Pergola Posts Depth and Concrete Mixture

[john2k]

I'm building a pergola, I have 3meter 6x6 posts. Planning to dig it down to depth of 70cm, is that enough? Also, I know lots of people use Postcrete for the posts, but I was thinking to use old school mixing ballast with cement to get a good concrete mixture. Someone recommended 1 part cement : 5 parts Ballast for a good strong result and water-tight concrete.

Any recommendations? Is there such thing as faster curing cement to mix with the ballast?

 

[flameport]

70cm is plenty. Less would probably do.

to use old school mixing ballast with cement

If you want to create loads of extra work, mess and wasted materials by mixing concrete yourself then go ahead.
Fast curing cement and various additives to achieve the same are available.

Alternatively, postcrete is ready mixed, sets in 10 minutes and is one bag per post hole.

 

[r896neo]

One bag per posthole? One bag per post hole??

Anyway back to the topic. The ballast mix is fine but you may need to prop the posts while it goes off, especially as its a big 6x6".

As above 70cm is probably overkill for a pergola. Its not under the same pressure as a fence. 20" would be fine or ideally 2ft if the digging isn't too bad.

Don't be tempted to use extra rapid cement, it is far too rapid for this job. You can use a rapid hardener/plugger but it wont be quick enough to make a noticeable difference.

Mixing your own is more of a faff but cheaper and gives a much stronger more dense concrete. Although strength isn't actually that important in this usage. Lots of postcretes are really glorified mortar and have almost no stone in them.

 

[john2k]

Thanks for your replies. Mixing cement and ballast is also far cheaper than the postcrete. What diameter should the post hole be for 6x6 post?

 

[r896neo]

As a general rule I aim for at least 4" all around but for a 6" post you'll probably need to go to 6" and you should prop or restrain it someway as it goes off. The bigger posts have a large surface area for wind and a very large swing weight if they do get a nudge.

Keep the mix very stiff and then you can really ram it in which will help, then add water once set. I certainly don't mean mix it all bone dry you must mix it properly with water just keep it as stiff as you can.

 

[conny]

Postcrete is £5.18 for a 20kg bag in B&Q
Not sure how it compares with sand, cement and ballast though.

 

[r896neo]

You need to shop around conny that's pretty dear.

To make up 150kg of concrete would require 3 25kg bags of gravel at £3.90 and 2 sand at £3.40 Then a bag of cement at £4.

So that's 150kg for £11.30 as opposed to 150kg of postcrete costing between £32 and 40. Considering that 150kg is probably only going to do 4 posts its a fair difference when doing a big fence with 20 posts.

 

[Tigercubrider]

Generally a pergola has two or four posts

 

[r896neo]

Your pergolas clearly aren't as grand as mine .

It was for general info for everyones benefit as conny's comment was solely regarding postcrete costs and comparison. Im generous that way.