We chose a gambril (barn style) roof to give more room on the second floor. I designed the roof trusses using a CAD program and put them together in my garage.
The trusses are made from 2x6's and are connected together with 1/2" plywood gussets.
The trusses are on 24" centers. They are fastened to the cabin with a brackets that I found at the hardware store. The bracket allow screws to be placed into the truss and into the plate of the second floor. They provide a secure attachment without having to toe nail.
Then we sheathed the roof with 1/2 inch plywood, covered with 30 lb. tar paper.