The shortening allowance of a hip jack rafter at the upper end is what fraction of the 45° thickness of the hip rafter?

The shortening allowance of a hip jack rafter at the upper end is indeed half the thickness of the hip rafter when measured at a 45° angle. This is due to the geometric relationships involved in roofing pitches. When a rafter is set at a 45° angle, the effective length contributing to the load transfer and structural integrity is altered, necessitating an adjustment to ensure proper fit and support.

When analyzing the hip rafter, which typically runs diagonally from the peak of a roof down to the eave, the angle and thickness have implications for how other rafters (such as hip jack rafters) should be cut and fitted. Utilizing half the thickness of the hip rafter simplifies the construction process and helps account for the material that would be ‘lost’ when making the bevel cut at the end of the hip jack rafter. This consideration ensures that the hip jack rafters bear their intended loads without compromising the overall structural integrity of the roofing system.

The selection of other options might stem from a misunderstanding of the relationship between the rafters or incorrect application of the principles of roofing and structural supports. Understanding this allowance is crucial for achieving precise cuts and maintaining the overall stability of the roof structure.