A dormer is an upright feature added to a roof to provide an additional window in an upstairs area. Dormers may be made in a variety of styles: one dormer with one window, one dormer with two or three windows, a long dormer with multiple windows or one with a combination of a window and a vent. Some have gable roofs, which slope on both sides. Others have flat, shed-style roofs. Some have mini-hip roofs, with a front that slopes down.
Plan the window dormer. Decide where on the roof you want it -- how high above the eave, how far down from the roof peak and how wide. Pick a dormer roof and a window style; window dimensions will affect the final design of the dormer. Check local building codes and residential restrictions on permits or rules affecting dormer additions. Consult an architect, if possible, since work will involve both the appearance and structural elements of the house.
Roughly mark the outline of the dormer on the roof. Make sure that it has a firm rafter on each side. Remove roofing on the chosen area down to the wood sheathing, leaving an area below the dormer intact. Outline the specific dormer area by snapping chalk lines on the sheathing; drive nails at the dormer corners and use those to hold the chalk line while you snap it.
Use a reciprocal saw or circular saw to cut through the sheathing and remove it to expose the roof rafters. Cut 2-by-4-inch boards to nail at the top of the dormer from the rafters on each side of the opening to the corresponding rafters on the other side of the roof. Cut off the rafters inside the dormer area between what will be the top and bottom of the dormer. Nail 2-by-6-inch header boards between the side rafters at the top and bottom of the dormer frame; also nail these to the ends of the rafters that were cut off.
Build a frame for the front of the dormer, with studs on each side and top and bottom plates, to form a rectangle. Add studs in between if the window will only fill part of the dormer. Make a rough frame for the window, with top and bottom header braces nailed to studs on each side and short studs nailed between those top and bottom headers and the top and bottom plates.
Erect the dormer front wall. Make sure it is plumb with a level and nail it to the header and the rafters on each side. Install 2-by-4 side boards, one nailed to the roof floor, the other from the top of the front wall to the rafter in the roof. Add studs as needed between those 2-by-4s for wall support.
Cut two dormer rafters for the front, using a rafter framing square to determine the angles. Nail those to the dormer side walls, sloping up, and finish with a ridge board at the top of those rafters from the roof header to the outside dormer wall. Add more short rafters as needed between the front wall and old roof. Make short cross braces with mitred ends to nail between the rafters, just below the ridge board.
Install the window, putting metal flashing around the edges; seal flashing with clear caulk on the outside. Put oriented strand board, or OSB, on the dormer roof and walls. Nail metal flashing at the valleys on the side of the dormer and metal drip edge on the sides of the dormer roof. Lay down roofing paper on the dormer and nail on shingles. Finish by nailing house siding and trim on the dormer walls. Replace shingles that were removed around the edges of the dormer.