Source: TOOLS OF THE TRADE Magazine
Publication date: July 11, 2006
By TOOLS OF THE TRADE Editors
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.
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