Installing a dormer in an attic is a stylish way to provide additional headroom and natural lighting in what is typically a very cramped space. This article will briefly discuss one of several methods of constructing a standard, gabled dormer.
This is a challenging project and should not be attempted unless you have previous experience with other remodeling projects. If you are unsure, consult with a professional.
Step 1 - Plan and Measure
Start by drawing out a layout to plan your work, incorporating details such as slope, headroom, and exterior design. Note that every roof is a little different, and standard dimensions may not apply. Include as much detail in your measurements as possible to avoid errors.
Using the dimensions you've decided on, trace the layout on the attic floor to be sure the interior wall will be square with the exterior walls. Next, trace the underside of the roof; to ensure accuracy, use a plumb bob.
Step 2 - Make the Rough Opening in the Roof
Begin this step by removing the roofing (shingles, roofing paper, sheathing) from a span of three or four rafters, exposing the attic. Locate one side wall of the dormer at an existing rafter. Double this rafter by nailing an identical rafter to its side. Measure across, perpendicular to the rafters, the width of the dormer, from inside wall to inside wall, and install another doubled rafter. These will act as the bottom wall plates for the side walls and will function as your support beams for the dormer.
Install one header perpendicular to and between the two doubled rafters where the dormer ridge will meet the roof line, and then install another one that will act as a sill for the dormer window.
There will be at least one existing rafter between the doubled rafters that will have to be cut back so it butts perpendicular to the header. Similarly, a rafter tail will be installed at the lower header, aligned with the other rafter tails and the existing rafter, so it appears as a continuous rafter was interrupted by the rectangular opening.
Step 3 - Build the Dormer Walls and Ceiling
Nothing larger than 2x6 lumber will be needed to construct the dormer itself. You can locate the dormer’s top plate elevation by measuring up from the attic floor. Build up the side and front walls by installing corner studs and a doubled, 2x4 top plate onto the bottom plates (doubled rafters) and the sill. Cut the top plates at an angle equal to the roof pitch where they attach to the main roof.
The studs along the side walls will have angled cuts at the bottom end, and square cuts at the top. Check that the plates are level and the corners are plumb. Create a rough opening for the dormer window by installing a header and trim boards. If you want the dormer to have a flat ceiling, install joists parallel to the front wall.
Step 4 - Build the Dormer Roof
Cut at least two of the dormer’s rafters and the ridge board. Install the two rafters at the gable end as a temporary support for the ridge. Make sure the ridge is straight and level. If the rafters are a good fit, cut and install those remaining at 16 inches on center. Install gable-end studs above the window header.
Step 5 - Make the Connection to the Main Roof
Snap a chalk line from the point where the dormer ridge meets the main roof to the point where the top plate of the side wall meets the main roof to determine the location of the valley. Install a 2x8 flat board against the main roof rafters along the valley so that the upper edge aligns with the chalk line. This will act as a nailer for the jack rafters that form the valley.
The ridge and the jack rafters will have angled cuts so that they "feather" into the roof plane. Getting these cuts right might require some trial and error.
Step 6 - Finish and Trim
Install plywood sheathing and roofing to the dormer’s frame the same as you would for any wood frame construction. Install the dormer window and add the siding and trim.
Step 7 - Install Flashing (Optional)
To prevent potential water damage and mold, you may want to install flashing. This final touch is not very complicated and will also give your dormers a completed look.