Framing gabled dormers and shed

Dormers are very popular in homes on the east coast. They are used to allow more space in a steeply sloped roof system. For most dormers, you will need to frame the roof first, then lift the gables and place the ridge beam. It is also important to understand where the roof load is designed to distribute the weight over the joists and floor joists. You will then know where your support points are so that you can install additional truss girders to carry the load and transfer it to the foundation of the house.

Framing gabled dormers

Gable dormers are built perpendicular to the existing roof plane, extend to the edge of the existing roof line, and include a gable roof. Gable dormers are designed so that the weight of the dormer beams is carried through the side walls and onto the existing roof beams. Knowing this, you will want to double or triple the roof rafters on each side of the new dormer so that the dormer side walls have strong support to rest on. By doing this, you will not have to add additional joists under the attic subfloor.

Similar to framing a new skylight, you will need to cut and cap the existing rafters that go through the dormer opening. You can use a jigsaw to get the job done.

If you are framing a wide gabled dormer, you can expect the cargo to require additional attention. You may need to use steel I-beams in the flooring system to support the roof. You may also need to add additional joists to the first story walls to help carry the weight to the foundation. You can do all of this before cutting the roof to limit the amount of time your home is exposed to outside elements.

Shed dormers

Framing a shed dormer is easier than framing a gabled dormer. The entire weight of the shed dormer is placed on the end wall rather than the side walls, so there are no loading issues if the end wall is framed on top of the exterior walls of the house. If the dormer end wall is located away from the exterior wall, the floor joist will need to be doubled or tripled to provide adequate support.

The top of the shed dormer may meet the main ridge or existing roof. If the top is below the ridge, you will need to install a header to support the load of the dormer joists. The header must be the correct size to support the tail joists and connect the joists and ridge. The dormer side walls of a shed do not support any dormer weight, so you will not need to reinforce the joists on either side under the side walls.

To save on the amount of time your home is open, try to have all of your supplies pre-cut. You should be able to frame a gable or shed dormer in less than a day.