Decking construction details

Hi,

I'm planning a DIY decking project, and have a couple of questions that I can't find answered elsewhere, I'd be very grateful for any help anyone can offer.

First, it seems that all deck designs have short support posts underneath (e.g. concreted in) and separate posts for the balustrade. Is there any reason these are not the same posts? I'd like some of the balustrade posts to be strong enough to support a rather large umbrella (that will be moved from post to post, as required). In the wind, that'll put a fair stress on the post, so I was thinking these posts, at least, should run all the way through into the ground, or to concreted-in post feet. What is the reason this is not usually used as a construction technique, or have I just not come across it? Will 100mm treated fence posts look a bit rough, is it about appearance?

Second, I'd like the deck to be 2x4 joists sat on 2x6 joist support members. The 2x4s would be 400mm or less on centre (for, say, 32mm deck boards), and the 2x6 would be probably around 1.2m between centres, so it seems to me the 2x4 would be sufficient, but deck designs usually use 2x6 for the joists. Any thoughts? I can do 2x6 for both layers, if required, but it looked over-engineered.

Cheers, Ben

 

Thanks for the swift reply, that's really helpful