Framing a Bay-Window Roof
It's only a small hip roof, but its 45-degree corners befuddle many carpenters. Drawing bay roofs out full scale is key to fast and accurate framing.
Synopsis: Framing a hipped roof for a bay window is harder than it looks. This article explains how to lay out framing members with the help of full-scale graphic templates.
For as long as architects have been drawing bay windows, carpenters have been scratching their heads about framing the roofs. Victorian builders sometimes got around the problem by letting two-story bays die into a projection of the main roof above. Tract builders in the 1950s did likewise by tucking bay windows under overhanging second floors or wide eaves.
When a bay bumps out on its own, however, it needs a miniature hip roof to keep out the elements. Because the corners of bay windows aren’t square, neither is the roof above, and figuring the rafter cuts isn’t straightforward. I’ve built many of these roofs and have worked out a system that does the job without guesswork.
The skeleton of a bay roof breaks down into two parts. The first part is the cornice, an assembly of horizontal lookouts tied together by subfascias. (A subfascia will receive a finished material, in this case aluminum coil stock. If you’re planning to install a painted wood fascia, you can substitute the finished fascia material for the subfascia.) A horizontal ledger carries the lookouts where they attach to the building.
The common rafters for the middle roof, hip rafters and jack rafters comprise the second part of the skeleton. The side roofs also require sloped ledgers to support the sheathing where it meets the building.
Bays can be site-framed or manufactured units. When the walls of a bay are framed on site, the horizontal lookouts double as ceiling joists. In that case, the lookouts bear directly on the wall’s top plates.
When I’m installing a manufactured bay, as…