in a qualified concreter:
- Structural footings or foundations — these must meet the NZ Building Code and may require an engineer's input
- Large pours (3 m³+) where timing and finishing become critical
- Exposed aggregate, coloured, or polished concrete — specialist finishing skills are needed
- Sloped or complex sites with drainage requirements
A professional concreter will also have established relationships with local plants and know the best mixes for your region's conditions. You can find qualified local concreters on tradedirectory.co.nz.
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