To roof a run, you are basically constructing a shed with no walls.
It needs rafters AND purlins, the one at right angles to the other, both of which will probably be spaced at 2' (altho it depends on your exact construction). (There is another way of doing it, but it doesn't save you THAT much lumber since it all needs to be bigger)
The manufacturer of Suntuf and Palruf has a brochure that details the support specifications needed for their products, and presumably mfrs of other plastic and metal roofing materials do too -- CONSULT THEM AND FOLLOW INSTRUCTIONS. The stuff is pretty saggy, and expensive enough you don't want to ruin a whole roof's worth by incorrect construction. (I really LIKE it, you just have to use it as intended)
There is no issue of strength of the roofing panels, only the issue of strength of how you've got them SUPPORTED. So, see above paragraph
If the roof is flat, you will need to use *approved* sealants at all seams, and the roof will still probably leak, plus you will spend MUCH more money on lumber constructing a stronger supporting structure; thus it is smarter to have some reasonable pitch like at least 2:12.
The supporting structure needs to be built to withstand your local snow loads, so do not just happily go off building plans designed for, like, Alabama
Good luck, have fun,
Pat