Treating the bottom of fence posts before they go in the ground is one of the most important steps you can take to extend the life of your fence. Untreated or poorly treated timber in contact with soil and moisture will rot far sooner than treated timber — often within just a few years. The good news is that with the right products and a bit of preparation, this is a straightforward DIY job.
Why Post Bottoms Are So Vulnerable
The section of a fence post that sits in the ground — typically 600 mm to 900 mm deep depending on post height and soil conditions — is exposed to constant moisture, soil bacteria, and fungi. This in-ground zone is where timber decay starts fastest. Even timber that arrives from the merchant with a factory treatment can have its cut ends exposed during installation, leaving raw, untreated wood vulnerable.
In New Zealand's varied climate — from the wet West Coast to humid Northland and freeze-thaw conditions in Central Otago — getting this right matters even more. BRANZ research consistently highlights moisture management as the key factor in timber durability.
Choose the Right Timber to Begin With
Before you even think about additional treatment, start with the correct hazard class timber. For fence posts in ground contact, you need timber rated to H4 or H5 under the NZ Timber Treatment Standard:
- H4 — suitable for in-ground contact in non-critical applications (most residential fencing)
- H5 — for in-ground contact in more severe conditions or where longer service life is required
Look for the treatment brand stamp on the timber. Common treated pine from merchants like PlaceMakers or Mitre 10 will typically be CCA (copper chrome arsenate) or LOSP treated. Always check the hazard class label before purchasing.
Treating Cut Ends — The Critical Step
When you cut a treated post to length, the cut end exposes raw, untreated timber. This is the most common cause of premature rot. You must treat all cut ends before installation.
What to Use
- End-grain preservative — purpose-made products such as Tanalith End Grain Preservative or similar copper-based end-grain treatments are available at most timber merchants and hardware stores. Apply generously with a brush, allow to soak in, and apply a second coat.
- Copper naphthenate-based preservatives — widely available and effective for end-grain treatment. Check the product label confirms suitability for in-ground use.
- Bituminous paint / black bitumen sealant — an older but still widely used method. Products like Sika's bitumen-based coatings (available from Sika NZ) can be painted onto the bottom 150–200 mm of the post. This creates a moisture barrier rather than a biocide treatment, so it works best in addition to a preservative, not instead of one.
Application Method
- Cut your post to the required length with a clean, square cut.
- Stand the post end-down in a shallow tray or bucket and brush on two generous coats of end-grain preservative, allowing the first coat to soak in fully before applying the second.
- Once the preservative has dried, apply a coat of bituminous sealant to the bottom 150–200 mm of the post for added moisture protection.
- Allow everything to dry fully before installation.
Soaking for Extra Protection
For even better results — particularly in wet or high-decay-risk areas — consider soaking the post bottom in preservative. Stand posts end-down in a drum or bucket of copper-based preservative for 24–48 hours. This allows deeper penetration into the end grain than brush application alone.
Post Installation Tips That Help
Treatment alone isn't the whole story. How you install the post also affects longevity:
- Concrete collars should be shaped to shed water away from the post — a flat or concave top traps moisture right where you don't want it.
- Gravel backfill around the base of the post (before concreting) improves drainage.
- Avoid burying the post deeper than necessary — the less timber in the ground, the less exposure to decay.
- Keep the above-ground portion painted or oiled to reduce moisture uptake from above.
A Quick Comparison of Treatment Options
| Method | Protects Against | DIY Friendly | Best For |
|---|---|---|---|
| End-grain preservative (copper-based) | Fungi, bacteria | Yes | All cut ends |
| Bituminous sealant | Moisture ingress | Yes | Extra moisture barrier |
| Soaking in preservative | Fungi, bacteria | Yes (with care) | High-risk/wet sites |
| Factory H4/H5 treatment | Fungi, bacteria, insects | N/A — pre-done | Full post body |
Safety When Handling Timber Preservatives
Many timber preservatives contain copper compounds and other chemicals. Always:
- Wear gloves, safety glasses, and old clothing when applying preservatives
- Work in a well-ventilated area
- Follow the product's safety data sheet (SDS)
- Dispose of waste product and containers according to your local council's guidelines
Refer to WorkSafe NZ for guidance on safe handling of chemical products.
When to Call a Professional
Fencing is generally DIY-friendly and does not typically require a building consent for standard residential boundary fences under 2 metres in height (though always confirm with your local council, as rules vary). However, consider hiring a qualified fencer if:
- Your site has difficult ground conditions (rock, high water table, steep slopes)
- You're installing retaining fence posts where structural loads are involved
- You're unsure about the correct post sizing, spacing, or depth for your fence type
A professional fencer will have the experience and equipment to get posts in straight, at the right depth, and properly treated. You can find qualified local fencers on tradedirectory.co.nz.
