Regarding using plastic card for the roof girders, don't be so hasty to under estimate their strength. Elsewhere on this site there is a thread of someone who's build a bridge in Manchester out of the stuff.. he then stress tested it.. with about 15 cans of beer!!! The rough calculations showed it was actually as strong (in scale terms) as the original bridge was estimated to be, possibly stronger because he didn't have the final cross members in place when he did the test.