I want to frame a gable roof onto my (to be constructed) 12'x24' shed/storage/workhouse. To gain more headroom--by a longer rafter run-- I want to extend the ceiling joists to about 18" beyond the wall top plates on both sides; and nail the rafter seats to the joist ends. This will give me a longer rafter run and more loft space. I like a gable over a gambrel because it's easier to construct, work on, and doesn't collect as many leaves and branches. Ques: Will the joists support the roof weight w/o sagging? Seems like I've seen two-story houses where the top floor extended out beyond the lower story.
I think that I know what you want to do and it sounds like an okay idea. You will probably have to treat the ceiling joists the same as you would floor joists that cantilever over a wall and carry roof load. I think that 2x10's are probably required for this load . What are you planning on using for the ceiling joists?
Maybe a better way is to carry the roof load directly onto the walls by putting in a 2x4 vertical after you've done what you are talking about. Don't forget your gussets. Glued and stapled.