shed dormer

huangyhg

shed dormer
there is a 25' x 9'-9" section of an existing roof that will be replaced with a symmetrical shed dormer on both sides that sit on 6'6" tall knee walls with stone masonry foundation walls below the knee walls.  the existing ridge beam elevation needs to be maintained, therefore i have a 12 hor. to 5 vert. roof pitch.  the distance from the bottom of the 2x12 ridge beam to the top of the knee wall is approximately 5 feet.  the proposed rafters will be 2x10 @ 16" o.c.  the shed dormers are in a loft area where the owner would like to maintain a vaulted type ceiling.  therefore, three of the four sides of the shed dormer will have masonry walls below them, and one will not.  i am concerned about the horizontal forces that will result from the roof pitch and the lack of vertical height between the ridge beam and the knee wall.  i was thinking of using 2x4 collar ties 32" o.c. w/ 5/8" outer sheathing (for added diaphragm strength), but i am not sure this will be sufficient.  please advise.  thanks in advance.         
analyze for the forces in 3d. ensure then the tie effect ensures that the global deformations, long term, are likely to stay limited to what you think is acceptable and by the constructive setup unable to damage the supporting walls.