A dormer is a vertical window with a roof, set into a sloping roof of a house, used to add light and living spaces to upstairs areas. There are many variations, from basic height and width to roof style and exterior finish. Most dormers have roofs that match the house roof, but some have round or cathedral roofs. Some houses have just one large dormer; many have two smaller ones spaced across the roof. Adding one over a front porch takes construction expertise and experience; it is not a job for beginners.
Design your dormer addition. Check building regulations for any permits or restrictions. Sketch out the basic location on the roof, the height and width you want and the style of roof, window and trim. Buy your window first, since everything else will fit around this. Then lay out your dormer inside the house, in the attic or room where it will be located. Make sure all corners are square. Then use a plumb bob to transfer the exterior dimensions to the ceiling. Drive nails through the roof at the corners to mark the outline.
Remove shingles in the marked area, then snap a chalk line between the corners to make the cut-out lines. Then use a circular saw or reciprocal saw to cut through the roof decking (nail a temporary scaffold board at the bottom of the roof for personal support). Leave any overhang sheathing uncut. Then pry off the old sheathing. How big an opening you cut will depend on what size dormer you plan. Remove sheathing along rafter lines so you can access the rafters and other framing inside your new dormer area.
Nail 2-by-4-inch horizontal supports between the rafters at the edge of the dormer area and corresponding rafters on the opposite side of the roof. Cut off the rafters inside the dormer area at what will be the peak of the dormer roof, then nail a 2-by-6 header across that top. Nail three short rafters between that header and the existing roof ridge board. Cut rafters evenly across the bottom opening for the dormer, cut a 2-by-6 footer board and nail it in place, to the full rafters on either side and the tops of the cut-off rafters.
Build the front wall inside the attic or house. Put a full 2-by-4 header across the top and put 2 by 4s down each side and around the rough window opening. Extend the window-framing 2-by-4s below the roof line and nail on a 2-by-4 footer that will be nailed to the rafters on either side. Cut and mount studs at the top to hold a rafter for the new roof. Put that front wall in place, brace it temporarily, then cut 2-by-4s to nail on the roof along the outer walls of the dormer. Cut additional 2-by-4s and studs in varying lengths, to frame from the front wall to the old roof.
Install a ridge board for the dormer, from the header plate on the old roof to the top of the dormer front wall, extending beyond the wall. Then cut and nail in 2-by-4 rafters from the ridge board to the dormer wall caps; notch them to fit the wall caps and extend them slightly beyond the wall. Then nail 2-by-4 horizontal braces between the rafters. Cut and nail in 2-by-4s along the ends of the rafters on each side; extend these to match the ridge board extension, then add 2-by-4s diagonally to connect ridge and side boards. Cut and add short braces between the end rafter and the extension.
Install your window in the rough opening. Put metal flashing around the sides. Then sheath the dormer roof and sides (and any exposed area of the old roof) with oriented strand board (OSB). Nail metal flashing along the sides of the dormer and the new roof valleys and metal drip edge on the roof sides. Then cover the dormer with roofing paper and shingles, tying new shingles into the old roof and replacing any removed shingles. Fold shingles over the new roof valleys. Put house siding on the edges and gable of the new dormer and finish with edge and window trim boards.
You can vary these plans to make two or more dormers, to make dormers with very large windows or with two or three smaller windows. Because you will be affecting the integrity of the original roof, have an architect draw or approve plans for a dormer addition. You can add trim to make dormers as simple or as ornate as you wish, to match the style of house.
Be very careful working on a roof. Nail temporary scaffold boards at roof bottoms to provide secure footing.