For most residential sleeper retaining walls in New Zealand, posts are spaced 1.2 m to 2.4 m apart, depending on the wall height, the size of the posts, the type of timber sleepers used, and the soil conditions on your site. As a general rule, the taller the wall and the heavier the retained soil, the closer together your posts need to be.
Why Post Spacing Matters
Posts are the structural backbone of a sleeper retaining wall. They transfer the lateral load from the retained soil into the ground via the buried section of the post. If posts are spaced too far apart, the sleepers spanning between them will flex, crack, or fail — and the wall can collapse. Getting the spacing right from the start is critical for safety and longevity.
General Spacing Guidelines
While every site is different, the following table gives a practical starting point for H4 or H5 treated timber posts in typical NZ residential conditions:
| Wall Height (retained soil) | Typical Post Spacing | Minimum Post Size |
|---|---|---|
| Up to 600 mm | Up to 2.4 m | 100 × 100 mm |
| 600 mm – 1.0 m | 1.8 m | 125 × 125 mm |
| 1.0 m – 1.5 m | 1.2 m – 1.5 m | 150 × 150 mm |
| Over 1.5 m | Engineer's design required | Engineer's specification |
Important: These figures are indicative only. Your actual design must account for soil type, surcharge loads (driveways, buildings, slopes above), and local ground conditions. Always confirm with a qualified professional.
Post Embedment Depth
The buried depth of the post is just as important as the spacing. A common rule of thumb is to bury the post to half the exposed wall height, plus 300 mm — so for a 1.0 m wall, you'd aim for at least 800 mm in the ground. In loose or sandy soils, you may need to go deeper or use a concrete footing. In rock or very firm ground, your engineer may allow a shallower embedment.
Timber Treatment Requirements
In New Zealand, all timber used in ground contact must meet the correct hazard class under NZS 3640. For posts set in the ground, you need H5 treated timber (e.g. CCA or equivalent). Sleepers that sit above ground but are exposed to the weather should be at least H4 treated. Using the wrong treatment class is a common DIY mistake that leads to premature rot and structural failure.
Check with your local timber merchant or visit PlaceMakers or Mitre 10 to confirm you're buying the correct treatment class for your application.
Do You Need a Building Consent?
This is one of the most important questions to answer before you start. Under the NZ Building Code and Schedule 1 of the Building Act 2004, retaining walls that retain more than 1.5 m of ground generally require a building consent. Walls under 1.5 m may be exempt — but only if they do not support a surcharge (such as a vehicle load, building, or another retaining wall) and meet other conditions.
The rules can be nuanced, so always check with your local council before starting work. You can also find guidance on exemptions at MBIE Building Performance.
Soil Type and Site Conditions
NZ soils vary enormously — from the firm volcanic clays of Auckland to the loose silts of Canterbury and the expansive clays of Wellington. Wet, loose, or expansive soils exert significantly more lateral pressure on a retaining wall than well-drained, firm soils. If your site has:
- High water table or poor drainage — you'll need closer post spacing and good drainage behind the wall (ag-pipe and gravel backfill are standard practice)
- Sloping ground above the wall — the surcharge load increases dramatically and an engineer's input is strongly recommended
- Seismic risk — New Zealand is an earthquake-prone country; walls over 1.0 m in higher seismic zones should be reviewed by a structural engineer
BRANZ publishes useful guidance on retaining wall construction and drainage best practice for NZ conditions.
Drainage Behind the Wall
No matter what your post spacing is, a sleeper retaining wall without proper drainage will fail prematurely. Always install:
- A layer of free-draining gravel (AP20 or similar) directly behind the sleepers
- A perforated ag-pipe at the base of the wall, sloped to an outlet
- Geotextile fabric between the gravel and the retained soil to prevent fines migration
When to Call a Professional
You should engage a Licensed Building Practitioner (LBP) or a structural engineer if:
- Your wall will retain more than 1.0 m of soil (and certainly anything over 1.5 m)
- There is a building, driveway, or other structure within the zone of influence above the wall
- Your ground conditions are poor, wet, or unstable
- You are unsure whether a building consent is required
For walls requiring consent, the design may need to be prepared or reviewed by a Chartered Professional Engineer (CPEng). You can find qualified retaining wall builders and engineers in your area on tradedirectory.co.nz.
For guidance on licensed building work and what LBPs can do, visit Licensed Building Practitioners.
Licensed Building Practitioner (LBP)
