Quick answer: Postcrete sets in 5–10 minutes and fully cures within 1 hour. For a standard 100mm fence post in a 200mm × 600mm hole, you'll need 1 bag of 20kg Postcrete. Pour water into the hole first, add the powder around the post, plumb it within 3 minutes, and don't disturb it for an hour.
Postcrete is the fastest way to set a fence post in the UK. It's a pre-mixed concrete designed for posts — no mixing in a wheelbarrow, no two-day waits. This guide covers exactly how long it takes to set, how many bags you need per post, and the step-by-step method we use ourselves.
How long does Postcrete take to set?
| Stage | Time | What you can do |
|---|---|---|
| Initial set | 5–10 minutes | Position and plumb the post — must be done before this point |
| Hard set | 30 minutes | Remove temporary bracing |
| Full cure | 1 hour | Hang panels, attach rails, full load |
| Cold weather (under 5°C) | Up to 2 hours | Wait longer before loading the post |
Compared to traditional concrete (which needs 24–48 hours to cure), Postcrete lets you finish a fence the same day you start it. That's the entire reason it exists.
How many bags of Postcrete per fence post?
This depends on the post size, the wind exposure, and the hole dimensions. Use this table as your starting point:
| Hole width | Hole depth | Duty | Bags of 20kg Postcrete |
|---|---|---|---|
| 150mm (6") | 600mm (24") | Very Light — trellis, picket | ½ bag |
| 200mm (8") | 600mm (24") | Light — sheltered panels | 1 bag |
| 250mm (10") | 600mm (24") | Medium — standard 6ft fence | 2 bags |
| 300mm (12") | 600mm (24") | Heavy — coastal, exposed sites | 3 bags |
For a standard 6ft (1.83m) fence panel on a 100mm × 100mm post in average UK conditions, plan for 2 bags per post. Order an extra 10% on top of your total — you'd rather have one spare bag than be one bag short halfway through the run.
How to use Postcrete: step-by-step
1. Dig the hole
Depth: 600mm (2ft) for a standard 1.8m fence post. Width: roughly 3 times the post width — so 200–300mm wide for a 100mm post. The post should sit in the centre with room for Postcrete on all sides.
2. Add water first
Fill the hole with water to about one third of its depth. This is the most-skipped step and the reason most failed sets happen. Dry powder on dry ground won't cure properly.
3. Position the post
Drop the post in. Rough-align it now — you'll have time to fine-tune in the next step.
4. Pour the Postcrete
Tip the powder evenly around the post until it covers the water and reaches roughly ground level. Don't mix it in — Postcrete self-mixes. If the hole looks dry on top, sprinkle a little extra water over the powder to start the reaction.
5. Plumb the post within 3 minutes
Use a spirit level on two adjacent faces. The post must be perfectly vertical before the 5-minute mark — once Postcrete starts to harden, you can't adjust it. A second person makes this a lot easier.
6. Brace and wait
Use scrap timber or fence post braces to hold the post upright. Walk away for 30–60 minutes. Don't lean on it, don't attach panels, don't rush.
7. Backfill
Once cured, top the hole with soil, gravel, or grass. Slope the soil away from the post to keep water from pooling at the base.
Postcrete FAQs
Can I use Postcrete in the rain?
Yes — light rain is fine and won't affect the cure. Heavy rain or standing water in the hole will dilute the mix and weaken the set. If it's pouring, cover the hole with a tarp for the first 30 minutes.
Can I use Postcrete in cold weather?
Postcrete works down to 0°C but cures slower below 5°C. In freezing conditions, allow up to 2 hours before loading the post and avoid using it if frost is forecast within 24 hours of the pour.
Can you put Postcrete on top of Postcrete?
Yes. If you under-filled a hole or need to top it up, you can pour fresh Postcrete on top of cured Postcrete with no bonding issues. Wet the surface lightly before adding the new layer.
Does Postcrete expand?
Postcrete expands very slightly during the cure — typically 1–2% — which helps it grip the post and the surrounding soil. It won't crack a normal post hole, but don't use it inside a brick or block structure where there's no room to expand.
Can I use Postcrete instead of concrete?
For fence posts, gates, signs, and washing line posts — yes, and it's faster. For load-bearing structures, foundations, slabs, or anything bigger than a single post, use traditional concrete. Postcrete is designed for column-shaped pours, not flat surfaces.
How long before I can hang fence panels?
One hour after the pour in normal conditions. Two hours in cold weather. Hanging panels too early will pull the post out of plumb before the Postcrete has fully cured.
How many bags of Postcrete per concrete post?
Concrete posts are heavier than timber and usually require a bigger hole — plan for 2–3 bags per post in standard conditions, 3 bags for exposed sites.
Is Postcrete the same as ready-mix concrete?
No. Standard ready-mix concrete is a wet mix designed for slabs, footings, and large pours, and takes 24–48 hours to cure. Postcrete is a dry, pre-blended powder formulated specifically for fence post installation and cures in under an hour.
Postcrete tips from the yard
- Always wear PPE. The dry powder kicks up dust — eye protection and a dust mask matter even on outdoor pours.
- Brace before you pour, not after. Once the cure starts you have minutes, not hours.
- Buy one extra bag per fence run. Splitting bags between posts always leaves you one short on the last hole.
- Don't lean on the post for 30 minutes. The cure is fast but it isn't instant — pressure during the first half hour is the most common cause of crooked posts.
- Wet the hole properly. If you skip the water step, the powder won't cure evenly and you'll get a weak post.
Buy Postcrete from Nottage Timber Merchants
We stock 20kg bags of Postcrete for collection or delivery across South Wales. Trade pricing available — call 01656 745959 or order online.
For larger fence projects, we also supply pressure-treated fence posts, fence panels, and decking timber — everything you need for a full fence build in one delivery.