For most residential deck post footings in New Zealand, a 1:2:3 mix (one part cement, two parts sand, three parts aggregate) producing approximately 17–20 MPa is the standard recommendation. Many builders simply use a pre-blended 20 MPa bagged concrete mix, which takes the guesswork out of proportioning on site. That said, the right mix is only part of the picture — footing depth, diameter, and soil conditions all play an equally important role.
Why Footing Strength Matters
Deck post footings transfer the load of your deck — and everyone on it — down into the ground. An undersized or weak footing can shift, crack, or sink, leading to a deck that becomes unsafe over time. In New Zealand, residential decks are governed by NZS 3604:2011 (Timber Framed Buildings), which sets out prescriptive requirements for footing sizes based on ground conditions and the load the deck carries. Your local council may also have specific requirements, so it's always worth checking with them before you start.
Standard Concrete Mix Ratios Explained
Concrete strength is measured in megapascals (MPa) — the higher the number, the stronger the mix. Here's a quick guide to common ratios and their typical uses:
| Mix Ratio (cement:sand:aggregate) | Approx. Strength | Typical Use |
|---|---|---|
| 1:3:6 | ~10 MPa | Blinding layers, non-structural fill |
| 1:2.5:4 | ~15 MPa | Light garden paths, non-load-bearing |
| 1:2:3 | ~20 MPa | Deck post footings, house slabs |
| 1:1.5:3 | ~25 MPa | Higher-load structural applications |
For deck post footings, a 1:2:3 ratio (20 MPa) is the widely accepted minimum. If your deck is elevated, large, or sits on poor or expansive soil (common in parts of Auckland and the Waikato), your builder may specify 25 MPa for added confidence.
Bagged Mix vs. Mixing Your Own
Pre-blended bagged concrete
For most DIY homeowners, a pre-blended 20 MPa bagged concrete mix from Bunnings or Mitre 10 is the easiest and most reliable option. The proportions are already correct — you just add water according to the bag instructions. Bags typically come in 20 kg sizes, and you can calculate how many you need based on your footing volume.
Mixing your own
If you're pouring a larger number of footings, you may choose to batch your own mix using a cement mixer. Use:
- 1 part Portland cement
- 2 parts clean, washed sand (no clay or organic matter)
- 3 parts 20 mm crushed aggregate
- Water — aim for a water-to-cement ratio of around 0.5 (roughly 20–25 litres per 40 kg of cement — check the bag instructions or confirm with your supplier, as the exact quantity depends on the specific product). The mix should be workable but not sloppy; it should hold its shape when squeezed.
Avoid adding too much water — it weakens the final strength significantly.
Footing Size and Depth Requirements
The mix ratio is only one part of the equation. Under NZS 3604, footing dimensions depend on:
- Ground bearing capacity (your soil type)
- The tributary area each post supports
- Whether the ground is subject to frost (relevant in Central Otago, Mackenzie Basin, and other inland areas)
As a general guide for a standard residential deck on firm ground, footings are often 300–400 mm in diameter and at least 400–600 mm deep — but always confirm with your council or a qualified builder. In areas with expansive clay soils or poor drainage, deeper footings may be required.
Mixing and Pouring Tips
- Dig your holes cleanly and remove any loose material from the base before pouring.
- Use a cardboard tube form (sonotube) to keep the footing shape neat above ground level.
- Keep the mix consistent — don't let it dry out between batches.
- Cure properly — keep the concrete damp for at least 3–7 days after pouring, especially in hot or windy conditions. This is particularly important in the NZ summer.
- Don't pour in heavy rain — it dilutes the mix and weakens the surface.
- Allow at least 7 days before loading the footing, and ideally 28 days for full strength before placing significant load.
Do You Need a Building Consent?
In New Zealand, many decks under a certain height are exempt from building consent under Schedule 1 of the Building Act 2004, but the rules are specific and depend on deck height, proximity to boundaries, and other factors. Always check with your local council before starting work — don't assume your project is exempt. You can find general guidance at MBIE Building Performance.
When to Call a Professional
While mixing and pouring concrete footings is within reach of a capable DIYer, there are situations where you should bring in a qualified Licensed Building Practitioner (LBP):
- Your deck is more than 1.5 m above ground level
- The ground is unstable, sloping steeply, or has poor bearing capacity
- You're unsure about footing sizes or the applicable NZS 3604 span tables
- Your council requires producer statements or engineering sign-off
- The project requires a building consent (which must be carried out or supervised by an LBP for restricted building work)
You can find qualified local deck builders and LBPs on tradedirectory.co.nz. Getting a professional to at least check your footing design before you pour can save costly mistakes down the track.
Licensed Building Practitioner (LBP)
