Start With the Hole, Not the Post

Most fence posts that end up leaning do not fail because the timber was poor. They fail because the hole was too shallow, too narrow, or filled with something that traps water against the wood. A post set into concrete properly will hold a straight line for fifteen years or more. A post dropped into a spade-wide hole with a bit of soil rammed around it will move within a season, usually after the first winter gales.

The rule of thumb is simple: roughly a third of the post should finish up underground. For a 1.8m post carrying standard panels, that means a hole about 600mm deep. If the fence is taller, sits on exposed ground, or the soil is heavy clay, go to 750mm. On a sloping site, always measure depth from the lower ground level, not the higher one, otherwise the post looks fine in summer and lifts in the wet.

Digging a Hole That Will Actually Hold

A hole should be about three times the width of the post. For a 100mm post, that is a 300mm hole, which gives the concrete enough mass to resist sideways pressure from wind and from the panels themselves. Use a post-hole spade or an auger for speed, then widen the bottom slightly with a narrow graft to create a bell shape. That bell of concrete is what stops the post being levered upward.

• Keep the sides of the hole rough rather than glassy smooth, as concrete keys better to a textured surface.
• Remove roots and loose rubble; they rot, settle and leave voids.
• On clay, dig a little deeper and wider than you think you need, and break up the base.
• Space posts so panel widths fit exactly, allowing a small gap for expansion.

Check the bottom of each hole with a tape measure. Depths drift after the tenth hole, and it is far easier to correct now than to argue with a leaning fence later.

Gravel First, Always

Put 100–150mm of clean gravel or coarse hardcore in the bottom of each hole and tamp it flat. This does two jobs: it gives water somewhere to go, and it lifts the end of the post clear of the wet. A post sitting on soil at the bottom of a hole will rot from the base up, no matter how good the concrete is.

Sit the post on the gravel and check it against your string line before adding anything else. If the ground is very wet, a small paving slab or a piece of engineering brick under the post makes a firm, free-draining footing. Do not, under any circumstances, fill the bottom of the hole with soil from the digging. It will settle, and the post will drop with it.

Mixing and Pouring the Concrete

You have two sensible options: a strong site mix, or a fast-setting post mix that you pour dry and then soak. A site mix of one part cement to four or five parts ballast, brought to a stiff, crumbly consistency, is ideal. Wet concrete is weaker concrete, and it pushes posts out of plumb as it slumps.

• Pour in stages of around 150mm and compact each layer with a length of timber to remove air pockets.
• Keep checking the post with a spirit level on two faces as you fill.
• Finish the concrete 50–75mm above ground level and slope it away from the post so water runs off.
• Never fill the hole to the top of the post, and leave the timber clear of the concrete line.

For gate posts, go deeper and use more concrete. A pair of hung gates pulls hard, so holes of 900mm or more with generous footings are money well spent.

Brace It and Leave It Alone

Concrete does not set square; the post has to be held square while it does. Cut timber battens at roughly 45 degrees and fix them to the post with a clamp or screw, driving the other end firmly into the ground. Two battens per post, at right angles to each other, will hold it steady. Run a string line along the whole run and check every post against that line, not just against the one you fitted last.

Allow roughly a day for a fast-setting mix and two to three days for a site mix before removing the braces and hanging panels. In cold weather, cover the concrete with plastic or hessian to stop frost damaging it, and keep pets and children off the new line. Once cured, leave the concrete exposed rather than burying it. A small gravel collar around the base looks tidy and keeps soil and mulch away from the timber.

Small Details That Make It Last

Before you drop a post in, cut the top to a slight angle or fit a cap so water runs off. Treat any cut end with a good wood preservative, and avoid nails or screws near ground level where moisture gathers. If your boundary sits on a slope, step the fence in sections rather than following the gradient, which keeps the panels under less strain and the posts easier to set.

Finally, check the line of your boundary and any local restrictions before you start digging. Knowing exactly where your land ends will save arguments later. Do the digging properly, take your time with the levels, and the fence will quietly do its job for years while you get on with the garden.