If you have decided to build a shed dormer on your house to allow for extra light or extra headspace, shed dormers are easier to build than gable dormers. Shed dormers have a flat, pitched roof, set at a complementary angle to the existing roofline. A gabled dormer, on the other hand, is not flat, but "gabled" which refers to two sides coming together to form a peak. Either one will serve the intended purpose, but each style has a different aesthetic effect.
Step 1 - Layout
After determining the size of shed dormers you would like, remove the shingles, and use a measuring tape to draw the layout of the dormer on the roof with chalk. Using this outline as a guide, use a drill to create a pilot hole at each of the 4 corners that you have marked, being certain to drill straight down through the ceiling. The holes in the ceiling will establish the 4-corner points on the ceiling inside the attic and avoid potential measuring mistakes. Alternatively, make exact measurements inside by using a known ‘tie-in’ that is easily established inside and out. Be sure to place the dormer such that the top allows for adequate headroom and the slope of the finished dormer is at least a 4-inch vertical drop for every 12-inch horizontal.
Step 2 - Cut Ceiling
On the roof, tape plastic over the pilot holes to keep the interior dry. With the chalk line, snap the layout on the ceiling described by the pilot holes in Step 1. Using the saw, cut the ceiling along the plumb lines. This allows you to see the rafters clearly.
Step 3 - Framing the Dormer and Modifications to Existing Structure
If you have decided that the top of the shed dormers will be at the existing roof ridgeline, the rafters of the dormer can be nailed to the ridge. If the top of the shed dormers falls below the existing ridgeline, you need to nail or screw and glue new boards to the existing rafters to help them carry the extra load.
You'll also need to install headers between the rafters at the top and bottom of the dormer opening. Pre-cut all the rafters and headers if they are required, using the saw. Determine the height of the headwall, to meet the slope described in Step 2. Construct the headwall framing on the floor, in preparation for attachment after the roof is removed.
It is best to complete as much of the work as possible before removing the roof, minimizing the time the attic is exposed to the weather.
Step 4 - Cutting Existing Roof
On the roof, using the pilot holes drilled in Step 1, snap the lines that will denote the opening in the roof for the dormer. Cut the shed dormers opening with a reciprocating saw. Be extremely careful not to damage the existing structure. Once the roof is opened, raise the end wall, attach the headers, sidewalls, and rafters for the dormer--sizes as determined from the design. As the sidewalls on the shed dormers carry no load, they can rest on either the existing floor or on the existing rafters. If you set them on the rafters, there will be no interference inside the shed dormers' space. You can finish the walls of the dormer with whatever siding matches the rest of your house.