Stepped retaining walls are one of the most practical ways to tame a steep NZ section — they divide a slope into a series of level terraces, reduce soil pressure on any single wall, and can dramatically improve usable outdoor space. Whether you're working with timber, concrete blocks, or natural stone, the principles of good planning, solid foundations, and proper drainage apply to every project.
Understand the Consent Rules First
Before you lift a shovel, check your building consent obligations. Under the NZ Building Code, retaining walls that retain more than 1.5 metres of ground generally require a building consent. Some walls under 1.5 m may be exempt under Schedule 1 of the Building Act — but this depends on surcharge loading (e.g., driveways, buildings nearby), soil type, and your local council's rules. Always confirm with your local council before starting, as exemptions vary. If your wall is on or near a boundary, resource consent under the Resource Management Act may also apply.
For walls requiring consent, you'll likely need engineering input from a geotechnical or structural engineer, especially on steep or unstable ground.
Plan Your Terraces
The key advantage of stepped walls is that you spread the retained height across multiple shorter walls, each easier and cheaper to build than one tall wall. A good rule of thumb:
- Keep each individual wall under 1.0–1.2 m where possible to stay in simpler territory structurally.
- Space terraces so the horizontal distance between walls is at least equal to the height of the lower wall — this reduces the walls loading each other.
- Sketch your cross-section on paper first, noting total fall, number of steps, and terrace widths.
For a very steep section, you may need a licensed building practitioner (LBP) or engineer to sign off on the design. Find qualified local retaining wall builders on tradedirectory.co.nz.
Choose Your Materials
Common retaining wall materials in NZ include:
| Material | Best for | Typical wall height |
|---|---|---|
| Treated timber (H5 rated) | Informal garden walls | Up to ~1.2 m |
| Concrete segmental blocks | Curved or straight walls | Up to ~1.5 m (unreinforced) |
| Reinforced concrete block | Taller engineered walls | 1.5 m+ with engineering |
| Natural stone / boulders | Sloped, naturalistic settings | Varies |
| Poured concrete | High-load or engineered walls | Any height with engineering |
For timber, use H5 treated timber rated for ground contact. For concrete blocks, products from suppliers like Firth or similar are widely available at Bunnings and Mitre 10.
Excavate and Prepare Foundations
Solid foundations are non-negotiable on steep ground:
- Cut into the slope to create a level bench for each wall. Remove any topsoil, roots, or loose material.
- Dig a foundation trench at least 200–300 mm deep (deeper in soft or clay soils) for the first course of blocks or timber.
- Compact the base with a plate compactor or hand tamper.
- Lay a compacted AP20 gravel base (at least 100 mm) to aid drainage and provide a firm footing.
- Check for level across the full length before laying the first course.
Build the Walls
Batter and Setback
Most segmental block walls are designed with a slight batter (lean back into the slope) — typically 10–15 mm per course. This improves stability. Follow the manufacturer's specifications for the product you're using.
Backfill and Drainage — Critical Step
Poor drainage is the number one cause of retaining wall failure in NZ. As you build each wall:
- Place a 100 mm perforated ag-pipe (agricultural drain) at the base of the wall, wrapped in geotextile filter fabric.
- Backfill directly behind the wall with free-draining gravel (AP20 or similar) for at least 300 mm.
- Ensure the ag-pipe drains to a legal outlet — a stormwater drain, swale, or daylight at the boundary. Check with your council on stormwater discharge rules.
- Never backfill with clay directly against the wall.
Geogrid Reinforcement
For taller segmental block walls (generally over 1.0 m), geogrid reinforcement layers are embedded into the backfill at specified intervals to tie the wall into the slope. Follow the block manufacturer's engineering tables or get a design from an engineer.
Work Safely on Steep Ground
Steep sections present real hazards — unstable soil, falling material, and machinery risks. Key safety points:
- Never undercut a slope without temporary support — unsupported banks can collapse without warning.
- Check for underground services (power, water, gas) before digging — call Before U Dig (0800 248 344).
- Use appropriate PPE: steel-capped boots, gloves, eye protection, and a hard hat if machinery is overhead.
- Review WorkSafe NZ guidance on excavation safety.
Finishing the Terraces
Once walls are complete and backfilled:
- Top up terraces with quality topsoil and compact lightly.
- Plant groundcovers or install turf promptly — bare soil on a slope erodes quickly in NZ rain.
- Consider root barriers if planting trees near walls, as roots can destabilise structures over time.
When to Call a Professional
You should engage a qualified retaining wall builder or structural/geotechnical engineer if:
- Any wall will retain more than 1.5 m of ground.
- The site has unstable, expansive (clay), or waterlogged soils.
- There is a building, driveway, or boundary fence within the zone of influence of the wall.
- Your council requires engineering sign-off for consent.
- You're unsure about drainage design or geogrid requirements.
An LBP or engineer can also help you avoid costly mistakes — a failed retaining wall on a steep section can cause serious property damage or injury. Search for experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
