Thank you for your feedback.
Some very valid points - thank you!
I have been looking into this and I would definitely add large cross sections in the corners at the strongest angle of support.
This would tie the whole structure together and combined with the decking floor and internal roof be plenty strong enough.
I plan to use a twin frame structure (with insulation in between)
With the twin parallel tubes joined for strength. If they are 3" apart this would give the strength of a much larger rib section than the original.
I also plan to cross brace internally as well from corner to corner.
I design complex and lightweight, but strong systems and to me this construction should be at least as strong if not stronger.
But as always I bow to experience and if anyone can see a hole in my plans I would very much like to hear from you...
I wish I could post pictures as you would see my roof frame/deck idea is a cross braced twin tube system with corner braces.
One of the main reasons I fancy doing this apart from the much needed headroom (I am 6'3") is I can keep my windows where they are and go up above them.
Re the flat roof, I plan on being able to angle areas - for Solar and Snow.
The deck would be just above a lower roof which would be angled to allow water to run off it to the sides.