Corner posts are the backbone of any timber fence — get them wrong and the whole run will lean, twist, or fail prematurely. The good news is that with the right preparation, materials, and technique, setting a solid corner post is well within reach of a capable DIYer.
Why Corner Posts Need Special Attention
Corner posts carry tension from two directions at once. Unlike line posts, which are pulled from one side, a corner post is under constant lateral stress from both fence runs meeting at that point. This means they need to be deeper, larger in diameter, and better anchored than your standard line posts.
Choosing the Right Post
For most residential timber fences in New Zealand, a 100 mm × 100 mm (4×4) H5-treated timber post is the minimum recommended size for corners. H5 treatment is essential for any timber in ground contact — it resists rot and insect attack in our damp NZ conditions. Look for the NZS 3640 treatment mark on the timber, which confirms it meets the New Zealand standard for preservative treatment.
Post length depends on your fence height plus the burial depth. A general rule of thumb is to bury one-third of the total post length in the ground. For a 1.8 m fence, you'd typically use a 2.7 m post, burying around 900 mm.
Digging the Hole
Use a post-hole borer (manual or petrol-powered) rather than a spade — you want a clean, round hole with minimal soil disturbance at the sides. The hole diameter should be roughly three times the post width, so around 300 mm for a 100 mm post.
Minimum depth guide (as a general rule):
| Fence Height | Minimum Hole Depth |
|---|---|
| Up to 1.2 m | 600 mm |
| 1.2 m – 1.8 m | 750–900 mm |
| 1.8 m – 2.1 m | 900 mm+ |
Always check your local council requirements — some areas have specific rules around fence heights and setbacks, particularly on boundaries. Your local council can confirm whether a building consent is required; many standard residential boundary fences fall under Schedule 1 exemptions, but it pays to check with MBIE Building Performance or your council directly.
Preparing the Base
Before dropping the post in, add 100–150 mm of compacted gravel or crushed rock to the base of the hole. This improves drainage and prevents the post end from sitting in standing water — a common cause of premature rot even in treated timber.
Setting and Bracing the Post
- Lower the post into the hole and rest it on the gravel base.
- Check for plumb using a spirit level on two adjacent faces. A post that's even slightly out of plumb will look noticeably wrong once the fence is built.
- Brace the post in position using two temporary timber braces nailed to stakes in the ground at 90 degrees to each other. This holds it perfectly plumb while the concrete cures.
- Double-check alignment — use a string line to confirm the post sits exactly where the two fence runs will meet.
Mixing and Pouring Concrete
For corner posts, concrete is strongly recommended over compacted soil or gravel alone. Use a 20 MPa mix or a bagged rapid-set post mix (available at Bunnings or Mitre 10). Rapid-set mixes are convenient for DIY — you pour dry mix into the hole, add water, and they set firm within 20–40 minutes.
Tips for pouring:
- Fill the hole in layers, tamping with a rod or stick to remove air pockets.
- Mound the concrete slightly above ground level and slope it away from the post — this sheds water and reduces rot risk at the critical ground-line zone.
- Keep the braces in place until the concrete has fully cured (at least 24–48 hours for standard mix; check the bag for rapid-set products).
Allowing for Fence Run Alignment
Once your corner post is set and cured, run a string line from the corner post to each end post along both fence runs. This gives you the reference line for setting all your line posts accurately. Take your time here — a straight string line makes the rest of the job much easier.
Dealing with Rocky or Sloping Ground
New Zealand's varied terrain can throw up challenges. If you hit rock before reaching the required depth, you may need to use a star picket or rock anchor in combination with a timber post, or hire a contractor with a rock-drilling attachment. On sloping sections, stepped fencing (where each panel drops in steps) or raked fencing (panels follow the slope) both require careful corner post planning — it's worth sketching this out before you dig.
When to Call a Professional
While setting fence posts is generally DIY-friendly, there are situations where it's worth bringing in a qualified fencer:
- Retaining walls combined with fencing — if your fence also acts as a retaining structure of certain heights, this may require a building consent and engineering input.
- Rocky or unstable ground — specialist equipment and experience are needed.
- Boundary disputes — always confirm the legal boundary with a licensed surveyor before digging.
- Large or complex projects — a professional fencer will have the tools and experience to get it done efficiently and correctly.
You can find qualified local fencers on tradedirectory.co.nz to get quotes and advice for your specific project.
