How to Mix Concrete Footings for Retaining Wall Posts Correctly

Mixing concrete footings for retaining wall posts correctly is one of the most important steps in building a wall that will stand the test of time. Get the mix wrong — too wet, too dry, or the wrong ratio — and you risk posts shifting, leaning, or failing under load. Here's a straightforward guide to doing it right in a New Zealand context.

Why Footings Matter So Much

Retaining walls hold back soil, and that soil exerts significant lateral (sideways) pressure on posts — especially after rain. A well-mixed, properly sized footing anchors each post firmly in the ground and distributes that load safely. In New Zealand, retaining walls over 1.5 metres high (or in certain ground conditions) require a building consent, so always check with your local council before you start. Walls under 1.5 m may be exempt under Schedule 1 of the Building Act, but confirm this with your council as site conditions, soil type, and proximity to boundaries or buildings all affect the rules. You can find guidance at MBIE Building Performance.

Choosing the Right Concrete Mix

For post footings, you want a structural mix — strong enough to resist movement and moisture. The standard DIY mix ratio by volume is:

This 1:2:3 mix produces concrete in the range of 20–25 MPa when properly cured — suitable for most residential post footings. Keep water to a minimum: the mix should be firm and workable, not sloppy. A rough test is the slump test — a handful squeezed should hold its shape without crumbling or oozing water.

Alternatively, for smaller jobs or single posts, pre-mixed bagged concrete (available at Bunnings or Mitre 10) is a convenient and consistent option. Follow the manufacturer's water ratio on the bag exactly.

How Much Concrete Do You Need?

Calculate the volume of each footing hole (cylinder formula: π × radius² × depth), then subtract the post volume. As a rough guide for a 100 mm × 100 mm timber post:

• Footing hole diameter: 3× the post width minimum (300 mm)
• Footing depth: Generally one-third of the total post length, or a minimum of 600 mm — whichever is greater. In softer or wetter soils, go deeper.
• Volume example: A 300 mm diameter × 800 mm deep hole = approximately 0.057 m³ of concrete per post

For larger walls or H-section steel posts, your footing dimensions will be bigger — consult your engineer's specifications.

Step-by-Step Mixing Process

Using a Concrete Mixer

1. Add half the water to the drum first.
2. Add aggregate, then sand, then cement.
3. Mix for 3–5 minutes until uniform in colour and texture.
4. Add remaining water gradually — stop when the mix is workable but not wet.

Mixing by Hand (Small Batches)

1. Combine dry materials thoroughly on a mixing board or in a wheelbarrow.
2. Form a well in the centre and add water in stages.
3. Fold from the outside in, turning the mix repeatedly until fully combined.
4. Avoid adding excess water — it weakens the final strength significantly.

Placing the Concrete

• Set your post first, braced plumb (perfectly vertical) with temporary timber braces.
• Pour concrete in layers, rodding or tamping each layer to remove air pockets.
• Mound the concrete slightly above ground level and slope it away from the post — this sheds water and reduces rot at the base.
• For timber posts, use H5-treated timber (rated for ground contact) to comply with NZ durability requirements under the Building Code.
• Allow at least 24–48 hours before removing braces, and avoid loading the wall for 7 days minimum. Full cure takes 28 days.

NZ Climate Considerations

In colder South Island regions or during winter, concrete can take longer to cure and may be damaged by frost if it freezes before setting. Avoid pouring in temperatures below 5°C without protection. In wet conditions, cover fresh concrete with plastic sheeting. BRANZ has useful guidance on concrete in NZ conditions.

Consent and Engineering Requirements

If your retaining wall is over 1.5 metres in height, retains a slope above, is near a boundary, or is in a flood-prone or unstable area, you will almost certainly need a building consent and may need engineer-designed footings. Do not guess footing sizes for consented walls — an engineer will specify the exact dimensions, reinforcing, and concrete strength (often 25 MPa or higher). Check MBIE Building Performance and your local council for specifics.

When to Call a Professional

While mixing and pouring concrete footings is a task many capable DIYers can handle for smaller walls, there are situations where you should engage a qualified retaining wall builder or Licensed Building Practitioner (LBP):

• The wall exceeds 1.5 m in height or requires a building consent
• The ground is unstable, waterlogged, or on a steep slope
• You're unsure about soil bearing capacity or drainage
• The wall is near a boundary, structure, or services
• Engineer-specified reinforcing or anchor bolts are required

You can find qualified local retaining wall builders on tradedirectory.co.nz. For LBP verification, visit Licensed Building Practitioners.

Sources & Further Reading

• MBIE Building Performance – Retaining Walls & Consents
• BRANZ – Building Research NZ
• Licensed Building Practitioners
• Bunnings – Concrete & Bagged Mix Products
• Mitre 10 – Concrete & Masonry Supplies
• Standards NZ – NZS 3604 Timber-Framed Buildings