Originally Posted by Dionysia
. . . . and there is no room under the bottom of the roof slope to put up 2x6s.
Actually, 2 x 6s could be sistered onto each 2 x 4 ceiling joist by taper-cutting the ends where the roof slope interferes. But a stiffer ceiling will result if each 2 x 4 joist would have a 2 x 4 flat member glued/screwed to its bottom. Crunching the (approximate) numbers for the section modulus of each configuration, it works out to 13.62 cu. in. vs. 10.63 cu. in., or a difference of approximately 28% stiffer with the flat 2 x 4s on the bottom.