Deck Support Posts - How Deep?

[Sturge42]

Hello everyone,

I've been reading alot of posts on this site, but can't seem to find an answer to my question.

I'm building a deck that will be 17 - 20cm above the ground, and will be using 6x2 joists and 3x3 posts. Some of it will be over a concrete patio so I'll be using 6x2 offcuts to create legs and screw these down using angle brackets. My question is about the posts.

The soil in our garden is clay. We've had to level the ground so it's pretty much solid clay from the surface. As our posts are barely above the surface, how deep should the holes be?

Also, we dug a hole yesterday about 1 foot down which didn't seem enough, and this morning it's just a puddle. What would you use to hold the post in? I know a lot of people here use concrete of postcrete, but I'm worried it'll just be a big puddle for the post to stand in 6 months of the year.

Location is East Lancashire.

Thank you all for your help, Ashley.

 

[r896neo]

Your concern is valid. fill the hole fractionly above ground level and bench the mortar/concrete away to shed water from the base of the post.

 

[Sturge42]

Thanks for the reply! How deep should the hole be? Given it will be only 20cm above ground, I was thinking between 12 and 18 inches.

 

[r896neo]

thats fine, its not like a fence post which needs depth to stop it moving laterally. all your pressure is downward force so just get down into good firm virgin ground. Unless its fill or made up ground 12'' would be fine