Making Deck support posts strong - horrible horrible clay!

[samm1978]

Hi

Sorry for the duplicate post - need help.

Needed some advice on how I should be going about getting my posts in for the deck. Got my ledger on and have now dug my post holes. They are 700mm deep and 300mm wide belling out to about 400mm. Only thing is after about 300mm down the soil is pure clay.

Not sure what I should do to make sure my posts don't sink. I'm even seeing advice online saying decking support posts shouldn't even be dug in but sit on top??

I'm planning to pound as much type 1 subbase into the bottom of the holes as I can to bring depth up to ~600mm and then put a 100mm medium density block at the bottom for the post to sit on before concreting it in.

Any advice would be greatly appreciated.

 

[john4703]

I'm not in any way an expert but when I built my deck I dug the holes then attached the posts to the joists and used temporary supports under the joists while I filled the holes with concrete. I then left it for over a week for the concrete to set.
That was 6 years ago and nothing has moved.
I think this would work on clay as the mass of concrete should be enough, I'd not bother with type one or a block as I'd rather have concrete in that space. (but possibly others will arrive to tell me I am wrong)

 

[gregers]

in the past ive dug a hole filled it with concrete and bedded in a post then once dry cut it to the required height.