Vass wrote:
free standing decking (i.e. not fixed to wall of house)
20 foot x 12 foot, blinking heavy it will be
I'm using 9" x 1.5" joists and I can hardly pick each one up as they
are currently 15 foot long

Sounds like you already have them, but they are way over spec for what you
are doing.

A 6 x 2 will span around 8 - 10 ft.

I propose to use 15 posts each 4"x 4" with 5 foot centres
the deck will be raised slightly approx 1 foot off the ground
allowing space under the joists as the ground is not level

I'd agree with 4 x 4 posts, but I don't reckon you will need that many.
Still, it isn't gonna fall down!

So, question is, do I
1. sink the posts into concrete?
2. dig down to hard stuff, chuck in a breeze block and free stand
them on this (as per Tommy Ground Force)

I've built bigger decks than that, and I use a combination of methods;

Use a post hole digger http://www.wickes.co.uk/invt/501584 to get a nice
round hole, go down about 16 - 18", drop half a brick in the bottom, drop in
post & fill with postcrete.

I do this for the posts at corners and around the edges of the deck.

For extra support in the middle I use 600 x 600 standard paving slabs
(around £2:50). Drop these in place & level them, post stands on this, cut
post to height & bolt joist to it.

I've recently started using these
http://www.screwfix.com/app/sfd/cat/...31526&ts=86105 to bolt
joists to posts and fix noggins. You need a good drill driver - min 14.4v
with gearbox or a mains driver to put them in, but they are fast and bloody
strong!


--
Dave
The Medway Handyman
www.medwayhandyman.co.uk
01634 717930
07850 597257