Corner fence posts are the hardest-working posts in any fence line — they anchor the tension from two directions at once. Done correctly, a well-braced corner post will hold firm for decades; done poorly, it will lean, twist, and eventually pull the whole fence out of line. The good news is that with the right technique and materials, this is a very achievable DIY job.
Why Corner Posts Need Special Bracing
A standard line post only resists lateral pressure from one direction. A corner post (also called a strainer post) must resist the combined tension of fence wires or rails pulling from two different angles. Without adequate bracing, the post will lean inward toward the corner over time — a problem that gets worse the tighter the fence wire is strained.
The solution is a diagonal strainer brace (sometimes called a prop brace or angle brace) on each run of fence leaving the corner. This transfers the pulling force down into the ground rather than letting it topple the post.
Choosing the Right Post Size
For a corner, always go one size up from your standard line posts:
- Light residential fencing (paling, picket): minimum 100 × 100 mm H4-treated timber post, or a 65 mm round steel post.
- Farm or stock fencing: minimum 125 × 125 mm H5-treated timber, or a heavy-wall steel post.
- Retaining or high-tension wire fencing: consult a fencing professional — post sizing depends on wire gauge, fence height, and soil type.
Always use H4 or H5 treated timber for in-ground posts in NZ conditions. Check with your timber supplier or PlaceMakers or Mitre 10 for the correct treatment class for your region and soil type.
Setting the Corner Post in Concrete
- Dig the hole to at least one-third of the post length, plus 100 mm for a gravel drainage base. For a 1.8 m fence, that means a hole roughly 700–800 mm deep.
- Add 100 mm of compacted gravel to the base for drainage — this reduces rot and frost heave.
- Set the post plumb in both directions using a spirit level, and brace it temporarily with timber offcuts while the concrete cures.
- Pour concrete (a standard 20 MPa mix works well) around the post, slightly mounding it above ground level and sloping it away from the post to shed water.
- Allow at least 24–48 hours before applying any tension to the fence.
Installing the Diagonal Strainer Brace
This is the key step that separates a solid corner from a wobbly one. You'll need a strainer brace running from the corner post back along each fence line.
What you need
- A strainer post (same size as your corner post) set 2–3 m back along each fence run
- A horizontal rail (the strainer rail or top rail) connecting the two posts at the top
- A diagonal timber brace or wire and tension rod running from the base of the corner post to the top of the strainer post
Step-by-step
- Set your strainer post 2–3 m back along each fence run, concreted in the same way as the corner post.
- Fix a horizontal rail between the top of the corner post and the top of the strainer post. Use 100 × 50 mm H4 timber or a steel rail, notched or bolted securely into each post.
- Install the diagonal brace: run a length of 4 mm high-tensile wire (or a timber diagonal) from the bottom of the strainer post up to the top of the corner post, crossing the horizontal rail. Twist the wire tight using a straining bolt or wire strainer — this creates a rigid triangle that locks the system.
- Repeat this brace assembly on each fence run leaving the corner.
| Component | Typical Size (timber fence) | Purpose |
|---|---|---|
| Corner post | 125 × 125 mm H4/H5 | Main anchor point |
| Strainer post | 100 × 100 mm H4/H5 | Transfers load to ground |
| Horizontal rail | 100 × 50 mm H4 | Connects posts, carries brace |
| Diagonal wire/brace | 4 mm HT wire or 75 × 50 mm timber | Resists tension pull |
Getting the Angle Right
The diagonal brace is most effective at roughly 45 degrees. If your strainer post is 2 m back and your fence is 1.8 m tall, you're close to that ideal angle. Going shorter than 1.5 m back significantly reduces the brace's effectiveness.
Soil Conditions Matter
In soft, sandy, or wet soils (common in parts of Auckland, Northland, and the West Coast), you may need to go deeper or use a wider concrete footing. In rocky ground, a rock anchor or expanding bolt anchor may be more practical than digging. If you're unsure, BRANZ has guidance on ground conditions and foundation performance.
When to Call a Professional
While bracing a corner post is generally DIY-friendly, there are situations where it pays to bring in a qualified fencer:
- Retaining fences over 1.5 m high may require a building consent — always check with your local council first, as requirements vary. MBIE Building Performance has information on Schedule 1 exemptions.
- Boundary fences close to neighbouring properties can involve legal obligations under the Fencing Act 1978.
- High-tension farm fencing with multiple wire strands requires specialist knowledge of wire gauges and straining equipment.
- If your soil is unstable, sloped, or waterlogged, a professional assessment is worthwhile.
You can find qualified, local fencing professionals on tradedirectory.co.nz to get the job done right.
Sources & Further Reading
- MBIE Building Performance – When you need a building consent
- BRANZ – Building research and technical guidance
- PlaceMakers – Timber and fencing supplies
- Mitre 10 – Fencing materials and hardware
- WorkSafe NZ – Safe digging and site safety
- Licensed Building Practitioners – Finding licensed tradespeople
