A gable dormer is a small room with a peaked roof built perpendicular to the main roof line. Constructing a gable dormer is slightly more complicated than constructing a shed dormer. The biggest differences are that the top plates of the sidewalls are level and a ridge board supports the roof peak and pairs of rafters on each side. Construction of a gable dormer, however, proceeds in the same order as a shed dormer.
Before you start, strengthen the floor with additional joists and install the proper subfloor to support the dormer framing.
Finding the Slope
Before you begin construction of a gable dormer, draw a section (or profile) view of it. Find the pitch of your existing roof and draw it to scale on graph paper. Then draw in the sidewalls to scale and determine the pitch of the dormer roof. The pitch of the dormer roof is the amount of rise in inches (measured from the top plate of the dormer sidewall) for every horizontal foot from the edge of the wall to the centerline of the roof peak. The dormer ridge line is horizontal. The pitch of your dormer roof will depend on the look of the dormer itself and the pitch of the main roof. Make sketches on tracing paper to get the design right. Then draw the dormer on your final selection plan.
(a) Lay out the dormer location on the attic floor and transfer the outline to the rafters with a plumb bob.
(b) Mark the roof exterior at the corners and cut the opening through the roof.
(c) Build temporary support walls, mark and cut the rafters at the edge of the opening. Install double headers at the top and bottom of the opening.
(d) Frame the front wall as you would a shed dormer and the sidewalls with a doubled top plate, overlapping the front wall cap plates on the sidewalls and extending the top plates of the sidewalls 13 ½ inches past the front wall. This extension will support the rake rafters.
Installing the Ridge
The ridge is supported at the top by the header and at the bottom by a 2×4 ridge stud that is fastened to the front wall top plate.
First, cut the ridge to length (the length of the sidewalls, plus the front-wall framing thickness, plus 1 foot for the overhang). Assemble the ridge and ridge stud. The length of the ridge stud equals the rise of the dormer roof minus the width of the ridge board. Cut the ridge stud and toe-nail it 12 inches from the outer end of the ridge board. Lift the assembly into place and fasten the top end to the header in a joist hanger. Toenail the ridge stud to the top plate of the front wall width 10d nails. Level the ridge. Mark the ridge and top plates at 16-inch intervals.
Cutting the Rafters
There are four kinds of rafters in a gable dormer, and each type requires different cuts. Label them as you cut them.
Rake rafters – These two are at the front of the dormer and form the overhang.
Common rafters – These run from the front wall to the valley.
Valley rafters – These run from the intersection of the ridge and header to the sidewalls.
Jack rafters – These short rafters run from the ridge and header to the valley rafters.
Lay your framing square on one end of a common rafter with the pitch measurements intersecting at the edge. Mark a line at this point and cut the rafter on the line. Hold the rafter snug against the ridge board and mark the point where the rafter touches the top plate of the sidewall. Lay the framing square at this mark and outline the bird’s mouth.
At a point 12 inches from the bottom of the bird’s mouth, mark a line parallel to the bird’s mouth. Cut the rafter on this line and cut out the bird’s mouth. Test-fit the rafter, make adjustments as necessary, and use it as a template to cut the remaining rafters.
Cut the rake rafters in the same fashion, ¾ inch longer than the common rafters. Measure from the intersection of the ridge and the header to the point where the sidewall meets the main roof rafters. Cut the valley rafters to length and miter the ends.
Measure both sets of jack rafters and miter-cut them to length. Toenail all rafters into place with 10d nails. The rake rafters are face-nailed to the ridge and toenailed flush with the front of the top plates of the sidewalls.
Sheathe the wall framing and the roof, then finish with roofing and siding, and install the window.
