Why wet ground is usually the culprit

Most loose fence posts in Britain are not really the victim of wind. They are the victim of water. A post set into ground that stays saturated through winter will lose its grip long before it rots through, because water does two things at once: it softens the soil around the post so it can move, and it weakens the timber itself over the following seasons. On heavy clay, water sits in the hole like a bucket, so the post effectively floats. On sandy or gravelly soils, drainage is much better but the soil particles themselves are round and smooth, so they slide past the post and let it rock.

Add frost heave into the mix and the damage accelerates. Each freeze lifts the post a few millimetres, then the thaw drops it back somewhere slightly different. After a couple of winters you have a fence that leans, rattles in the wind and lets the panels sag towards the neighbour's side. The good news is that almost all of it is fixable, and if you correct the drainage at the same time as you re-set the post, the repair will outlast the fence.

First, work out what you are dealing with

Before you buy anything, find out whether the post is loose, rotten, or both. Grip the post at knee height and rock it firmly. Then probe the timber with a slim screwdriver about 150mm above and below ground level, where rot always starts.

• Firm timber, moving post: the hole has simply lost its grip. You can re-pack and re-secure it.
• Soft, spongy timber or a screwdriver that sinks in easily: the post is rotten at the base. Replacing it, or fitting a post repair spur, is the only sensible answer.
• Ground visibly waterlogged after rain: fix the drainage first, or the new fix will fail in the same way.
• Posts rocking in a long run of fence: check whether the rails and gravel boards are holding water against the base.

Concrete, post spikes or gravel boards?

Each method has a place, and the right one depends on your soil and how much time you have.

• Concrete: the most durable option in soft or wet ground, and the one that suits heavy clay. A properly shaped concrete collar holds a post far more firmly than soil alone. Use a post mix or a 4:1 ballast and cement blend, and remember it sets hard rather than gripping softly, so the post must be braced plumb until it cures.
• Post spikes or metal sockets: quick, tidy and rot-free, because the timber never touches soil. In firm, well-drained ground they are excellent. In wet clay or waterlogged loam, however, the spike itself will work loose, and a spike driven into saturated ground simply cannot resist lateral load.
• Gravel boards: not a fixing method on their own, but a genuinely effective way to keep the bottom of the fence out of the wet zone. By raising the panels and the bottom rail above ground level, they stop splash-back and soil contact doing their worst.

Getting the drainage right

This is the step most people skip, and it is why repairs fail twice. When you dig out around a post, go deeper than the original hole and widen the bottom, so you finish with a bell shape rather than a cylinder. That bell of concrete or compacted hardcore resists lifting far better than a straight-sided plug.

Line the base of the hole with around 75mm of gravel or 10mm shingle. This gives water somewhere to go rather than letting it pool against the timber. If your ground is genuinely wet, consider a shallow French drain running past the fence line, filled with shingle and wrapped in a permeable membrane, and set the base of the post on a couple of bricks or a paving slab so it never sits in standing water. Finish the concrete with a slight dome above ground level so rain runs off and away from the post, not into it.

Re-setting a post step by step

• Dig a hole roughly three times the width of the post and at least 600mm deep, or a third of the post's above-ground height.
• Remove any rotten timber and treat the cut or damaged end with a suitable wood preservative.
• Place 75mm of shingle, then set the post in and brace it plumb in two directions.
• Pour in the mix, working it down with a length of batten to remove air pockets, and slope the top away from the post.
• Leave it undisturbed for at least 24 hours, and avoid hanging panels or gates on it for several days.
• Apply a bead of silicone or a proprietary cap where water could collect on the top of the post.

Keeping timber sound through the seasons

Once the fence is stable, a little routine care goes a long way. Check the run after gales and again in early spring, and re-firm any post that has developed a wobble before it works the concrete loose. Keep soil, leaf mould and gravel board debris away from the base of posts, since these hold moisture against the timber. An annual coat of a water-repellent timber treatment on exposed faces, particularly cut ends and the bottom 300mm, will slow the rot that eventually claims most fences. If a gate hangs on a post, fit a diagonal brace or a gate wheel so the weight is not carried entirely by the ground. Do these small things and a fence that has lasted fifteen years will comfortably see out another fifteen.