How to Add Dormers

Preparation

• 1

  Mark the layout of the dormer on the attic floor, the front and sidewalls, and the rafters. Make the walls of the dormer square with the exterior walls of the attic. Drop a plumb bob from the underside of the roof to the floor to determine the inside edge of each wall and mark.

• 2

  Remove the shingles from the installation area. Cut the opening with a reciprocating saw. Use a pry bar and hammer to remove the materials.

• 3

  Follow the layout on the floor for the front wall. Cut the rafters at the top of the opening and bottom with a reciprocating saw. Create double rafters on each side of the opening by nailing a new rafter to the existing rafter.

• 4

  Cut two 2 by 6 inch boards to fabricate a double header. Nail the two boards together with 10d nails and secure across the cut rafters at the bottom of the opening. Install a similar header across the cut rafters at the top. Use 16d nails to secure the components.

Dormer Construction

• 5

  Build the front wall of the dormer, which includes the rough opening for the window. The edges of the new rafters rest on top of this wall. Base the height of the wall on the slope of the rafters leading from the top header, such as 4 inches per foot of run. Refer to the plans for exact specifications.

• 6

  Cut the top and bottom plates from 2 by 4 inch lumber. Cut the studs for the wall the length of the wall height minus 3 inches which is the thickness of the plates.

• 7

  Build the rough opening for the window between the two king studs on each side of the opening. Make the dimensions the recommended opening size plus 3 inches. Cut two 2 by 6 inch boards to fabricate the header to go above the opening. Install the header flush against the top plate.

• 8

  Cut two horizontal boards from 2 by 4 inch lumber. Nail the two boards together and secure between the kings studs to create the bottom of the rough opening. Cut a trimmer from 2 by 4 lumber. Install the trimmer on each side, between the header and sill, and the bottom of the sill to bottom plate. Place the wall section against the bottom header and check with a carpenter’s level for straightness and plumb. Fasten to the floor with 16d nails.

• 9

  Cut the new rafters and install. Nail the rafters to the header at the top and to the top of the new wall. Use lumber that matches the existing rafters.

• 10

  Cut the 2 by 4 inch studs for the sidewalls of the dormer. Make the length of the studs the measurement from the top edge of the dormer rafter to the surface of the double rafters. Place the studs with the top end flat against the inside of the end rafters and fasten to the dormer end rafters and double rafters with 16d nails.

• 11

  Cover the rafters and studs with exterior grade plywood. Use the same dimension lumber as used on the existing roof and siding.

• 12

  Staple roofing felt to the roof sheathing. Install matching shingles in accordance with the manufacturer’s instruction. Cover the dormer walls with house wrap and install matching exterior siding. Proceed with the window installation and complete the interior finishing work.