Framing a dormer is like framing a little house on top of a roof. Here is how you frame a dormer.
1. Make sure that where the side walls of the dormer are going to sit that the trusses or rafters are doubled to take the load and space them the out out dimension of the dormer.
2. Install the head outs for the ridge and bottom wall. Use a doubler.
3. Frame the side walls first on top of the doubled rafters after sheeting it of course. Then frame the front walls.
4. Sheer the walls and flash the side walls with 6 inch l metal. Make sure that the the side flashing laps over the bottom flashing. This is the proper roofing procedure.
5. Run the trusses like regular until you get to the last truss. Run a dummy ridge into the doubler and nail the other end into the end truss.
6. Run the valleys and cut the valley jacks into them.
7. Bird block it as standard.
8. Run the fascia
9. Sheet the roof
10. Install the front window
11. These are the major steps involved in framing a typical doghouse dormer.