re: Progress, not Perfection, lol

Steve
4/12 is minimum for 3 tab roofing for it’s warranty not to be voided. the more pitch you have the less load you will have because only so much snow can stay on a steep pitched roof. Tie rafters will help but will take up room for interior storage.
having the rafters 16oc will help too, so would 2×6″ rafters .using #1 2by material will help. Any of the previous suggestions will help, all of them might be an over kill,but I like over killing builds ,it lessons the chance of having to come back later to fix something.