Article summary
Why fence posts fail first and the drainage and installation techniques that stop rot before it starts.
- Category:
- maintenance
- Read time:
- 3 min
- Published:
- 8 December 2025
- Written by:
- My Homes Fencing Expert
Quick answer: fence posts rot first because they sit half in wet ground and half in air, so prevention comes down to the right timber and good drainage — use in-ground-rated H4 posts (H5 for the wettest or coastal sites), stop water pooling around the base, and shape any concrete collar so it sheds water away from the timber rather than holding it against it. Get the post right and the rest of the fence easily outlives it.
In Auckland's damp clay soils, the post is almost always the first thing to go. Here's why, and how to make sure yours last.
Why Posts Fail Before Anything Else
Palings and rails sit up in the air where they dry out between showers, but the post spans the worst possible zone — the point where it passes from wet soil into the air. That ground line stays alternately soaked and damp, which is exactly the condition timber-rotting fungi love, so posts break down there long before the boards above them do.
This is why a fence is only ever as good as its posts. You can replace a cracked paling in minutes, but a rotten post means digging out concrete and re-setting — so it pays to get the post specification and installation right the first time.
Timber Grade and Setting Posts Right
In-ground fence posts should be H4-treated as a minimum — that's the hazard grade specifically rated for timber in ground contact. For very wet, poorly drained or coastal sites, step up to H5. Ordinary H3.2 timber (fine for rails and palings above ground) is not rated for burial and will rot early if used as a post. Never bury a freshly cut, untreated end either — cuts expose untreated inner timber.
When setting the post, concrete gives the strength to resist leaning, but it must be finished so water runs off: crown the top of the concrete collar so it slopes away from the post rather than forming a dish that holds a puddle against the timber. A layer of gravel at the bottom of the hole helps water drain away from the base instead of sitting there.
Drainage and Ongoing Habits
After installation, the enemy is standing water. Keep soil, mulch and garden beds below the treated line, don't let a downpipe discharge next to a post, and clear leaves and debris that trap moisture at the base. On a boundary that's naturally boggy, a little extra drainage at install time — gravel, or directing runoff away — prevents rot for decades.
These habits cost almost nothing but make the difference between posts that fail in five years and posts that last twenty-plus. If you already have one or two posts going soft, it's usually cheaper to replace them now than to wait until they take neighbouring bays down with them.
Frequently Asked Questions
H4 is the minimum for posts in ground contact, and H5 is better for very wet, poorly drained or coastal sites. H3.2 is only rated for above-ground use like rails and palings, so it rots early if used as a buried post.
Concrete is best for strength and resisting lean, but the top of the collar should be crowned to shed water away from the post. Adding gravel at the bottom of the hole helps drainage so water doesn't sit against the timber base.
With H4 or H5 timber, good drainage and water kept away from the base, a fence post can last 15–25 years or more — often outlasting the palings and rails it supports.
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