dromers are really just made form a stud work frame with a beam as the head, if the window is 1.2 or less two 4x2 on edge is ok for the head then your ceiling joist sit on that

you just need to work out the hight of the dormers i.e 4" head the the hight of the window then your up stand for lead out side remember the tiles and batten, you need about 4" above the tile

mark