Postcrete: Setting Times, How Much To Use, Plus FAQs

What is Postcrete?

Postcrete is a type of ready-to-use cement made specifically for fence posts. It is pre-mixed and is made of cement, sand, and additives that help it secure wood, concrete, and metal posts to the ground. Postcrete sets extremely quickly and allows landscapers to efficiently fix posts to the ground.

It is a Blue Circle brand and this type of cement may also be referred to as Postfix, although Postcrete is the most popular in the UK.

What is Postcrete Used For?

Postcrete is designed specifically for fixing fencing, decking, and gate posts to the ground. It can also be used to secure rotary washing lines. Postcrete is preferred by landscapers as it is supplied pre-mixed and only needs to have water added before it begins to work.

It should not be used for foundations or shed bases as it is not strong enough and will be difficult to mix as it sets so quickly. Ready-mixed concrete would be a more suitable and convenient material for people that do not want to mix concrete themselves.

The manufacturer also warns against using Postcrete for general concreting, floor-levelling screeds, bricklaying mortar, footings, grout or render.

How Long Does Postcrete Take to Set?

Front of Postcrete bag showing average setting time

It takes 5-10 minutes for Postcrete to set. However, it will continue to harden and gain strength over the next 10-12 hours. You should wait 24 hours to ensure it is fully set before resuming your fencing project or hanging gates. These timings apply at temperatures above 3°C and should not be used in cold weather below this minimum.

This is much quicker than regular cement, which takes up to 48 hours to fully set.

How Much is Postcrete Per 20kg Bag?

Postcrete costs £6.75 per 20kg bag, which works out at £0.34 per kg. These costs are based on retail prices from builders merchants and include VAT. Customers can get a more competitive price if they sign up for a trade account or buy in bulk.

How Much Postcrete to Use Per Post?

Bag of Postcrete showing you will need one bag per post

Blue Circle recommends using one 20kg bag of Postcrete per 4×4 post. However, landscapers in the industry often suggest adding up to two bags per hole to make sure the cement is strong enough to survive stormy weather. The optimal amount of Postcrete will also vary depending on the size of your fence post and the depth of the hole.

Although adding more than one bag per post will increase the initial cost of your fencing job, it will be worth it in the long run when they stay put despite high winds. If you are securing a gate post, which is subject to much more wear and tear than a fence post, you should use 3-4 bags to make sure it is secure.

There is no need to add ballast aggregate to Postcrete, it is more than strong enough for the job.

How to Break Up and Remove Postcrete

The easiest and quickest way to break up and remove Postcrete is using a demolition hammer. These are available to hire and cost as little as £27 to rent. Removing old concrete can be time-consuming without the proper tools and hiring suitable equipment will save you hours and hours.

How Much Water for Postcrete?

Postcrete water instructions on the back of the bag

You should first fill your hole with water up to a third of the depth before adding Postcrete. Any more water than this and the concrete will be too wet, which will reduce its strength and take longer to dry. If your Postcrete is cracking or not setting fully, it is usually caused by excess water.

How Big Should a Fence Post Hole be For Postcrete?

Holes for posts that are going to be secured using Postcrete can be calculated by dividing the height of your fence post by three and multiplying the width of the post by 3. For instance, if the post is 2400 x 75mm, your hole should be 800mm deep and 225mm wide.

The table below shows how deep and wide your hole should be based on the size of commonly used fence posts in the UK.

Size of fence post being used (mm)How deep the hole for the post needs to be (mm)How wide the hole for the post needs to be (mm)
1800 x 100600300
1800 x 75600225
2400 x 100800300
2400 x 75800225
3000 x 1001000300
3000 x 751000225

How Long Does Postcrete Last Before Going Off?

Postcrete manufacturer instructions on use by date and storage

Postcrete lasts for 300 days from the date it was packaged before going off. You can find this best-before date on the back of the Postcrete bag. After this date, the Postcrete will still work as intended, however, it may cause skin irritation due to increased levels of chromium. 

The chromium levels in cement increase over time and over 300 days may be over the legal limit. This is the sole reason for the use-by date on Postcrete. If the Postcrete is still a powder and does not contain any lumps, it will still work as normal.

Take extra precautions to ensure the Postcrete does not come into contact with your skin if you are using an out of date product.

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