What Is a Roof Rafter?

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Rafters are the support members for roofing systems.
Rafters are the support members for roofing systems. (Image: Thinkstock/Comstock/Getty Images)

Roof rafters are structural components of a building's roof. Consider them the underlying framework that supports the roof decking and shingles. Any building with a roof is likely to have rafters, although the design of the rafter system can vary and depends on the type of building.

Supporting the Rafters

The rafter rests on a supporting wall, which transfers the weight of the roof to the building's foundation. The rafters commonly rest on the exterior walls of the building but may also be supported by load-bearing interior walls. The rafter is set at an angle on a sloped roof and horizontally on a flat roof.

At the Ridge

Some rafter systems use a ridge beam, while other systems attach the two sides of the rafter to each other at the peak or ridge of the building. If a ridge beam is used, it is commonly built in place first with supporting end rafters. The remaining rafters are then fastened individually to the ridge beam and plate at the top of the supporting exterior walls.

Trusses

A truss rafter is built separately from the building and set in place as an individual unit. Truss rafters can be purchased prebuilt or constructed on site. The truss rafter includes internal bracing between a ceiling joist and the top edge of the rafter. This makes the truss rafter self-supporting, requiring support on only the two exterior walls. Trusses are heavier than rafters and must be lifted into place by mechanized equipment or a large crew.

Calculating Slope

The slope of a roof utilizes two measurements: the horizontal distance the rafter covers and the rise from the level at the top of the wall to the peak of the roof. If the roof building measures 10 feet from the wall to a point directly under the peak, and 6 feet from the peak of the roof to a line between the tops of the walls, the roof would have a rise of 6 feet and a run of 10.

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