Setting a post in concrete is one of the most fundamental carpentry tasks for any deck or fence project. Done correctly, it gives your structure a solid, long-lasting foundation that can handle New Zealand's varied weather — from the wet winters of the West Coast to the dry, baking summers of Hawke's Bay. There are two main approaches: direct burial (setting the post itself into concrete) and post anchors/brackets (setting a metal connector into the concrete and bolting the post to it). Both are widely used in NZ, and the right choice depends on your situation.
Do You Need a Building Consent?
Before you dig a single hole, check whether your project needs a building consent. Under Schedule 1 of the Building Act 2004, some low decks and fences are exempt — but the rules have specific limits around height, proximity to boundaries, and other factors. For example, a deck more than 1.5 m above ground generally requires consent. Fences over 2.5 m typically need consent too. Always confirm with your local council before starting, as getting this wrong can cause real headaches when you sell your home. MBIE Building Performance has clear guidance on what's exempt and what isn't.
What You'll Need
- Posts — treated timber rated H4 or H5 for ground contact (check the James Hardie or supplier spec sheets for composite options)
- Concrete — bagged rapid-set or standard mix (e.g., from Bunnings or Mitre 10), or ready-mix for larger jobs
- Post-hole borer or spade
- Spirit level and string line
- Temporary bracing timber and stakes
- Post anchors/brackets (if using the anchor method)
- Tape measure, pencil, safety glasses, gloves
Method 1: Direct Burial in Concrete
This is the traditional approach and is still common for fences and some deck posts.
Step 1 – Dig the Hole
As a general guide, bury the post to at least one-third of its above-ground height, with a minimum of 600 mm depth for most fence posts. Deck posts may need to go deeper depending on your engineer's or council's requirements. Make the hole diameter roughly three times the post width — so about 200–250 mm for a 90 mm post.
Step 2 – Add Drainage
Pour 100–150 mm of compacted gravel into the base of the hole. This helps water drain away from the post end and reduces rot, even with treated timber.
Step 3 – Set and Brace the Post
Place the post in the hole and use temporary braces nailed to stakes to hold it perfectly plumb. Check with a spirit level on two adjacent faces. A string line between posts helps keep everything aligned.
Step 4 – Pour the Concrete
Mix your concrete to a firm, workable consistency — not sloppy. Pour it in around the post, rodding it with a stick to remove air pockets. Mound the concrete slightly above ground level and slope it away from the post so rainwater sheds away rather than pooling at the base. This is critical for post longevity.
Step 5 – Cure and Check
Leave bracing in place for at least 24–48 hours (longer in cold weather). Rapid-set mixes can speed this up. Recheck plumb before the concrete fully sets.
Method 2: Post Anchors Set in Concrete
Post anchors (also called post bases or stirrups) are increasingly popular for decks because they keep the timber post above ground, dramatically reducing moisture exposure and rot risk. This method is also easier to replace posts later.
- Dig and pour a concrete pad or footing to the required depth and diameter.
- While the concrete is still wet, set the anchor bolt or stirrup in position, checking it's perfectly located and plumb using a template or string line.
- Allow the concrete to fully cure (typically 3–7 days for structural footings).
- Bolt the post into the anchor bracket using the manufacturer's specified fixings.
For deck structures, your engineer or the NZS 3604 timber-framed buildings standard will specify footing sizes and embedment depths. Always follow these — don't guess on structural footings.
Timber Treatment: Getting It Right
For any post going into or near the ground, you must use H4 or H5 treated timber in accordance with NZS 3640. H4 is suitable for ground contact in most situations; H5 is used where there's a higher decay or termite risk. Using the wrong treatment class can void warranties and cause premature failure. Check with your timber supplier or PlaceMakers if you're unsure.
Concrete Mix Guide
| Application | Suggested Mix Ratio (cement:sand:aggregate) | Notes |
|---|---|---|
| Fence posts | 1:2:3 or bagged general-purpose mix | Rapid-set bags are convenient |
| Deck post footings | Engineer-specified or 1:1.5:3 minimum | Check consent documents |
| Large structural footings | Ready-mix (specified strength, e.g. 17.5 MPa) | Get a quote from a local supplier |
Tips for a Long-Lasting Result
- Never skip the gravel base — drainage is the number one factor in post longevity.
- Seal the end grain of cut timber posts with an appropriate preservative end-grain sealer before installation.
- Don't backfill with soil alone — concrete gives far better lateral stability.
- In coastal or high-wind zones, your council or engineer may require larger footings or specific anchor designs. Check BRANZ guidance for wind zone requirements.
- Keep concrete off the post face where possible — trapped moisture between concrete and timber accelerates decay.
When to Call a Professional
If your deck requires a building consent, you'll likely need plans prepared and may need to use a Licensed Building Practitioner (LBP) for the restricted building work. Structural decks over 1.5 m high almost always fall into this category. An LBP carpenter can ensure your post footings meet the engineer's specifications and the NZ Building Code. For any work involving electrical connections (outdoor lighting, power points), a registered electrician must do that work — see the EWRB for what's restricted. You can find qualified local carpenters and concreters on tradedirectory.co.nz.