I would use continuous top plates, skip the CG's which is how I read your post. By using the continuous top plate your rafters are independent of the post, simplifying the work and raising.
Also note/read that #7 has a tricky aspect which can be overcome by using the rafters in #9 and alluded to by Jim Rogers, I believe.
Rafter spacing on a 20' building and only 16' wide you could use 2' but I would fall on the 4' side, nice, neat and half the work. Full 1" boards and roof top guard with the metal on next. No purlins needed. That is what I am doing and commencing cutting in the morning.
If you use collar beams will you have enough head clearance on a narrow building, have you drawn it yet? Might as well use the space. How far below the eve line are you planning on placing the tie beam?
I don't see this as all that big of a frame, a nice starter to learn on. Now you have $.04 worth in debt notes, not silver or gold, btw.
Tim
