How to use the hip rafter calculator
- Common run is half the building width, the same run you'd use for the common rafters. On a regular hip roof the hip runs diagonally from the corner of the walls to the end of the ridge.
- Pitch is the pitch of the common rafters. The hip's own slope is shallower because it travels farther (diagonally) to reach the same height.
- Jack O.C. is the spacing of the jack rafters along the wall plate, usually the same as the commons.
Why the hip uses 17 instead of 12
For every 12" a common rafter runs, the hip travels the diagonal of a 12" square in plan: 12 × √2 = 16.97", which framers round to 17". Rise is the same for both. So a 6/12 roof has a hip pitch of 6 in 17, and the hip length per foot of common run is √(16.97² + 6²) = 18.00". That's the second line of the framing square's rafter table.
To mark the hip plumb cut with a framing square, use 17 on the blade and the pitch (6) on the tongue. On a speed square, pivot to the pitch on the hip/val scale. The calculator gives the angle in degrees for a bevel gauge or digital square.
Jack rafters and the common difference
Each jack is shorter than the one beside it by the same amount: the O.C. spacing times the common rafter factor. At 6/12 and 16" O.C. that's 16 × 1.118 = 17.89" (17 7/8"), which matches the "difference in length of jacks, 16-inch centers" line on a framing square. The table lists every pair from the longest down to the corner.
Deductions before you cut
- Hip at the ridge: shorten by half the 45° thickness of the ridge (or common rafters, depending on how your hip frames in), measured level.
- Jacks at the hip: shorten each jack by half the 45° thickness of the hip. For a 1½" hip that's about 1 1/16", measured level. Then make the 45° side (cheek) cut.
- Hip at the plate: the hip seat must be dropped or backed so its top edges sit in plane with the roof.
Valleys use the same lengths and angles as hips. A valley's jacks run from the valley up to the ridge.
FAQ
How do you calculate hip rafter length?
Multiply the common run (in feet) by the hip length per foot of run, √(16.97² + pitch²). For a 12' run at 6/12 that is 12 × 18.00 = 216", or 18'-0". Then make deductions for the ridge.
Why is hip rafter pitch 17 instead of 12?
A hip runs diagonally across the corner, so for every 12" of common run it travels 12 × √2 ≈ 16.97" in plan. Its pitch is written as rise in 17, for example 6 in 17 for a 6/12 roof.
What is the common difference of jack rafters?
It's how much shorter each jack is than the next. It equals the O.C. spacing times the common rafter slope factor: 17⅞" at 6/12 on 16" centers, or 26 13/16" at 6/12 on 24" centers.
Are valley rafters the same length as hip rafters?
For a regular roof with equal pitches, yes. Hip and valley rafters share the same run, length per foot and plumb angle. They differ only in how the ends and top edges are cut.
Does this work for unequal pitch roofs?
No. This calculator assumes a regular hip with the same pitch on both sides, so the hip runs at 45° in plan. Irregular (bastard) hips need different math.