How to Fix a Fence Post That Is Rotting at the Base

A fence post rotting at the base is one of the most common fencing problems in New Zealand, especially in our damp, humid regions. The good news is that depending on how far the rot has progressed, you may be able to repair the post rather than replace it entirely — saving time and money.

Assess the Damage First

Before reaching for tools, work out how bad the rot really is. Push a screwdriver or sharp spike firmly into the post at and just above ground level. If the timber feels spongy, crumbles easily, or the screwdriver sinks in more than 10–15 mm with light pressure, the rot is significant.

• Surface or minor rot (less than 20–25% of the post cross-section affected): a repair splice or post repair spike may be sufficient.
• Moderate to severe rot (post is structurally compromised, leans, or crumbles through most of its cross-section): full post replacement is the safer, longer-lasting option.

Also check whether the post is load-bearing — for example, a gate post or a corner post on a retaining fence. These carry more stress and should generally be fully replaced if significantly rotted.

Method 1 – Post Repair Spike (Quickest Fix)

A post repair spike (sometimes called a post saver or repair stake) is a galvanised steel bracket that you drive into the ground alongside the rotted post. The good upper section of the post is bolted to the spike, effectively bypassing the rotten base.

1. Dig or break away the concrete footing around the base of the existing post.
2. Cut off the rotted section of the post cleanly with a handsaw, just above where sound timber begins.
3. Drive the repair spike into the ground next to the cut post using a sledgehammer and a driving block (to protect the spike top).
4. Bolt the existing post to the spike using the supplied hardware, checking for plumb with a spirit level.
5. Backfill and compact around the spike.

This method works well for standard paling or picket fences. Repair spikes are available at Bunnings and Mitre 10 in various sizes — choose one rated for your post dimensions.

Method 2 – Timber Splice (Traditional Repair)

A timber splice involves sistering a new length of treated timber alongside the rotted base and bolting the two together. This is a good option when you want to keep the original post height and the rot is confined to the bottom 300–500 mm.

1. Excavate around the post base to expose the rotted section.
2. Cut away the worst of the rotted timber, leaving sound wood above ground.
3. Cut a new length of H4 or H5 treated timber (see below) to sit from below ground level up past the rot zone by at least 300 mm into sound timber.
4. Bolt the splice to the existing post using two or more galvanised coach bolts.
5. Re-concrete the base, ensuring the splice piece is set at least 450–600 mm into the ground (deeper in loose or sandy soils).

Method 3 – Full Post Replacement (Best Long-Term Result)

When rot is severe, full replacement is the right call.

1. Remove fence rails and palings attached to the post.
2. Break out the old concrete footing using a cold chisel and hammer, or hire a rotary hammer drill with a chisel bit.
3. Dig or auger a new hole — typically 450–600 mm deep for a standard 1.2–1.8 m fence (a general rule of thumb is one-third of the post length in the ground, but check your specific situation).
4. Set the new post in concrete, checking plumb on two faces with a spirit level. Brace it while the concrete cures (at least 24–48 hours before reattaching rails).
5. Reattach rails and palings using hot-dipped galvanised or stainless steel fixings to prevent rust staining and corrosion.

Choosing the Right Treated Timber

This is critical in New Zealand. The Hazard Class of the timber must match its end use:

Always look for the Treated Timber stamp and confirm the hazard class before purchasing. PlaceMakers and Mitre 10 stock a good range of treated fence posts.

Protecting the New Post

• Apply an end-grain preservative (such as a copper naphthenate-based product) to any cut ends before installation — this is where moisture enters most readily.
• Slope the concrete collar slightly away from the post at ground level so water sheds away rather than pooling at the base.
• Consider a post cap on top to prevent water sitting on the end grain.

For paint or stain finishing on above-ground timber, Resene and Wattyl both offer exterior timber coatings suitable for fences.

Do You Need a Building Consent?

Most standard residential boundary fences under 2.5 m high are exempt from building consent under Schedule 1 of the Building Act — but always confirm with your local council, as exemptions can depend on location, fence type, and proximity to boundaries or retaining structures. Check MBIE Building Performance for general guidance on Schedule 1 exemptions.

When to Call a Professional

Consider hiring a qualified fencing tradesperson or handyman if:

• Multiple posts are rotted and the whole fence run needs attention.
• The fence is a retaining fence holding back a bank — this may involve engineering requirements.
• You're not comfortable working with concrete, post-hole augers, or heavy timber.
• The fence is on a boundary shared with a neighbour (there may be legal obligations around cost-sharing under the Fencing Act 1978).

You can find experienced local fencing tradespeople and handymen on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Building Act Schedule 1 exemptions
• BRANZ – Timber in construction
• Resene – Exterior timber coatings
• Bunnings – Fencing products and hardware
• WorkSafe NZ – Safe use of tools and equipment
• Standards NZ – NZS 3604 Timber-framed buildings