How do I replace a rotten fence post without taking the fence down?

Fence posts rot at the ground line, not at the bottom, because that is where timber is alternately wet and full of air. The rest of the post is usually sound, which is why a fence can be perfectly good above and lethal at the base. Replacing a single post is a half-day job that saves the whole run.

Safety

What you'll need

• A replacement post, 100mm x 100mm for a 1.8m fence
• Postcrete or a bag of ballast and cement
• A post hole digger or narrow spade
• A crowbar or digging bar
• Two long timber props and a spirit level
• A pruning saw or reciprocating saw (optional)

Step by step

1. Confirm the post is actually the problem.Grip it and rock. If the panel moves and the post stays put, the panel fixings have failed. If the post rocks in the ground, the base has gone. Push a screwdriver into the post at ground level — if it sinks in, the timber is rotten.
2. Prop both panels before releasing anything.A timber prop under the outer end of each panel, screwed or wedged, so nothing drops when the post comes out. Panels are heavier than they look and their edges split easily.
3. Release the panels from the post.Unscrew the panel clips, or knock the arris rail brackets off on a close-board fence. Take photos first so you refix in the same order.
4. Cut the post off at ground level.A saw cut just above the concrete gives you a flat stump to lever against and gets the post out of the way of the panels.
5. Get the concrete plug out.Dig round it to expose the sides, then lever it with a digging bar using a brick as a fulcrum. A concrete collar around a 100mm post is usually 250-300mm across and lifts once it is free on three sides. If it will not move, break the top off and dig the rest out.
6. Clean and deepen the hole if needed.Aim for a third of the total post length in the ground — 600mm for a 1.8m fence, 750mm for a 2.4m post. Add 100mm of compacted gravel or scalpings at the bottom so water drains away from the end grain.
7. Stand the new post and brace it.Two braces at right angles, screwed to the post and pegged to the ground. Check plumb on two faces and check the top aligns with the neighbouring posts before you pour anything.
8. Fill with concrete and haunch it above ground.Postcrete is poured dry into water in the hole and sets in around ten minutes; a mixed 1:2:4 concrete is cheaper and slower. Either way, finish the top of the collar 25mm above soil level and slope it away from the post so rain runs off rather than sitting against the timber.
9. Leave it before loading it.Postcrete takes a couple of hours before you can refix panels, ordinary concrete a full day. Leave the braces on for 24 hours regardless.
10. Refix the panels and treat the cut ends.Any sawn face on a pressure-treated post is untreated timber. Brush end-grain preserver on the top of the post and on any cuts before fitting a post cap.

Tips

• Take the panel out on the neighbour's side of the fence if the clips are on that face — it saves fighting the panel through a gap.
• If the old concrete plug will not budge, set the new post 100mm to one side and let the old plug stay buried. The panel gap changes by a few millimetres and nobody notices.
• Buy a post 300mm longer than you think you need. Trimming the top is easy; a short post is scrap.

Common mistakes

• Filling the hole flush or dished at the top — A flat or hollow concrete top holds water against the post at exactly the point where posts rot. The collar must be proud of the ground and sloped away.
• Concreting the base of the post solid with no drainage — Water that gets in cannot get out and the end grain sits in it. A gravel layer under the post lets it drain and adds years.

If it doesn't work

Questions people ask