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Roofing Calculations
ROOF FRAMING DEFINITIONS
Ridge: The horizontal line running the length of the roof where the two roof planes meet. It is the
uppermost part of the roof that the rafters connect at. (Sometimes referred to as the king rafter)
Plate: The horizontal wall member that the ceiling joist and rafters sit on and fasten to.
Rise: The vertical distance measured from the the top of the plate to the top of the ridge.
Span: The horizontal distance measured between the outside edges of the wall’s top plates.
Run: The horizontal distance between the outside edge of the wall’s top plate and the center of the
ridge. In most cases, where the pitfch of the roof is the same on both sides, the run is half of the span.
Pitch / Slope: Angle of the rafters in relation to the plate.
It can be written in two formats:
Ratio of unit rise to unit run
(expressed as inches of rise over a foot of run)
Example: 7/12 or 7 Inch
or
Angle of rafters, in degrees
Example: 30.26˚
Rafters Types:
• Common Rafter: Connects the plate to the ridge and is perpendicular to the ridge.
• Hip Rafter: The hip rafter extends from the corner of two wall plates to the ridge at an angle other
than 90˚. The hip rafter is an external angle of two planes.
• Valley Rafter: The valley rafter extends from the corner of two wall plates to the ridge at an angle
other than 90˚. The valley rafter is an internal angle of two planes.
• Jack Rafters: Rafters that connect the hip or valley rafter to the wall plate.
• Irregular Hip/Valley Jacks: Jack rafters found in dual pitch or irregular roofs.
Hip Rafters
Ridge
Valley Jack Rafter
Valley Rafter
Hip Rafter
Rake Wall
Common Rafters
Hip Jack Rafter
Ridge
Rafter
Plate
Pitch
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