Dormers are very popular in homes on the East coast. They are used to allow for more space in a steep pitched roofing system. For most dormers, you will need to frame the roof first and then raise the gables and set the ridge beam. It is also important to understand where the roof load is designed to distribute the weight on the rafters and floor joists. Then you will know where your bearing points are so you can install extra framing beams to carry the load and transfer down to the home’s foundation.
Framing Gable Dormers
Gable dormers are built perpendicular to the existing roof plane, extends out the edge of the existing roof line and includes a gable roof. Gable dormers are designed to have the weight of the dormer rafters carried through the side walls and on to the existing roofs rafters. Knowing this, you will want to double or triple the roof rafters on either side of the new dormer so the side walls of the dormer have a strong support to rest on. By doing this you will not have to add additional joists under the attic sub-floor.
Just like framing a new skylight, you will need to cut and head the existing rafters that run through the opening of the dormer. You can use a recip saw to do the job.
If you are framing a wide gable dormer, you can expect the load to require extra attention. You may need to use steel I-beams on the floor system to hold up the roof. You may also need to add extra beams in your first floor walls to help carry the weight to the foundation. You can do all of this before you cut into your roof to limit the amount of time your home is exposed to outside elements.
Framing a shed dormer is easier than framing a gable dormer. All the weight of the shed dormer is placed on the end wall instead of the side walls so there is no load-bearing issue if the end wall is framed on top of the exterior walls of the home. If the end wall of the dormer is located away from the exterior wall, the floor joist will need to be doubled or trippled to provide adequate support.
The top of the shed dormer can either meet with the main ridge or existing roof. If the top meets below the ridge, you will need to install a header to support the load of the dormer rafters. The header must be correctly sized to carry the tail joists and connect the rafters and the ridge. The side walls of a shed dormer do not carry any weight of the dormer so you will not need to reinforce the rafters on either side under the sidewalls.
To save on the amount of time your home is open, try and have all of your materials pre-cut. You should be able to frame a gable or shed dormer in less than a day.