How do I fit concrete posts and slotted fence panels?

Setting concrete posts to a string line, getting the spacing exactly right for slotted panels, and why the spacing has no tolerance.

Difficulty: intermediate
Time: 5 hr
Read: 3 min
Safety: caution

Short answer

Set the two end posts first, run a string line, then space every post at 1880mm centres for a standard 1830mm panel. Panels slide down the slots from above, so every post must be plumb and the gaps identical — there is no adjustment once the concrete has gone off.

Concrete posts outlast timber several times over and hold panels in slots rather than on clips, which means the panel can be lifted out and replaced in seconds. The catch is that the spacing has to be right the first time. A timber post fence forgives 10mm; a slotted concrete run does not.

Safety

Concrete posts are heavy — a 2.4m post weighs around 60kg and is awkward to control while standing it in a hole. Lift with help. Check for buried services before digging, and dig the first spade depth by hand. Wear gloves and eye protection when cutting or drilling concrete.

What you'll need

Concrete posts, 100mm x 100mm, 2.4m for a 1.8m fence
Concrete gravel boards (optional)
Ballast and cement, or postcrete
A string line, spirit level and tape
A helper for lifting

Where a gravel board sits, and what it saves

Step by step

Set out the line and check the spacing works.Measure the full run and divide it. A 1830mm panel needs 1880mm post centres, giving a 1780mm clear gap between posts. If the run does not divide evenly, plan the odd panel at one end and cut it to fit.
Dig the end holes first.600mm deep for a 1.8m fence, 300mm square. Concrete posts need the same depth rule as timber — a third of the post in the ground.
Set the two end posts and string between them.Plumb on two faces, braced, concreted. Once these are set and hard, a string line at the top face of both gives you the line and height for everything in between.
Dig and set the intermediate posts to the line.Measure centre to centre each time from the previous post rather than cumulatively from the end, and check each new post against the string for line and height.
Drop the gravel board in before the panel.The board slides down the same slot. It must go in before the panel above it, so do not fit panels until the boards are in.
Slide the panels down the slots.Two people, one at each end, lowering the panel down both slots together. If it jams, it is not square — lift and try again rather than forcing it.
Pack any slack in the slot.A panel that rattles in the slot will wear. Timber wedges or a bead of exterior sealant at the top of the slot stops the movement.
Fit post caps if the posts are open-topped.Some concrete posts have a moulded weathered top; plain ones benefit from a cap that sheds water off the top face.

Tips

Set every post to a taut string line at the top rather than eyeballing it. A concrete fence shows every wobble.
Buy one spare panel. A slotted run cannot take a slightly different panel width later if the pattern is discontinued.
End posts and corner posts have slots on one or two faces only — check you have ordered the right ones before digging.

Common mistakes

Setting posts before checking the whole run divides evenly — You end up with a 400mm gap at one end and no way to fill it neatly. Plan the whole run on paper before the first hole.
Measuring cumulatively down the run — Small errors compound and the last few gaps end up wrong. Measure each gap from the post you just set.

If it doesn't work

Panel will not drop into the slot

Cause: Posts slightly out of parallel, or slot fouled — Fix: Clean the slots first. If the gap is genuinely tight, plane the panel batten. If the gap is wide, pack with treated timber.

Fence steps up and down along its length

Cause: Posts set to ground level rather than to a string line — Fix: On a level site, set all post tops to one line. On a slope, step deliberately at each post rather than drifting.

Post leaning after a season

Cause: Insufficient depth or concrete for the exposure — Fix: Concrete posts are heavier than timber and need the full collar. Dig out and reset with a 300mm square collar to full depth.

Questions people ask

Can I use timber panels with concrete posts?

Yes — standard timber panels are made to slide into concrete post slots. The panel is timber, only the posts and gravel boards are concrete.

What spacing do concrete posts need?

1880mm centre to centre for a standard 1830mm (6ft) panel, which leaves a 1780mm clear opening. Confirm against the actual panels you have bought before digging.