Building a retaining wall on a corner or curved section boundary is more complex than a straight run, but it's very achievable with the right planning and materials. The key is understanding how corners and curves affect structural loads, drainage, and — critically — whether your project needs a building consent under the NZ Building Code.
Do You Need a Building Consent?
In New Zealand, retaining walls are subject to Schedule 1 of the Building Act 2004, which sets out exemptions from building consent. As a general guide, a retaining wall that retains no more than 1.5 metres of ground and does not support a surcharge (such as a driveway, vehicle load, or building) may be exempt — but this depends on your specific site conditions and local council rules.
Corner and curved walls can create additional lateral forces and complex load paths, so your council may require engineering input even for walls under 1.5 m. Always check with your local council before you start. You can find general guidance on consent requirements at MBIE Building Performance.
Planning Your Corner or Curved Wall
Before you lift a spade, spend time on planning:
- Survey the boundary. Confirm the exact boundary line with your property title or a licensed surveyor. Walls built over the boundary can cause legal disputes.
- Check soil type and slope. Expansive clay soils or steep slopes increase lateral pressure significantly.
- Identify services. Locate underground pipes, cables, and drains before digging. Call Dial Before You Dig (0800 248 344) or check with your council.
- Consider surcharge loads. A driveway, vehicle parking, or structure near the top of the wall dramatically increases the forces the wall must resist.
- Drainage plan. Poor drainage is the number one cause of retaining wall failure in NZ. Plan for agricultural drain coil (ag pipe) and free-draining backfill (such as AP20 chip) from the outset.
Choosing the Right Material for Corners and Curves
Not all retaining wall materials handle corners and curves equally well. Here's a quick comparison:
| Material | Corners | Curves | DIY Friendly? |
|---|---|---|---|
| Concrete block (e.g. Besser block) | Good with cut/mitred units | Difficult | Moderate |
| Segmental retaining wall blocks | Excellent — purpose-made corner units | Good — small units allow gentle curves | Yes |
| Timber (treated H5 posts & rails) | Good with framed corners | Difficult | Yes |
| Poured concrete | Excellent | Excellent | No — specialist work |
| Natural stone | Moderate | Good — dry-stack curves well | Moderate |
Segmental concrete blocks (the interlocking type available at Bunnings or Mitre 10) are often the most practical DIY choice for corners and gentle curves, as manufacturers supply purpose-made corner units and the small block size naturally accommodates curves.
Building a Corner Section
Footings and Base Course
A solid, level base course is critical — especially at corners where two wall runs meet. Excavate at least 150–200 mm deeper than your first course and compact the base with a plate compactor. Use a compacted AP20 gravel base rather than native soil where possible.
- Start laying from the corner outward in both directions so the corner unit locks in first.
- Use a builder's square to confirm 90° corners, or set out your angle with string lines.
- For angled (non-90°) corners, you may need to cut units with an angle grinder or hire a masonry saw.
Batter and Setback
Most segmental block systems require a batter (slight lean back into the slope) of around 1° per course. Follow the manufacturer's specifications exactly — this is what gives the wall its structural integrity.
Building a Curved Section
- For convex curves (bowing outward), small segmental blocks work well — simply fan the units apart slightly at the front face.
- For concave curves (bowing inward), units are brought closer together at the front. Tighter curves may require cutting.
- Use a string line and pegs or a garden hose laid on the ground to mark out your curve before excavating.
- Geogrid reinforcement (used in taller segmental walls) must be cut and overlapped carefully around curves — follow the manufacturer's instructions precisely, as this is a structural element.
Drainage — Don't Skip This Step
All retaining walls must have adequate drainage to prevent hydrostatic pressure building up behind the wall. For a corner or curved wall:
- Lay 100 mm ag pipe along the base of the wall, wrapped in geotextile sock, sloping to a legal discharge point.
- Backfill the first 300–500 mm behind the wall with free-draining aggregate (AP20 chip).
- Install weep holes if using a solid block or concrete wall — typically every 1.2–1.8 m along the base course.
For guidance on drainage and moisture management, BRANZ is an excellent NZ-specific resource.
When to Call a Professional
You should engage a qualified retaining wall builder or structural engineer if:
- The wall will exceed 1.5 m in height (building consent almost certainly required).
- The wall is near a boundary, building foundation, or driveway.
- The site has poor or unstable soils, a steep slope, or a high water table.
- You're unsure about the structural design, especially for curved or complex corner geometry.
- Your council requires engineering sign-off.
A Licensed Building Practitioner (LBP) may be required to carry out or supervise certain restricted building work. Check the LBP register to find a qualified professional. You can also find experienced local retaining wall builders on tradedirectory.co.nz.
Safety note: Retaining wall collapses can cause serious injury or death. Never underestimate the forces involved, particularly on sloped or corner sites. If in doubt, get professional advice before you build.
Licensed Building Practitioner (LBP)
