How to Use Postcrete for Fence Posts: A Fencer's Guide

If you have ever set a fence post on a wet Tuesday in February, you will know why postcrete is the trade's go-to. It is fast, clean and forgiving, and done right it holds a post solid for decades. Done wrong, you will be back in six months re-setting a leaning line. Here is how we do it on site, with the numbers we actually use.

How much postcrete per post

Postcrete comes in 20kg bags, and each bag sets roughly one fence post in a standard 600mm deep hole for a 75x75mm or 100x100mm timber post. The rule we work to is one bag per post for posts up to 1.8m out of the ground, on decent ground. For taller posts or gate posts that take load, we use two. The bag is not magic; it is just pre-mixed rapid concrete with a set accelerator. You still need the right hole depth and diameter, or no amount of postcrete will save you.

How long postcrete takes to set

Postcrete is quick. In normal weather, above about 7 degrees, it goes off in roughly 5 to 10 minutes once water hits it, and it is firm enough to support a post within 20 to 40 minutes. In cold weather, say below 5 degrees, allow it twice as long and do not be tempted to pour water onto frozen ground. Full structural strength takes around 24 hours, but for a fence post that is not carrying a hanging load you can usually fit the panels the same day if you are careful. In summer, keep an eye on the mix going off in the bucket before you pour.

How many bags per post by post size

Here is the breakdown we use, assuming a hole about three times the post width and 600mm deep for posts under 2m, 750mm for taller:

• 75x75mm timber post, 1.8m fence: 1 bag.
• 100x100mm timber post, 1.8m fence: 1 to 2 bags depending on hole size.
• 100x100mm gate post or 2.4m post: 2 bags minimum, 3 if it is a double gate.
• 100x100mm corner or straining post taking wire: 2 to 3 bags.
• Concrete slotted post, 1.8m: 1 bag is plenty, the post itself is heavy.

For metal palisade posts we dig deeper, 750 to 900mm, and use two to three bags because the post is slim but tall and acts like a lever in the wind.

Step by step: mixing and pouring

1. Dig the hole to three times the post width and 600mm deep minimum. Clean the bottom so you are not setting into mud.
2. Drop the post in, pack a little loose dry mix or pea gravel around the base so water can drain, and prop it plumb in two directions.
3. Pour the postcrete in dry around the post, filling to about two thirds of the hole. Do not mix it in a bucket first; it sets in the ground.
4. Add clean water steadily, about 2.5 to 3 litres per 20kg bag, pouring it on top of the dry mix so it soaks down. Stop when you see water pooling on the surface.
5. Jab a stick or the edge of a spade down the sides a few times to release trapped air and let the wet mix fill the gaps.
6. Tidy the top with a trowel, sloping it slightly away from the post so water runs off rather than sitting in a puddle round the timber.
7. Hold the post plumb for five minutes, then leave it. Do not lean the panels against it for at least an hour.

Common mistakes

The ones we see most are setting posts into holes that are too shallow, using a single bag on a 2.4m post on a windy site, and adding too much water so the mix slumps and never properly cures. Another is pouring postcrete into a hole full of standing water; it dilutes the mix and you get a weak set. Always bail the hole out first. And do not backfill with topsoil round a gate post; use the postcrete or a proper dry mix, or the post will lean the first time the gate swings.

When to use a concrete mix instead

Postcrete is brilliant for straightforward fence runs, but reach for a proper ballast and cement mix, or a hand-mixed concrete to a 1:2:4 ratio, when a post is taking real load: a heavy double driveway gate, a tall palisade run on an exposed site, or a corner straining post on a long closeboard line. Concrete mix takes longer to go off, a few hours to handle and days to fully cure, but it sets far stronger and resists the leverage a gate puts on a post. Our rule: if it swings or it carries a latch, we dig deeper and use a real concrete mix rather than a rapid bag.