Yes, old concrete blocks can be used to build a garden retaining wall — and it's a popular, cost-effective choice for many Kiwi homeowners. However, whether it's a straightforward weekend project or something that requires a building consent and an engineer depends largely on how high the wall will be and the conditions of your site.
Assessing Your Old Concrete Blocks First
Before you start stacking, take a close look at the blocks you plan to reuse. Not all old concrete blocks are suitable for retaining work.
- Check for cracks or spalling: Surface chips are usually fine, but deep cracks or crumbling concrete can compromise structural integrity.
- Look for signs of contamination: Blocks from demolition sites may have been exposed to chemicals, asbestos-containing materials, or other hazardous substances. If you're unsure of the block's history, check with WorkSafe NZ for guidance on handling potentially contaminated materials.
- Confirm the block type: Standard hollow concrete masonry units (CMUs), solid dense blocks, and decorative garden blocks all behave differently under lateral soil pressure. Hollow blocks used for retaining generally need to be filled with concrete and reinforced with steel rod.
- Check dimensions are consistent: Mismatched sizes make it harder to build a stable, level wall.
Height Limits and Building Consent
This is the most critical factor. In New Zealand, retaining walls are governed by the NZ Building Code and local council rules.
- Retaining walls up to 1.5 metres in height may be exempt from building consent under Schedule 1 of the Building Act — but this depends on your specific council, the surcharge (load) behind the wall, proximity to boundaries, and site conditions such as slope and soil type.
- Walls over 1.5 metres, or those close to a boundary, near a building, or on unstable ground, will almost certainly require a building consent and may need to be designed by a Chartered Professional Engineer (CPEng).
- Always confirm with your local council before you start. Rules vary between territorial authorities, and getting it wrong can mean costly remediation or issues when you sell your property.
For authoritative guidance, refer to MBIE Building Performance, which publishes clear information on consent requirements and the Building Code.
Key Design Principles for Block Retaining Walls
Even for a modest garden wall, good design makes the difference between a wall that lasts decades and one that fails after the first heavy rain.
Batter (Lean)
Build your wall with a slight batter — leaning it back into the slope by roughly 25–50 mm per 300 mm of height. This helps counteract the lateral pressure of the retained soil.
Drainage is Non-Negotiable
Poor drainage is the number one cause of retaining wall failure in New Zealand's wet climate. Water build-up behind the wall dramatically increases pressure on the blocks.
- Lay a free-draining aggregate (such as AP20 chip) behind the wall.
- Install agricultural drainage pipe (ag pipe) at the base of the wall, sloped to daylight or a stormwater outlet.
- Leave weep holes at the base of the wall — gaps between blocks every 1–1.5 metres — to allow water to escape.
Foundation
Dig a solid foundation trench and set your first course of blocks below ground level — typically at least 100–150 mm, or deeper on softer soils. A compacted base of AP40 or similar aggregate helps prevent settlement.
Reinforcement
For walls over about 600–800 mm, hollow blocks should be filled with concrete and reinforced with deformed steel bar (rebar). Seek advice from a retaining wall builder or engineer on the correct rod size and spacing.
What You Can and Can't Do Yourself
| Task | DIY Allowed? | Notes |
|---|---|---|
| Walls under 1.5 m (no consent required) | Generally yes | Confirm with your council first |
| Walls over 1.5 m | No — consent required | Engineer design likely needed |
| Drainage and aggregate backfill | Yes | Follow good practice |
| Connecting to stormwater drainage | Restricted | May require a licensed drainlayer — check with PGDB |
| Structural design/engineering | No | Must be a CPEng for consented walls |
For research on best-practice construction methods, BRANZ is an excellent NZ-specific resource.
Sourcing Additional Blocks and Materials
If you need more blocks to complete your wall, recycled concrete blocks are often available from demolition salvage yards. New concrete blocks and all the drainage materials you'll need — ag pipe, geotextile fabric, aggregate — are available from Mitre 10, Bunnings, or PlaceMakers.
When to Call a Professional
Retaining walls do more than look tidy — they hold back tonnes of soil and must be built to last. Call a qualified retaining wall builder or structural engineer if:
- Your wall will be over 1.0 metre and you're not confident in your building skills.
- The site has poor or unstable soil, a steep slope, or high groundwater.
- The wall is near a boundary, driveway, or structure.
- You need a building consent — an engineer's design will be required.
- You're unsure whether your old blocks are structurally sound.
A licensed professional will ensure your wall meets the NZ Building Code and won't create a hazard or a liability. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
