Doghouse Dormers: Framing From the Ground Up
Assembling parts on the ground and cutting the roof at the last minute provide a low-stress route to a high-value project.
Synopsis: Doghouse dormers can be the perfect accent to many house styles. Rhode Island builder and Fine Homebuilding contributing editor Rick Arnold shows his technique for cutting and assembling doghouse-dormer parts on the ground to minimize rooftop assembly time — and also to minimize the duration that a house’s interior is open and at risk to the elements. Arnold highlights the process with details about how he calculates a precise roof pitch, then uses the Pythagorean theorem to determine dimensions for framing members and dormer parts. Included are sidebars about dormer design and the California valley that Arnold uses when installing a doghouse dormer.
One of the most popular ways to open the dark, cramped upper level of a house is to build one or more dormers. From a homeowner’s point of view, these small additions can increase curb appeal and create more-hospitable living space in the top level of a house. From a builder’s point of view, dormers concentrate almost all aspects of residential framing into one small package. Like any building project, the success of constructing dormers depends on precise calculations, careful planning, and some smart assembly work. Trust me, the last thing you want is to be on the roof trying to figure out why the wing wall won’t fit while a rain cloud hovers overhead, waiting to take advantage of that big hole in the roof.
Over the years, I’ve built a number of dormers, improving the process each time. I’ve finally settled on a system that enables me to figure all the dormer measurements precisely, cut and assemble most of the pieces on the ground (or in the attic), and put off cutting a hole in the roof until the last minute—no blue tarp needed.
This system is safer because it reduces the amount of time I’m measuring and building on the roof, and it also minimizes the number of trips to the ground to make cuts. My system also lowers the level of stress that comes with having a roof that’s open to the elements for an extended period of time. The trade-off is that all the measurements have to be as accurate as possible. Many of the calculations build off one another, and a sloppy measurement in one step throws off the math in the many steps that follow. It sounds daunting, but most of the components of a doghouse dormer can be found by dividing the framing components into a series of triangles, and then calculating the missing dimensions.
Check the existing pitch on site
I always start a dormer project by verifying the pitch of the existing roof. I don’t want to assume that I know the pitch, then cut and build all the dormer components only to discover that the dormer doesn’t fit correctly. Trusting the plans is tempting, but you might end up missing an abnormality like the 7 1/8-in-12-pitch roof on this project.
The most accurate process is to measure the total rise and total run from inside the attic. This project took place in an attic that was being remodeled, so the rafters, the subfloor, and the main ridge all were exposed. This made it easy to get precise rise and run measurements. If the framing had not been exposed, I would have cut back the drywall at the proposed dormer opening and measured the roof pitch there. That said, I prefer to have the rafters exposed so that I can take the largest set of measurements possible. The longer the two sides of the triangle (rise and run) that you are measuring, the more accurate the numbers will be.
Roof pitch and window height determine the wing-wall dimensions
Once I know the pitch of the existing roof, I need to calculate one of the two sides that make up the dormer’s triangular wing wall. I begin with the height of the front wall, which often is driven by the window height.
On this project, the homeowner wanted the dormer window to be about 3 ft. 6 in. tall. That would require a rough opening of 3 ft. 7 1/2 in. to allow for window installation. As a rule, I keep dormer-window openings at least 6 in. above the main roof, leaving room for proper flashing, and about 6 in. below the top plate of the front wall so that the top of the window lines up near the bottom of the dormer’s soffit. Adding the numbers, I end up with a front-wall height of 4 ft. 7 1/2 in. Now I can combine the height of the wing wall with the pitch of the roof and simply solve for the missing pieces.
From Fine Homebuilding #186
For more information and diagrams on how to install a dormer, click the View PDF button below.