Bigger isn’t always better
The most common shed-dormer design mistake is to become greedy about increasing usable square footage. The result can be an overwhelming, boxy blemish because the roof is too flat, the dormer extends too far laterally, and it isn’t nested within the eaves of the primary roof (drawing below). By keeping a few pointers in mind, you can avoid such an unfortunate result and keep the new dormer from overwhelming the primary roof.
For one, pay attention to the dormer’s spring line within the primary roof. A dormer that springs from the primary roof ridge, while generally easier to build, can result in a hulking dormer—especially if the house is fairly wide and if the face of the dormer wall is coplanar with the exterior wall below. A dormer that springs from a line below the primary-roof ridge clearly appears subordinate to the primary roof and is less likely to overwhelm the roofline.
Roof pitch is another important variable. In general, a 11/2-story house with a roof pitch of 8-in-12 or steeper is suitable for a shed dormer. Of course, a shed dormer’s roof slope needs to be shallower than the primary-roof slope, but it’s best to keep the dormer-roof slope at least 4-in-12 or steeper. Any flatter, and the face of the dormer wall can appear awkwardly tall; the whole dormer might protrude by a disproportionate amount, almost bug-eyed from the roof.
If a shed dormer runs the whole length of the house, it competes for dominance with what had been considered the primary roof. Many Capes built in the past 20 years have a “shed dormer” in the rear that runs the width of the house, so all the dormer walls become extensions of the main walls of the house. The lack of hierarchy is awkward.