If you run an interior wall perpendicular to the ceiling joists on a roof system that is designed for a clear span wouldnt that be a non-bearing wall even though the ceiling joist are resting on it. I used 2x6 rafters and 2x8 ceiling joists which the inspector passed for the 18 ft span, on one end I added partition down beside stair case . The reason IM asking is I used a 2x4 flat ways for a door header which is approved for interior non bearing.