Sound advice, thanks. I’m beginning to think that the area I need to cover is too large to expect the panels to do the job of keeping water out, and by making such a large area out of panels I’ll be trapping an awful lot of heat under them - making a roof on the structure first, and then putting panels on over that will let me space them out so each row can shed it’s heat a bit.

I think my main lateral beams will be 2x8, as you say. My main length is too long for a single span, so overlapping 2x8s will give me the strength.