This should give you a quick run through of the axial thrust down the rafter and at the midlap of the ceiling joist ;
http://www.windyhilllogworks.com/Calcs/Sloped%20Rafter%20Design%20for%20Bending1.htm
Notice the rafter dimension required, also the thrust. Then kick the pitch up to a 4/12 or greater and notice the thrust.