Roof windows are usually installed in roofs with steeper angles, such as 20 to 55 degrees, while skylights often go into roofs with a more shallow angle. Unlike a typical fixed skylight, a roof window opens to admit fresh air.Consult the manufacturer’s specifications for the size of the rough opening needed for your roof window, which may be a few inches wider or longer than the actual roof window dimensions. Your framing project will focus on doubling the rafters and adding doubled headers and sills to maintain the structural strength of the roof.
Cut a piece of cardboard to the size of the roof window’s specified rough opening. Take it to the attic or top-floor room with an exposed ceiling.
Move the cardboard around on the underside of the rafters until you find a good placement for the roof window, ideally 1 1/2 inches from the side of at least one existing rafter. The manufacturer may specify the maximum height of the window from the floor. For example, you may need to have no more than about 7 feet between the floor and the top of the window to allow for an easy grasp of the control bar just under the top sash.
Mark the corners of the cardboard on the existing rafters or on the underside of the roof sheathing. Drill one hole in each corner up through the sheathing and the shingles. Measure the size of the rafter lumber, typically 2-by-6, 2-by-8 or 2-by-10. Measure the length of the rafters. Measure the angle of the plumb cut at its top and the birdsmouth cut at its bottom with a speed square.
Transfer the rafter’s length and the angles of the cuts at the ends to fresh 2-inch dimensional lumber to create two sister rafters, basically twins of the existing rafters. Cut the sister rafters with a circular saw along your marks. Screw them to the existing rafters flanking the rough opening, on the side of the rafter away from the opening, with 2 1/2-inch deck screws.
Tack -- that is, temporarily nail -- 2-by-4 braces across the rafters a foot above and below the marked rough opening to support the roof framing as you continue to remove parts of the roof deck and reinforce the rafters.
Move to the roof and snap a chalk line between the holes you drilled earlier in Step 3. Cut out the rough opening with a circular saw along the chalk lines. Discard the cutout section of roof deck and shingles.
Return to the attic or top floor. Measure the distance between the rafters nearest to the rough opening. Mark and cut four 2-inch pieces of dimensional lumber matching the rafter size to this measurement. Facenail one piece to another with 6d nails. Repeat the facenailing with the second pair of cut 2-by-4s. You’ve made a doubled header and a doubled sill. Toenail the doubled header between the sistered rafters with 12d nails at the top of the rough opening. Toenail the doubled sill similarly at the bottom of the rough opening.
Measure the distance between header and sill. Mark and cut a piece of rafter-sized lumber to this measurement. Face-nail this piece with 6d nails to the inside of the rafter that is 1 ½ inches from the rough opening, so that the piece lines up flush with the rough opening. If the other side of the rough opening doesn’t line up with a rafter, cut two more pieces of rafter-sized lumber to the same header-to-sill length. Facenail the boards together and toenail them between the header and sill, aligned with the final side rough opening.
Measure any original rafters interrupted by the rough opening. Mark and cut sister pieces to this measurement. Facenail these pieces to the interrupted rafters. Remove the temporary 2-by-4 braces. Measure, cut and facenail sister pieces to any purlins, or braces, supporting the rafters flanking the rough opening to complete your roof window framing project.