Why do fence posts always rot at ground level?

Because decay fungi need moisture and air together. Deep in the ground the timber is wet but short of air; above ground it dries. The band either side of the soil surface has both — and it is also where wind puts the most bending force on the post.

Does setting a post in concrete stop it rotting?

Not by itself. A collar finished flat or dished holds water in a cup around the timber and can make decay worse. The concrete needs to shed water away from the post, and the timber still has to be treated for ground contact.

Can I repair a rotted post without taking the panels out?

There are spur and splint methods that bolt a new section alongside the sound part of the post. They can work as a repair, but they rely on the remaining timber being genuinely sound above the decay — which is worth having checked rather than assumed on a post that has already failed once.

Should I replace timber posts with concrete ones?

It is a common choice because concrete posts do not decay at the ground line. They are heavier to install and less forgiving to alter later, and they need panels and gravel boards sized to fit their slots, so it is a decision to make for the whole run rather than one post.

Caution

Updated 30 August 2026

Fence Post Rotted At The Base

A timber post has gone soft and crumbly right at ground level, while the timber above and below still looks sound.

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Fencing and gate work, illustrative image

What's happening?

Timber posts almost always fail in a narrow band at the ground line, and the reason is oxygen. Wood-decay fungi need both moisture and air; well below ground the timber is wet but starved of air, and above ground it dries out, so the zone that has plenty of both is the few centimetres either side of the soil surface. That is precisely where the bending load from wind is greatest, which is why a post can look perfectly sound top and bottom and still snap. Preservative treatment only protects the depth it penetrated, so a post cut to length on site without its cut end being re-treated has an untreated face left in the ground, and a post whose treatment was never rated for ground contact was always going to fail there. Concrete does not stop this — a post set in a collar that holds water in a cup around the timber rots faster, not slower.

What usually causes it?

Wood-decay fungi thriving in the ground-line band where both moisture and air are available
Timber not treated to a standard suitable for ground contact
Cut ends and notches not re-treated, leaving untreated wood in the soil
A concrete collar finished level or dished so it holds water against the post rather than shedding it
Soil, mulch, gravel or debris built up around the post, extending the damp zone up the timber
A gravel board missing or rotted, so soil and vegetation sit directly against the post and panel
Long-term wetting from a downpipe, gully or a lawn that sits waterlogged in winter

Is it dangerous?

A rotted post is a fence waiting to fall. The failure mode is sudden — the post snaps at the ground line in a gust rather than tilting gradually — so a fence with soft posts should be treated as unstable even while it is still standing straight. Urgency rises where the fence borders a pavement, a road, a shared access or a neighbour's garden, where children play against it, or where it supports a gate, a light or any wiring.

Can I fix it myself?

The screwdriver test is the whole diagnosis. Push a screwdriver blade firmly into each post at ground level: sound treated timber resists, and rotted timber takes the blade with almost no pressure. Do every post in the run, because if one has gone at that height its neighbours are the same age and in the same ground. Clear soil, mulch and vegetation away from the post bases and cut back anything growing against them. Replacing a single post in a panel fence is achievable DIY if you can support the panels either side; replacing several, or resetting concrete, is heavier work than it looks.

When should you call a fencing and gate installer?

Call a fencing installer when a post is soft at ground level, when more than one post fails the probe, or when the post carries a gate. Do not wait for it to fall — the repair is straightforward and the failure is not. Also call where the fence has no gravel boards and soil sits against the timber, because replacing the post without addressing that repeats the same failure, and where a downpipe or wet ground is keeping the post bases permanently damp.

What will the fencing and gate installer actually do?

Probe every post in the run at ground level to establish which have decayed and which are sound
Support the panels either side, then remove the failed post and break out the old concrete
Fit a replacement post treated for ground contact, or a concrete or steel post where the run needs to last longer
Set it to a depth suited to the fence height, in a concrete collar shaped to shed water away from the timber, re-treating any cut ends and notches made on site
Fit or replace gravel boards so soil and vegetation no longer sit against the panel and post
Clear the post line of built-up soil, mulch and growth, and advise on drainage where the ground stays wet

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Gravel Board RottenLow risk Fence Leaning After High WindsCaution Concrete Fence Post Cracked Or SnappedCaution

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