Replacing an old timber retaining wall is a manageable project for a capable DIYer — but it requires careful planning, the right materials, and a solid understanding of NZ building rules. Done properly, a new retaining wall will protect your property for decades; done poorly, it can fail dangerously and cause costly damage.
Step 1: Check Whether You Need a Building Consent
This is the most important first step. In New Zealand, retaining walls are subject to the NZ Building Code and may require a building consent depending on their height and the surcharge (load) they carry.
Under Schedule 1 of the Building Act, retaining walls that retain no more than 1.5 metres of ground and carry no surcharge (e.g. no driveway, structure, or slope above) may be exempt from consent — but this is not guaranteed. Rules vary by council and site conditions.
Always confirm with your local council before starting work. If your wall is over 1.5 m, retains a slope, or supports a driveway or building, a consent is almost certainly required. Unpermitted work can cause serious problems when you sell your home.
Step 2: Assess the Site and Plan Your New Wall
Before demolition, take stock of what you're working with:
- Height and length of the existing wall
- Soil type behind the wall (clay soils exert more pressure)
- Drainage — is there evidence of water pooling or seepage?
- Surcharge loads — is there a fence, driveway, or structure above?
- Access for machinery and skip bins
Choose your replacement material at this stage. Common options include:
| Material | Pros | Cons |
|---|---|---|
| Treated timber (H5) | Cost-effective, natural look | Shorter lifespan than masonry |
| Concrete block | Durable, strong | Higher cost, needs skilled labour |
| Keystone/segmental block | DIY-friendly, no mortar | Limited height without engineering |
| Poured concrete | Very strong | Requires formwork, usually consented |
If you're replacing like-for-like with timber, use H5 treated timber rated for ground contact. Check with your supplier at Bunnings, Mitre 10, or PlaceMakers for appropriate product options.
Step 3: Organise Equipment and Safety Gear
You'll need:
- Spades, mattocks, and a wrecking bar
- Chainsaw or reciprocating saw (for cutting old timber)
- Post-hole borer (hire one or use a contractor)
- Level, string line, and tape measure
- PPE: gloves, safety glasses, steel-capped boots, and a dust mask
- Skip bin or trailer for waste disposal
Old CCA-treated timber (the green-coloured timber common before the early 2000s) may contain arsenic and chromium. Do not burn it. Dispose of it at an approved facility and check with your local council or WorkSafe NZ for safe handling guidance.
Step 4: Demolish the Old Wall
- Relieve pressure first — if possible, excavate some soil from behind the wall before removing it to reduce the risk of collapse.
- Remove any capping, fencing, or fixtures attached to the wall.
- Pull out or cut off old posts at ground level, then dig out the footings.
- Work in sections if the wall is long, so you're never exposing a large unsupported face of soil at once.
- Pile removed soil safely away from the edge — loose soil can slump quickly.
Step 5: Prepare the Foundation and Drainage
Good drainage is critical. Most retaining wall failures in NZ are caused by hydrostatic pressure — water building up behind the wall with nowhere to go.
- Excavate to a firm, level base for your new posts or footing.
- Lay a drainage layer of clean aggregate (AP20 chip is commonly used) behind where the wall will sit.
- Install agricultural drain pipe (ag pipe) at the base of the wall, sloped to daylight or a stormwater outlet.
- Use geotextile fabric to separate the aggregate from the surrounding soil and prevent silting.
For guidance on drainage requirements, BRANZ is an excellent NZ resource.
Step 6: Install the New Wall
For a timber wall:
- Set your posts — bore holes to the required depth (typically 1/3 to 1/2 the retained height below ground, but check with your designer or council). Set posts in concrete.
- Allow concrete to cure — at least 24–48 hours before loading.
- Fix horizontal timber rails (pales or planks) to the posts, working from the bottom up.
- Backfill in layers, compacting as you go and keeping the drainage aggregate layer intact.
- Install a top cap to shed water and protect end grain.
Ensure all fixings are hot-dipped galvanised or stainless steel — standard zinc-plated fixings will corrode rapidly in ground contact.
Step 7: Final Checks and Reinstatement
- Check the wall is plumb and level along its length.
- Confirm drainage outlets are clear and functioning.
- Reinstate any topsoil, lawn, or garden above the wall.
- If a consent was required, arrange a council inspection before backfilling (check your consent conditions).
When to Call a Professional
Hire a qualified retaining wall builder or structural engineer if:
- The wall is over 1.0–1.5 m in height or requires a building consent
- There is a structure, driveway, or significant slope above the wall
- The site has poor or unstable soils
- You're unsure about drainage design or structural requirements
- The wall is part of a boundary (neighbour disputes can arise)
A licensed building practitioner (LBP) may be required to carry out or supervise restricted building work — check at Licensed Building Practitioners. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
