As a general rule of thumb, timber retaining wall posts should be buried to at least one-third of their total length in the ground. So a 1.8 m post would need a minimum 600 mm embedment. However, this is a starting point — the actual depth required depends on wall height, soil conditions, surcharge loads, and NZ Building Code requirements. Getting this right is critical, because an under-embedded post is one of the most common causes of retaining wall failure.
The One-Third Rule Explained
The one-third rule is a well-established rule of thumb used across the trade. If your finished wall height above ground is, say, 900 mm, you'd typically use a post long enough to allow 450 mm or more below ground — meaning a total post length of around 1.35 m minimum. In practice, most builders round up and use a deeper embedment for added safety, particularly in softer soils.
For walls retaining more than 600 mm of soil, or where there are additional loads (like a driveway, vehicle traffic, or a structure nearby), the one-third rule alone is not sufficient. You'll need engineering input.
NZ Building Code and Consent Requirements
In New Zealand, retaining walls are subject to the NZ Building Code, and whether you need a building consent depends on the height of the wall and the conditions around it.
Under Schedule 1 of the Building Act, retaining walls that retain less than 1.5 m of ground and have no surcharge (e.g. no vehicle loads, no structures above) are generally exempt from building consent. However, this exemption has conditions, and rules can vary — always confirm with your local council before starting work.
Walls retaining 1.5 m or more, or any wall with a surcharge load, require a building consent and typically need to be designed by a Chartered Professional Engineer (CPEng). You can find more guidance at MBIE Building Performance.
Soil Type Makes a Big Difference
The bearing capacity and stability of your soil directly affects how deep posts need to go. Here's a general guide:
| Soil Type | Embedment Consideration |
|---|---|
| Firm clay or compacted gravel | Standard one-third rule may be adequate |
| Loose or sandy soil | Increase embedment depth — seek advice |
| Wet or waterlogged ground | Engineering input strongly recommended |
| Fill or disturbed ground | Treat as weak soil; get professional advice |
If you're unsure what type of soil you have, a local retaining wall builder or geotechnical engineer can assess it. New Zealand's varied geology — from Auckland's volcanic soils to Wellington's greywacke and Canterbury's alluvial gravels — means there's no single answer that fits every site.
Post Sizing and Timber Treatment
For timber retaining walls, posts are typically H5 treated timber (e.g. 150×150 mm or larger), which is rated for ground contact in New Zealand conditions. Never use untreated timber or lower treatment grades (H3, H4) for in-ground use — they will rot prematurely.
Common post sizes and typical applications:
- 100×100 mm H5 – light garden walls, low retained heights
- 150×150 mm H5 – standard residential retaining walls
- 200×200 mm H5 or larger – taller or heavily loaded walls
Always check the manufacturer's specifications and, for consented walls, follow the engineer's design. Suppliers like Bunnings, Mitre 10, and PlaceMakers stock a range of H5 treated timber posts.
Spacing and Backfill
Post spacing is typically 1.2 m to 1.8 m centres for standard residential walls, but again this depends on wall height and loading. Once posts are set, the hole should be backfilled with concrete (for walls over about 600 mm retained height) or well-compacted gravel. Concrete backfill significantly improves lateral resistance.
Drainage is equally important — a retaining wall without adequate drainage will build up hydrostatic pressure behind it, dramatically increasing the load on your posts. Install ag pipe (agricultural drainage pipe) at the base of the wall and use free-draining backfill (e.g. AP20 chip) behind the wall face.
A Practical Example
For a wall retaining 900 mm of soil with no surcharge:
- Minimum post embedment: ~450 mm (one-third rule)
- Recommended post embedment: 600–700 mm in firm ground, more in soft ground
- Total post length: at least 1.5–1.6 m
- Backfill: concrete around post base
- Timber: H5 treated, 150×150 mm minimum
For anything taller or more complex, get an engineer involved.
When to Call a Professional
You should engage a qualified retaining wall builder or structural engineer when:
- The wall will retain 1.5 m or more of soil
- There is a surcharge load (vehicle, building, or slope above)
- The ground is soft, wet, or unstable
- The wall is near a boundary, slope, or structure
- You need a building consent (required by law in these cases)
For consented work, the design must typically be completed or reviewed by a Licensed Building Practitioner (LBP) or engineer. See Licensed Building Practitioners for more information. You can find experienced local retaining wall builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
