Rafters are the boards that hold the roof of a building above the attic. They distribute the weight of the roof to the walls of the building without requiring vertical posts that would limit attic space. All stick-built roofs have common rafters and most stick-built roofs have hip rafters. More complex roof shapes use a combination of the common and hip rafters in addition to any of the five other kinds of rafters.
A common rafter is any rafter that goes from the wall of a building to the ridge board. In a gable end roof, common rafters are the only type of rafters used. A gable end roof is a roof where the ridge board runs the full length of the house and each end of the roof is a triangle that continues in the same plane as the wall.
A hip rafter forms the outside corner of a roof. Hip rafters are installed at a 45-degree angle to common rafters.
Hip jacks are rafters that run from an outside wall of the building to the hip rafter. Jacks bear that name for the simple reason that they support, or jack up, the rafters. Hip jacks can run parallel or perpendicular to the common rafters.
A standard hip roof has a ridge board shorter than the length of the house. Four hip rafters connect the two ends of the ridge board to the corners of the house. Common rafters run from the front and rear walls of the house to the ridge board. Hip jacks rang from the front, rear, and side walls into the hip rafters.
Other Rafter Types
Roofs that are more complex require additional types of rafters. A valley rafter defines the inside corner of a roof. Valley jacks run between the valley rafter and the ridge board. A cripple jack rafter connects a valley rafter to a hip rafter. The cripple jacks are only found in an inside corner. A flying hip rafter connects a lower ridge board to a higher ridge board.