Want to build a 12x24 shed roof attachment, all open sides, to an existing pole barn to park a tractor and a livestock trailer under. What size should the rafters be? I plan to build it like the barn (which uses trusses) using the rafters on 4 ft. centers with 2x4s perpendicular to them with the metal roofing on the 2x4. Will 2x6s work or should I go with 2x8s (the barn trusses use a 2x6 top cord)???