Jack Rafter Length Calculator

Generate a theoretical centerline length schedule for jack rafters along one regular 45-degree hip.

Calculate

Jack rafters are the shortened common rafters that run from the wall plate up to a hip. This tool builds a theoretical centerline schedule for one side adjacent to one regular 45-degree hip. It reports the length of each jack at its station along the wall, and the common difference between consecutive jacks.

Stations, start and end policy

Let the half-span equal the width divided by two. Starting from the corner, jacks are placed at each spacing interval (12, 16 or 24 inches on center, or a custom value) for every station strictly between zero and the half-span. The corner hip itself and the king common rafter at the half-span are not jack stations, and a station landing exactly at the half-span is excluded.

Length and common difference

For a station at distance x along the wall, the centerline length to the wall is x × √(1 + p²), the overhang adds a tail of overhang × √(1 + p²), and consecutive jacks differ by the spacing times √(1 + p²). The table below is generated for one side; mirrored schedules on the other faces do not, by themselves, establish a complete framing count.

What this schedule is not

This is a clean theoretical model. It does not give finished cutting lengths, cheek cuts, birdsmouths, physical hip-thickness deductions, stock optimization or any structural capacity. Use it to understand the pattern and to check a layout, then apply a verified cutting method for real lumber. You can export the schedule as CSV or print it.