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.
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.
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.