Perameters CA uses for thickness of a roof plane?

[Autumn1]

When CA auto generates a roof how does it determine size of Rafters (Thickness of roof). I assume that 2x6 lumber rafter would be the minimum size CA would create for a small (minimal span) roof. So there probably are set spans at which CA would generate using the next conv. lumber size (2x8 , 2x10 etc) ? Never occured to me that the thickness of a roof could be changed until started messing around with a "patio roof". I figured out how to re-define the space occupied by rafters so I could model a roof only a couple inches thick. I am just thinking of single plane simple rectangular "shed roof". (NOT like a 24' wide garage roof with trusses)

 

Wondering about alternative roofs other than conv. lumber and "comp shingles". Can CA model a "Translucent" roof such as those crummy 26" x 12' Poly Carbonate corrugated panels? Are there any commonly available metal frame roof with clear or translucent panels that are "good quality" or is that strictly a proprietary type of patio / deck cover that one would have to buy "designed, sold and installed "by others".   ?   Hope that makes sense. Thanks for any ideas.