How to Make a Roof Dormer

Identify the rafters for the outside edges of the dormer. Remove roofing on the existing roof to expose the rafters. Reinforce those rafters and the corresponding rafters on the other side of the roof with duplicates. Double or triple the outside rafters, depending on the size of the dormer.

Brace the dormer rafters with a horizontal board nailed to those rafters and to the corresponding rafters on the other side. Cut brace angles with a circular saw to the slope of the roof. Level braces with a level and fasten them with 16d framing nails driven with a hammer through both outside boards. Put braces on both sides of the dormer rafters for a large dormer.

Cut off rafters between those edges at the top and bottom of the dormer frame with a reciprocal saw, the length depending on the size of the dormer. Build two headers to go horizontally between dormer rafters at the top and bottom. Make these with lumber the same size as the rafters, typically 2-by-6-inch. Fasten them to the dormer rafters with metal rafter hangers.

Connect the rafters from the roof ridge board to the top header and from the bottom header to the house wall cap with framing nails. Drive the nails through the headers into the ends of the cut-off rafters.

Build a front wall with double 2-by-4-inch studs on the ends and doubled 2-by-4 top plates and a bottom plate. Frame any windows with a 2-by-4 on each side of the opening, a header and a sill between those at the top and bottom of the opening, and "trimmer" studs between the header and sill.

Erect that wall, set it plumb and nail the bottom plate to the bottom dormer header. Brace it temporarily with other boards nailed between the wall and the rafters.

Make side walls to go between the front wall and the dormer rafters. Use doubled 2-by-4s for the top plates and single 2-by-4s for bottom plates, angled to the slope of the roof. Put vertical studs between the plates at 16-inch intervals. Measure the spaces with a tape measure and cut studs to the specific lengths between plates. Fasten studs with framing nails.

Set the side walls in place and plumb them. Nail the bottom plates to the dormer rafters and nail the corner studs together with framing nails. Remove the temporary braces once walls are secured.

Frame a gable dormer roof by cutting two "valley" rafters that are 2-by-4-inch to connect the dormer rafters to the center of the top header. Allow 1 1/2 inches between the tops for a ridge board.

Cut common rafters to frame the dormer roof from the front wall to the house roof. Leave space for a ridge board and cut a triangle notch, called a birdsmouth, to fit over the dormer wall caps. Add 1 foot to each rafter length for an overhang.

Set the outer pair of rafters in place and nail them to the wall caps. Slide a 2-by-6-inch ridge board between the tops of the end and valley rafters. Level it and nail the rafters to it. Install other rafters at 24-inch intervals, nailed to the wall caps and ridge board. Trim the last pair of rafters to connect the ridge board to the valley rafters. Make short rafters of varying lengths to connect the top header to the valley rafters.

Add cross tie braces to the rafters about one-third of the way down from the peak. Cut 2-by-4s with ends angled to the slope of the rafters and nail the ties between the two rafters.

Cut a fascia or facing board to go on the ends of the rafter overhang between the front wall and the roof. Angle the roof end of the board to the slope of the roof. Nail the fascia to the ends of the rafters.

Cover the roof with 5/8-inch oriented strand board, called OSB. Place 4-by-8-foot sheets over the dormer framing and fasten OSB to the rafters with 8d galvanized nails. Trim any excess OSB with a reciprocal saw. Put OSB on both the dormer and main roof.

Install metal flashing on the valleys where the dormer roof meets the house roof, on all edges of the dormer roof and around the window. Fasten flashing with galvanized flashing nails. Lay roofing paper on both roofs, staple it in place with a construction stapler and finish the dormer with shingles.