Hip & Jack Rafter Cut Calculator

How to use it: enter your roof pitch (as rise and run, e.g. 6 and 12 for a "6 in 12" roof), the common rafter's run (wall plate to ridge), and your jack rafter spacing. This models one 45° hip corner - the most common case, where two walls meet at a right angle with the same pitch on both sides.

Formula. With the corner at the origin, the hip rafter runs to the point directly below where both common rafters would reach the ridge - the diagonal of a square corner, so its horizontal run is √2 times the common run:

Hip run = common run × √2, Hip length = √(hip run² + total rise²)

A jack rafter starting a distance a from the corner runs parallel to the common rafters and meets the hip exactly when its run equals a (since the hip's plan line is the corner's diagonal) - so jack length is simply proportional to its distance from the corner, forming an evenly stepped sequence:

Jack length = position × (common rafter length ÷ common run)

Worked example. With the defaults (6-in-12 pitch, 3600 mm common run, 600 mm jack spacing): total rise = 3600 × (6÷12) = 1800 mm; common rafter length = √(3600² + 1800²) ≈ 4024.9 mm. Hip run = 3600 × √2 ≈ 5091.2 mm; hip length = √(5091.2² + 1800²) = 5400 mm. The common difference is 600 × (4024.9 ÷ 3600) ≈ 670.8 mm, so the five jacks between the corner and the ridge run 670.8, 1341.6, 2012.5, 2683.3 and 3354.1 mm.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Roof pitch rise

6

Roof pitch run

12

Common rafter run

3600 mm

Jack rafter spacing

600 mm

Primary result: Hip rafter length: 5400 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This calculates rafter lengths and plumb-cut (vertical) angles only - it does not compute the hip rafter's backing angle (the bevel that keeps its top edge flush with both roof planes) or the jack rafters' side/cheek cuts (the compound angle where each jack meets the hip). Those are genuine compound-angle cuts that a framing square, rafter table, or dedicated framing software will get you more reliably than a generic formula here - always verify hip and jack cut angles against a rafter table or a local carpenter before cutting expensive lumber.

It assumes a simple 45° hip corner (two walls of equal pitch meeting at a right angle) and doesn't model irregular hips (unequal pitches or non-90° corners), overhang/tail length beyond the wall plate, ridge board thickness offsets, or seismic/wind connector requirements.