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.