I'm drawing out plans for a 12 x 18 shed which I plan to build in the spring. The pitch of the roof will be a 6 in 12, so the rise will be 3'. I was wondering if 2x4 rafters would be enough, or should I go with 2x6 rafters. Also, I was planning a 16" gable, is that enough, or too much? Any help would be greatly appreciated.