How deep a fence post should be, how wide the hole, and how to use Postcrete the way the bag says. The whole job in cutaway, every number sourced.
Chapters
- How to install a fence post with Postcrete
- Why fence posts lean
- Fence height rules UK
- What is underground before you dig
- Marking out the fence line
- Choosing a fence post
- Tools for setting a fence post
- How deep should a fence post be
- Gravel base
- Getting the post level
- How to use Postcrete
- When to hang the panels
- Next: post two
Most leaning fences have one thing in common: the posts were not set properly. Get the first post right, with the right depth, a solid base and Postcrete used the way the bag actually says, and the rest of the fence becomes much easier.
Before you dig: rules
Check your boundary and any local planning requirements first. In England you can put up a fence up to two metres without planning permission, but only one metre where it runs next to a road. Your title plan shows a general boundary, not a precise line, so talk to your neighbour and dig on your own side.
Before you dig: what is underground
Gas pipes, electricity cables and water supplies can all be buried where you are digging, and a 675 mm hole reaches the depth where they live. Ask your utility providers for their plans, but remember that not everything appears on a plan, especially the pipe to your own house, or cables running to sheds and garden lighting. You can hire a cable detector for the day. Either way, dig by hand with a spade, and stop and look as you go.
Mark out the line
Before the first hole, mark the line of the fence. A peg at each end, a string between them, and a mark where the first post goes. This first post sets the line for everything after it.
Choose your post
Use a pressure-treated post rated for ground contact (Use Class 4). Make sure it is long enough for your fence height with at least 600 mm below ground, roughly a quarter to a third of the post. If you need to shorten it, cut from the top and treat the exposed timber. Never bury a freshly cut end.
Dig the hole
Aim for a hole roughly three times the width of the post. The exact depth depends on the fence height, the soil and how exposed the site is, but for a six foot fence it is typically around 600 mm, plus a bit for gravel. Keep the sides straight and the bottom firm. You want a solid foundation, not a loose, funnel-shaped hole.
Gravel
Add a layer of gravel and compact it firmly. This gives the post a level base and keeps the end grain off wet soil, so water drains away instead of soaking into the timber and rotting it.
Position and level
Lower the post into the hole and check it is upright on two adjacent faces. Checking one face is not enough: a post can be dead upright from where you stand and leaning on the other side. A post level straps on and shows both faces at once. When you are happy, brace it with two timber off-cuts so it cannot move. Take your time here. A crooked first post makes everything else harder.
Postcrete: water first
Postcrete is a fast-setting mix designed to go straight into the hole without mixing. Fill the hole with water to about a third of its depth. Pour the powder evenly around the post until there is no standing water, then sprinkle a little water over the top. Recheck your level immediately, because it sets in five to ten minutes, so have everything ready before you start. Keep the top of the mix below ground level and slope it away from the post so water will not collect against the timber, then cover it with soil.
Before you load it
Fast-setting does not mean full strength. The manufacturer says you can hang panels ten minutes after it sets. Give it an hour at least, and if it is windy, leave it overnight.
What comes next
Next in the series: the second post, getting the spacing right for standard panels, and keeping everything in line with the string.
The job in 7 steps
- Check the rules and what is undergroundConfirm your boundary and the height limit (2 m in England, 1 m next to a road). Ask your utility providers for plans, remember the pipe to your own house is often not on them, and dig by hand with a spade.
- Mark the lineA peg at each end, a string between them, and a mark where the first post goes. The first post sets the line for everything after it.
- Dig the holeThree times the width of the post, about 600 mm deep for a 1.8 m fence, plus 75 mm for gravel. Straight sides, firm bottom.
- Add the gravelAbout 75 mm, compacted. The end grain sits on gravel, never on wet soil.
- Position, level and braceLower the post in, check it is upright on two adjacent faces, then brace it with two battens so it cannot move.
- Pour the PostcreteWater to a third of the hole depth first. Pour the powder evenly around the post until there is no standing water, sprinkle the top, recheck the level immediately. It sets in 5 to 10 minutes.
- Finish and waitKeep the top of the mix below ground level, slope it away from the post and cover it with soil when set. Give it an hour before hanging panels, overnight if windy.
The numbers in this video, and where they come from
Not covered on purpose: electrics, gas and structural work. Those need a qualified trade.
Questions people ask
How deep should a fence post be?
For a 1.8 m (6 ft) fence, about 600 mm in the ground, plus about 75 mm of gravel under the post, so a 675 mm hole. Add at least 600 mm to the height above ground when choosing the post length.
How wide should the hole be?
Three times the width of the post. For a 100 mm post that is a 300 mm hole, the same width all the way down. A hole that narrows towards the bottom leaves the post standing on a point.
Do you mix Postcrete?
No. Fill the hole with water to about a third of its depth, pour the dry powder in evenly around the post until no standing water is visible, then sprinkle a little water on top. It sets in 5 to 10 minutes.
How many bags of Postcrete per post?
Two 20 kg bags for a 100 mm post in a 300 x 600 mm hole, according to the Tarmac datasheet. Use the calculator on the materials page for your number of posts.
How long before you can hang the panels?
The manufacturer says ten minutes after it sets. Other brands say leave it overnight. Give it at least an hour, and if it is windy leave it until the next day.
Do I need planning permission for a fence?
In England, not for a fence up to 2 m, or 1 m where it is next to a road, unless your house is listed or in an area with an Article 4 direction. Scotland's rules differ. Check with your council if in doubt.