Jack Rafter Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Jack rafters step down evenly by a common difference: rafter length per foot times spacing in feet. At an 8:12 pitch with 16 in spacing and a 10 ft common run, each successive jack rafter is about 19.2 in shorter than the last, with a 33.7° plumb cut and a 45° side cut.
Key takeaways
- Each jack rafter is shorter than the last by a fixed common difference.
- Common difference = rafter length per foot of run × spacing in feet.
- Plumb cut matches the main roof pitch angle; side cut is 45° on a standard hip.
- Jack rafters still need a birdsmouth cut at the wall plate like common rafters.
Your measurements
Enter the values you know — results update live.
Rise per 12 inches of run.
Center-to-center spacing between jack rafters, typically 16 or 24 inches.
Horizontal run of the full common rafter from wall plate to ridge, at the hip or valley line.
Analysis
Live results from your measurements.
Show the working
- Rafter length per ft of run:
√(12² + 8²)= 14.42 in - Common difference:
14.42 × (16 ÷ 12)= 19.23 in - Common rafter length:
14.42 × 10= 144.22 in - Jack 1:
144.22 − 19.23= 124.99 in - Jack 2:
144.22 − 2 × 19.23= 105.76 in - Plumb cut angle:
atan(8 ÷ 12)= 33.69°
What this calculator does
Jack rafters are the shortened rafters that run from the wall top plate to a hip or valley rafter instead of to a ridge board. Because each jack rafter attaches to the hip at a progressively higher point as it gets closer to the corner, every jack rafter in a sequence is shorter than the one before it by a constant amount called the common difference. This calculator finds that common difference along with individual jack lengths, the plumb cut angle (same as the main roof pitch angle) and the 45° side cut needed where each jack rafter meets the hip or valley.
How to use it
- Enter the roof pitch as rise per 12 inches of run.
- Enter the on-center spacing between jack rafters, usually 16 or 24 inches.
- Enter the common rafter run — the horizontal distance from the wall to the ridge along the hip line.
- Read the common difference: how much shorter each successive jack rafter is than the last.
- Use the listed jack lengths (closest to farthest from the corner) to cut your first four jacks.
- Cut the top of each jack rafter at the plumb angle, then add a 45° side cut (compound angle) to fit flush against the hip or valley rafter.
The formula
Rafter length per foot of run
length per ft = √(12² + pitch²)
Hypotenuse of a 12 in run triangle at the given pitch, giving inches of rafter per foot of run.
Common difference
common difference = length per ft × (spacing in inches / 12)
How much shorter each successive jack rafter is than the one before it.
Worked example
8:12 pitch, 16 in on-center spacing, 10 ft common rafter run.
- Length per ft: √(144 + 64) = 14.42 in
- Common difference: 14.42 × (16 ÷ 12) = 19.22 in
- Common rafter length: 14.42 × 10 = 144.2 in
- Jack 1: 144.2 − 19.22 = 124.98 in
- Jack 2: 144.2 − 38.44 = 105.76 in
- Plumb angle: atan(8 ÷ 12) = 33.69°, side cut 45°
Each jack rafter is about 19.2 in shorter than the last, with the first jack cut to roughly 125 in, a 33.7° plumb cut and a 45° side cut.
Common difference by pitch at 16 in on-center spacing
| Pitch | Length per ft of run | Common difference (16 in O.C.) |
|---|---|---|
| 4:12 | 12.65 in | 16.86 in |
| 6:12 | 13.42 in | 17.89 in |
| 8:12 | 14.42 in | 19.22 in |
| 10:12 | 15.62 in | 20.83 in |
| 12:12 | 16.97 in | 22.63 in |
Common mistakes
- Using the common rafter length instead of subtracting the common difference for each jack.
- Forgetting the 45° side cut and only cutting a plumb cut, leaving a gap at the hip.
- Mixing up the hip rafter's own (steeper) backing angle with the jack rafter's side cut, which stays 45° on a standard hip.
- Measuring spacing along the slope instead of horizontal on-center spacing.
- Not accounting for the hip rafter's thickness when marking the shortest point of each jack.
Assumptions and limits
- The roof has a standard 90° corner with equal pitches on both adjacent roof planes.
- Jack rafters run perpendicular to the wall plate, same as common rafters.
- Side cut angle of 45° assumes a standard (not irregular) hip or valley.
- Rafter length per foot excludes ridge and hip rafter thickness (theoretical length to centerline).
Frequently asked questions
What is a jack rafter?
A jack rafter is a shortened rafter that spans from the wall top plate to a hip or valley rafter instead of running all the way to a ridge board.
What is the common difference in jack rafters?
It is the constant length each successive jack rafter is shorter than the one before it, caused by attaching higher up the sloped hip or valley rafter at each spacing interval.
What angle do you cut jack rafters at?
Cut the top with a plumb cut equal to the roof's pitch angle, then add a 45° side cut (compound miter) so the rafter face sits flush against the hip or valley.
Is the jack rafter side cut always 45 degrees?
On a standard hip roof with equal pitches meeting at a 90° corner, yes — the side cut is 45°. Irregular hips need a different side cut angle.
How do I find jack rafter length without a chart?
Multiply the rafter length per foot of run by the common run in feet for the common rafter, then subtract the common difference once for each step away from the common rafter.
What spacing is typical for jack rafters?
Jack rafters usually follow the same 16 in or 24 in on-center spacing as the rest of the roof's common rafters.
Do jack rafters need a birdsmouth cut?
Yes, same as common rafters, a birdsmouth notch is cut where each jack rafter crosses the wall top plate.
Can I use a speed square for jack rafter cuts?
Yes, a speed square or framing square can mark both the plumb cut angle and the 45° side cut once you know the pitch.
Last updated 2026-10-08. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.