How to Build a Roof Dormer

Step One Planning

Plan the dormer. This can be a difficult project, and is not a project for the novice remodeler. While you are working on the dormer, there will be an open space in your roof that allows the elements to get into your home. If installed incorrectly, the windows and joints of the dormer could leak. If positioned incorrectly, snow could build up or rain wash off in spots that cause other problems. A well-planned dormer can add beauty and value to your home, while a poorly done dormer can cause structural problems or, at the very least, detract from the look of your home.

Take the time to evaluate your existing roof. Try to incorporate your dormer into what is already there, creating a seamless appearance. Draw plans for the dormer, or buy them ready-made, and work out the placement and design of the dormer on paper. Take the plans with you when you go to buy supplies, and a salesperson can help you put together the materials list and give you a cost estimate. Make sure all of your supplies have been delivered or picked up before you start working.

Mark your layout

Mark the placement of the dormer walls on the attic floor. Make sure the walls are square with the outside walls of the house. Using a plumb bob, mark the inside edges of the walls on the underside of the roof. Drive nails up through the roof at the corners of the walls.

Cutting the hole

Climb onto the outside of the roof. Remove the shingles from the area where the dormer will be placed. Use the nails that you drove through the roof in step two as the guide for snapping chalk lines. Snap lines between each pair of nails, creating a square. Set your circular saw to a depth that will cut through the roof sheathing, and then cut along the chalk lines to open the roof.

The rafters

Using a plumb bob, transfer the lines that you drew on the attic floor to the rafters. Cut the old rafters along those lines, creating a hole where the dormer will be. Reinforce the opening by adding new rafters along the sides of the hole.

The front wall

Install a 2X6 header. This is a 2X6 board that stretches between the new rafters from step four, and is attached to the cut edge of the old rafters that have been cut. Build the frame for the front wall with it lying on the attic floor. Stand the frame up in the opening, and align it with the front wall that is drawn on the floor. After leveling the frame, nail it to the floor joists. Make sure the top of the wall is square and that the sides are plumb, or vertical, and brace them in place with 2X4s.

Side walls

Add a corner stud to the front wall. Cut the end of a 2X4 to match the roof slope angle. Place the 2X4 on top of the corner stud, with the angled end toward the roof. Level it, and then nail it to both the roof and the corner stud. Make sure it is level because this is the top plate for your wall. Add a bottom plate, which is a 2X4 that runs from the bottom of the top plate along the roof to the inside of the corner stud. The bottom plate should be attached flat to the roof. Now, you are ready to add wall studs on 16-inch centers, finishing the side wall. Follow the same procedure for the other side. Add a second top plate to both walls, overlapping the corners.

The roof ridge

Cut a 2X4 to the length between the attic floor and the bottom of your roof ridge, according to the measurements from your plans. Nail this to the outside of the header from step five. This is a temporary support for your 2X8 roof ridge. Position the roof ridge, with one end resting on the support. Making sure it is level, mark the top of the ridge on the side rafters. Remove the roof ridge. Cut a 2X8 to fit the length of the opening between the side rafters. Attach this header board between the rafters, with the top edge aligned with the marks that show the top of the roof ridge. Cut the ridge board to length, and notch it according to your soffit details. Nail it to the center of the header board, with the tops aligned and the roof ridge level.

The rafters

Cut your rafters and install them on 24-inch centers. Run a string across the top of the rafters and make a mark where it intersects the rafters from the original roof. Measure from that spot to where the ridge board of the dormer roof intersects with the header board. This is the length for your valley rafter. The valley rafter extends from the original roof’s rafter to the headboard, where it is positioned beside the ridge board. The ends need to be cut at an angle that allows them to lie flat. The valley jack rafter then needs to be cut, and installed, so it fits between the ridge board and the valley rafter, connecting the two and giving the valley rafter additional strength.

Finishing the roof

Add subfascia and 2X4 overhangs, and then cover the roof and walls with plywood. Complete soffit and overhang details according to your plans. Fit the roof trim and nail it, and then finish the overhang and soffits. Add roof edge molding and step flashing. Re-roof along the edge of the dormer. Cover the valley and roof edges with a strip of ice and water barrier. Install a preformed galvanized metal valley. Shingle the roof from the bottom up. Install the windows. Cover the sides of the dormer with building paper, and then install the siding. Nail up the rafter ties.