Mike Guertin, a contributing editor for Fine Homebuilding, replies: There are several ways to tie a shed dormer into an existing roof. The method you choose depends on a lot of factors: the depth of the existing rafters, the pitch of the roof, the width of the dormer, substructure support opportunities, aesthetic preference and any other details unique to your situation.
But let’s look at two dormer examples (see drawings). First, a shed dormer can tie into the existing roof either at the ridge or into the roof framing below the ridge. This choice is primarily aesthetic, and the structural details can be modified to accommodate your decision.
Dormers that originate at the ridge are easiest to frame. If the ridge is structural, you can make the dormer as wide as you want, the width limited only by the carrying capacity of the ridge. Collar ties can be situated at the desired ceiling height or omitted altogether, provided that the existing rafters and dormer rafters are sized properly for the span and loads.
When the ridge isn’t structural, collar ties are usually necessary between the dormer rafters and the existing rafters on the opposite side of the roof. These collar ties have to be sized and attached to resist the outward thrust acting on the front wall of the dormer.
Dormers that intersect the roof plane below the ridge can be framed in a couple of ways, but the most common method begins with a header inserted between doubled common rafters on the sides of the dormer at the point where the dormer roof meets the existing roof. Cripple rafters are cut and attached between the ridge and the header. The shed-roof rafters then fasten to the other side of the header.
This framing alternative is usually best for dormers up to 6 ft. wide depending on the size and length of the existing rafters. However, that width can be increased with properly sized and engineered support framing. Again, collar ties may be required depending on the particulars of your roof.
Metal framing connectors should also be used wherever shed-roof rafters attach to either a structural ridge or the header. And with any type of shed dormer, it’s always a good idea to let a structural engineer see the plans before you start tearing off shingles.