Things You'll Need
Screws, 2 inches long
Nails, 3 inches long
A dormer is a protruding mini-roof that extends out the side of your main roof. Dormers are added to attics or second floors to let in more light and to increase the amount of usable floor space by increasing the amount of headroom. There are two primary types of dormers: shed dormers, which look like a piece of the main roof has been raised to a flatter angle, and gable dormers, which have their own roof lines that lie perpendicular to the main roof line. Gable dormers are more aesthetically pleasing, while shed dormers are easier and more inexpensive to build.
Build a Shed Dormer
Remove the ceiling drywall and insulation from an area 1 foot higher and 2 feet wider than the area where the dormer will go. Cut through the drywall with a utility knife and pull it away from the rafters. Pull out the insulation by hand.
Remove the shingles or other roofing from the area where the dormer will go, using a pry bar. Cut through the roof sheathing along the lines where the dormer will go with a Sawzall or circular saw, then remove it using a pry bar or crowbar, but leave the rafters.
Support the rafters that will be getting cut by screwing a 2-by-6 across them with a drill at a height on the ceiling where it won't interfere with your work. Support the 2-by-6 with posts that extend down to the floor.
Cut the rafters off square, using the Sawzall or circular saw, and create a header by attaching a doubled-up piece of lumber of the same dimensions as the rafters to the new ends. Nail these two pieces of lumber together, using a hammer. The new header must butt up tightly against the first remaining rafter on each side of the gable hole. Screw through the new header into the cut off rafters, and through the uncut side rafters into the ends of the new header, using a drill in both cases.
Remove the exterior wall by cutting through it with the Sawzall, extending the dimensions of the hole in the roof vertically down to the floor.
Frame in a new wall built to the full height of the new dormer with 2-by-4s cut to length with the circular saw. If the original wall was a knee wall of 3 feet or so, the new wall will be around 3 feet taller, leaving 6 feet of headroom in the dormer space.
Install new rafters that extend out from the header to the top of the new wall, beginning at each end of the new header. Attach the rafters, using screws and a drill or nails and a hammer. This will leave a wedge-shaped vertical space between the old side rafter and the one you are installing.
Install new rafters spaced 16 inches on center, in the same manner that you installed the previous rafters.
Frame in the wedge-shaped spaces between the new roof and the old, using the same materials and techniques you used to install the new outer wall of the dormer.
Sheath in the new walls and the roof with 5/8-inch plywood. Screw the plywood to the studs and rafters with 2-inch screws.
Build a Gable Dormer
Cut the hole in the roof, using a Sawzall or a circular saw, so that the sides of the hole go up the roof at an angle and come to a point in the center.
Make the new wall that you install in the hole pointed so that its peak is at the same elevation as the peak of the hole you cut.
Frame the roof by installing a plank across these two points, then building in rafters running from this plank down to the edges of the hole on both sides.
Attach the gable dormer directly to the old roof until it reaches its full horizontal width, at which point it will continue onto wedge-shaped side walls similar to a shed dormer.
Check the weather to be sure you have several days without rain.