glad I could help.
You'll definitely want more than 2/12 if you get snow, if 3/12 is all you can get, brace it a LOT and what ever size boards you were going to use, go bigger and closer together. you're spanning more than 12 feet? I wouldn't go under a 2x8 and I'd put them 12" on center. As aart said, you don't want to lose a roof to a snow load and at 3/12 it's going to take a long time to run melt and run off, unless I'm reading that you're going to be out there clearing it by hand?
You'll definitely want more than 2/12 if you get snow, if 3/12 is all you can get, brace it a LOT and what ever size boards you were going to use, go bigger and closer together. you're spanning more than 12 feet? I wouldn't go under a 2x8 and I'd put them 12" on center. As aart said, you don't want to lose a roof to a snow load and at 3/12 it's going to take a long time to run melt and run off, unless I'm reading that you're going to be out there clearing it by hand?