Whenever adding any type of dormer, it's essential to determine the roof load and design the load path transfer to the existing floor joists or roof rafters. Then you can ensure adequate bearing points, or add framing and install beams to carry the loads through the walls down to the foundation. It wouldn't be a bad idea to consult a structural engineer for guidance, especially on larger dormers.
Gable dormers get their name from the gable roof covering them. These dormers are built perpendicular to the existing roof plane, with their ridges extending out from the existing roof. This means the sidewalls of the dormer bear and transfer the weight of the dormer rafters to the existing main roof rafters or the existing floor joists in the room/attic below. Which way you decide to support the dormer loads depends on the interior layout you're shooting for.
Bearing on the Rafters. My first choice is to install double or triple roof rafters on either side of the new dormer for the sidewalls to rest on. This eliminates the need to remove attic subflooring to insert additional joists. It also leaves the area beneath the common rafters uncluttered without any walls to interfere with open space and provides freedom to lay out partition walls later.
I locate the position of the dormer and do as much work as possible inside the attic before opening up the roof. First I insert double or triple rafters between the ridge and exterior wall flush to the inside edges of the dormer opening. Rolling them one at a time into position with a little help from a sledgehammer usually works. The dormer walls will bear on these rafters.
The existing common rafters that remain within the dormer opening area need to be cut and headed-off as if you were framing for a huge skylight opening. I use a recip saw to cut the rafters off at the top of the opening laying out the cuts to include the headers. The same goes for the bottom cuts.
Bearing on the Floor Joists. The other way to frame a gable dormer is to bear the sidewalls on the floor deck. You'll need to add floor joists directly beneath each wall to handle the loads. Installing extra joists is an easy process when the attic floor isn't covered, but when it is you have to cut the subflooring back enough to allow you to install additional joists, a task that multiplies if you are faced with adding joists in a finished room with a finished floor. Another downside to the full-height bearing walls is they can create a tunnel effect inside, especially when the ceilings will be high. Also, the portion of the walls beneath the common rafters have to remain in place and may drive interior layout since they can't be moved.
One advantage to bearing the walls on the floor is that the rafters don't have to be "sistered" for narrow-width dormers, so there's no struggle inserting them. Where possible, I size and locate dormers between existing rafters. For example, a 46-1/2-inch-wide gable dormer (exterior measurements) leaves only two rafters to be cut and headed-off, and that won't require doubling the common rafters on either side.
For wide gable dormers where the walls will carry Herculean loads, LVLs or steel I-beams may need to be inserted into the floor system to hold up the roof. And it's in these cases where I take extra care to follow the load path through the first floor and into the foundation with posts framed within bearing walls.
In either case, I frame the dormer walls, cut the rafters, and prep the ceiling joists before cutting open the roof. I also cut one end of the dormer ridge to match the common rafter pitch and leave the other end long for trimming in place. All this prep work minimizes the time the roof is exposed to the weather.
After the walls, ridge, and common rafters are set, I overlay the valley framing on top of the main roof sheathing to save from cutting a true valley. The roofing material will need to be stripped back for this step.
Framing a shed dormer is a simpler process than building a gable dormer. The end wall of a shed dormer carries the load rather than the sidewalls. There is generally no load-bearing issue when the end wall is framed directly over the exterior wall of the first floor. But if the dormer wall is positioned in from the exterior wall, the floor joists bearing the load must be adequate to carry the weight. The joists may need to be doubled-up to provide the support or engineered beams such as LVLs may need to be inserted to carry header loads transferred to the floor system.
I start by determining where the top of the dormer rafters will intersect with the main roof. They can either meet the main ridge or intersect with the existing roof below the ridge. When the intersection is below the ridge, a header needs to be inserted to support the dormer rafters. I double or triple the common roof rafters on either side of the dormer opening and insert a header between them at the intersection point. The header must be sized to carry the tail joists that fill between the header and the ridge and carry the dormer rafters.
Then I determine the height for the end wall that provides a rafter slope of at least 4 in 12 and allows for adequate headroom in the dormer. To minimize open time that the roof is exposed to the elements during the framing process, I pre-cut the dormer rafters and header (if needed) and frame up the end wall. Once the existing roof is cut out, I can raise the wall, set the rafters, and sheathe the dormer in a matter of hours ready to dry-in with sheathing and paper.
The sidewalls of a shed dormer don't carry any load, so they can either rest on the floor or the roof rafters with no additional framing. Rather than pre-cutting studs beforehand, I frame sidewalls after the new dormer rafters are set.
No matter whether I'm framing a gable or a shed dormer, I like to do as much planning and pre-cutting before opening up the roof. The shorter the time the roof is open before the new roof is tied into the old, the less time I have to spend keeping the house weather-tight. In most cases, I cut a roof open and frame up to a 12-foot-wide gable dormer or a 20-foot-wide shed dormer to the point of being dried-in, in less than two days with just one other worker, and very often in just one day.
–Mike Guertin is a home builder and remodeler in East Greenwich, R.I., and is a member of the JLC Live! and The Remodeling Show construction demonstration teams.