What Type of Concrete Mix Should I Use for a Retaining Wall Footing?

For most residential retaining wall footings in New Zealand, a 25 MPa concrete mix is the recommended starting point — it offers a solid balance of strength, workability, and durability. The right mix, however, depends on your wall height, soil conditions, and whether a building consent is required.

Understanding Concrete Strength (MPa)

Concrete strength is measured in megapascals (MPa), which indicates how much compressive load the cured concrete can handle. For footings, common options are:

For retaining wall footings, never go below 17.5 MPa, and in most practical situations 25 MPa is the sensible choice. If your wall is over 1.5 m high or in a high-load area, an engineer may specify 30 MPa or higher.

NZ Building Code and Consent Requirements

Before mixing a single bag of concrete, check whether your retaining wall needs a building consent. Under the NZ Building Code and Schedule 1 exemptions, retaining walls up to 1.5 m in height may be exempt from consent — but this depends on surcharge loading, proximity to boundaries, and your local council's rules. Always confirm with your council before starting work, as getting this wrong can be costly.

For walls requiring consent, a structural engineer will typically specify the exact concrete mix, footing dimensions, and reinforcing steel. Follow their specifications precisely — they take precedence over general guidance. You can find more on consent requirements at MBIE Building Performance.

Footing Size and Reinforcing

The footing is the foundation your wall sits on, so getting it right matters. As a general guide for a standard concrete block or timber retaining wall:

• Width: Typically 2–3 times the wall thickness
• Depth: At least 300–400 mm below finished ground level (deeper in frost-prone areas or poor soils)
• Reinforcing: 12 mm deformed bar (D12) is commonly used; your engineer or council will specify if consent is required

Always check with a qualified professional if you're unsure about sizing — soil type, drainage, and surcharge loads all affect the design.

Choosing Your Mix: Premix Bags vs. Ready-Mix Concrete

Premix Bags (DIY-Friendly)

For smaller footings, premixed concrete bags (available from Bunnings or Mitre 10) are convenient. Look for bags labelled 25 MPa or high-strength and follow the water ratio on the packaging carefully — too much water weakens the final product significantly.

Tips for using premix bags:

• Use a concrete mixer for anything more than a few bags
• Keep the water-to-cement ratio consistent
• Don't add extra water to make it easier to pour — use a plasticiser if needed
• Cure the concrete by keeping it damp for at least 3–7 days after pouring

Ready-Mix Concrete (Larger Jobs)

For footings longer than a few metres, ordering ready-mix concrete from a local supplier is more economical and gives you a consistent, quality-controlled mix. Specify 25 MPa (or your engineer's specified strength) and ask for a low-slump mix (around 80–100 mm slump) suitable for footings. Have your formwork ready before the truck arrives — you'll have limited time to pour and consolidate.

Drainage: Don't Overlook It

Poor drainage is one of the biggest causes of retaining wall failure in New Zealand. Hydrostatic pressure builds up behind a wall when water can't escape, putting enormous stress on the footing and wall structure. Make sure your design includes:

• Ag pipe (agricultural drainage pipe) behind the wall
• Free-draining backfill (gravel or scoria, not clay)
• Weep holes through the wall face

BRANZ has useful guidance on retaining wall drainage and construction best practice.

Mixing Tips for a Strong Footing

• Don't pour onto dry or dusty ground — dampen the base first to prevent the soil from drawing moisture out of the concrete
• Consolidate the mix by rodding or using a vibrator to remove air pockets
• Protect from rain immediately after pouring if wet weather is forecast
• Avoid pouring in freezing conditions — concrete won't cure properly below about 5°C

When to Call a Professional

Retaining walls can be deceptively complex. You should engage a licensed building practitioner (LBP) or structural engineer if:

• Your wall will be over 1.5 m high
• The wall is near a boundary, driveway, or structure
• You're on unstable, clay-heavy, or sloping ground
• A building consent is required
• You're unsure about any aspect of the design or footing specification

You can check LBP credentials at Licensed Building Practitioners. For qualified local concreters and retaining wall specialists, search tradedirectory.co.nz to find professionals in your area.

Sources & Further Reading

• MBIE Building Performance – Retaining Walls & Consents
• BRANZ – Building Research
• Licensed Building Practitioners
• Bunnings – Concrete & Masonry Products
• Mitre 10 – Concrete & Cement
• Standards NZ