The size of timber posts you need for a DIY retaining wall depends on several factors: the height of the wall, the spacing between posts, the type of soil being retained, and whether there are surcharge loads (like a driveway or building) above the wall. As a general guide, a low garden retaining wall up to 400 mm high might use 100 × 100 mm posts, while walls approaching the 1.5 m consent-exempt threshold typically require 150 × 150 mm or larger posts — but always verify with your local council before you build.
Understanding the Key Variables
No single post size fits every situation. The four main factors that determine what you need are:
- Wall height – measured from the bottom of the excavation to the top of the retained soil
- Post spacing – typically 1.2 m to 2.4 m centres for timber sleeper walls
- Soil type and loading – heavy clay or saturated soils exert more lateral pressure than free-draining sandy soils
- Surcharge loads – any structure, vehicle, or slope above the wall adds extra pressure
These variables interact. A taller wall, wider post spacing, or heavy surcharge all increase the bending moment on each post, requiring a larger or stronger section.
Common Timber Post Sizes as a Guide
The table below gives a broad starting point for H5-treated timber posts in typical residential conditions with posts at approximately 1.8 m centres. These are indicative only — always confirm sizing with a structural engineer or your council for anything beyond a simple garden wall.
| Retained Height | Indicative Post Size |
|---|---|
| Up to 400 mm | 100 × 100 mm |
| 400 mm – 800 mm | 125 × 125 mm |
| 800 mm – 1200 mm | 150 × 150 mm |
| 1200 mm – 1500 mm | 150 × 150 mm or 200 × 200 mm |
| Over 1500 mm | Engineer's design required |
Closer post spacing (e.g. 1.2 m centres) can allow a smaller post section; wider spacing demands a larger one.
Timber Treatment: H5 Is Essential
For any post going into the ground in New Zealand, you must use H5 hazard-level treated timber. H5 treatment is rated for ground contact and provides protection against rot, fungi, and insect attack in our damp NZ conditions. Using H4 or lower in-ground will lead to premature failure and a potentially dangerous wall.
Look for the treatment stamp on the timber and ask your supplier — Bunnings, Mitre 10, and PlaceMakers all stock H5 posts in a range of sizes.
Embedment Depth Matters as Much as Post Size
A correctly sized post that isn't buried deep enough will still fail. As a rule of thumb, posts should be embedded to at least one-third of the total post length (embedded depth plus above-ground height). For a wall retaining 1.0 m of soil, you'd typically need at least 500–600 mm of embedment in firm ground — more in soft or wet soils. The embedded section acts as a lever to resist the lateral soil pressure.
Building Consent and the 1.5 m Rule
In New Zealand, retaining walls are subject to the NZ Building Code and may require a building consent. Under Schedule 1 of the Building Act, retaining walls that retain up to 1.5 m of ground and have no surcharge from a building or vehicle may be exempt from consent — but this is not automatic in every situation.
If your wall:
- Retains more than 1.5 m of soil, or
- Has a building, driveway, or other structure within the zone of influence above it, or
- Is on a steep or unstable site
…then a building consent is very likely required, and an engineer's design will be needed. Always check with your local council before starting work — rules can vary by district and site conditions.
BRANZ is a useful resource for understanding best-practice construction details for retaining structures.
Drainage: Don't Overlook It
Poor drainage is one of the most common causes of retaining wall failure. Hydrostatic pressure from waterlogged soil can multiply the load on your posts dramatically. Always install:
- Coarse aggregate backfill (AP20 chip or similar) directly behind the wall
- Ag pipe (agricultural drain) at the base of the wall, sloped to daylight
- Weep holes or gaps between sleepers to allow water to escape
When to Call a Professional
While a low garden wall is a manageable DIY project, there are situations where you should engage a qualified retaining wall builder or structural engineer:
- Wall height exceeds 1.0–1.5 m, especially with any surcharge loading
- The site has unstable, expansive, or waterlogged soils
- A building consent is required (an LBP may need to carry out or supervise the work — see Licensed Building Practitioners)
- The wall is near a boundary, public footpath, or existing structure
- You're unsure about soil conditions or drainage
A failed retaining wall can cause serious property damage or injury, so it's worth getting expert advice early. You can find qualified local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
