Online Truss Hip Rafter Calculator - Calculate Hip Rafter Lengths Between Prebuilt Trusses
Enhance your truss construction projects with our Online Truss Hip Rafter Calculator. This tool is expertly designed to calculate the precise lengths of hip rafters needed to fit between prebuilt trusses. Ideal for construction professionals and DIY builders, it ensures that hip rafters integrate perfectly with existing truss structures, promoting structural integrity and aesthetic coherence. Access and print the required measurements for your projects, facilitating accurate and efficient building processes.
How to Use This Tool
Enter the building width and pitch
Type the span of the hip end and the roof pitch. These fix the geometry of the hip corner and the height of the girder truss location.
Set the girder truss setback
Enter how far the girder (the last full common truss) sits from the end wall - typically equal to half the span on a standard hip set. The tool draws the hip corner framing between the girder and the wall corners.
Review hip and jack lengths
Read each hip rafter length with its cheek cuts, and the stepped jack rafters that fill between the hip and the wall plate at your chosen spacing.
Print the measurements
Export or print the dimensioned plan and cut list so the hip end can be stick-framed on site to match the prebuilt truss package exactly.
Hip rafter math between trusses
On a truss roof, the hip end is often stick-framed: the truss plant delivers common trusses up to a girder, and hip rafters run from the building corners up to the girder or to a flat apex. The hip travels diagonally, so its unit run is 16.97 inches for every 12 inches of common run, and its length per foot of common run is sqrt(2 + (pitch/12)^2). The hip rises at the common pitch over its diagonal run, which is why an 8/12 roof puts the hip itself at only about 5.66/12.
Jack rafters between the hip and the plate shorten by a fixed amount each step: the on-center spacing times the common pitch multiplier. Cut the longest jack, subtract the common difference repeatedly, and the whole side is done. The calculator prints that ladder of lengths with the 45-degree cheek cut noted on each.
hip unit run = 16.97 in per 12 in of common run
hip length = common run x sqrt(2 + (pitch/12)^2)
hip pitch = (common pitch) / sqrt(2), e.g. 8/12 common = 5.66/12 on the hip
jack common difference = on-center spacing x sqrt(1 + (pitch/12)^2)
hip drop (lower the hip seat so sheathing planes clear a 1.5 in thick hip) = 0.75 x pitch / 16.97
Worked example: 28 ft hip end at 8/12, trusses 24 in o.c.
- Building width 28 ft, so the common run to the hip corner is 14 ft and the girder truss sits 14 ft from the end wall on a standard hip set.
- Hip rafter length: 14 x sqrt(2 + (8/12)^2) = 14 x sqrt(2.4444) = 14 x 1.5635 = 21.89 ft line length, about 21 ft 10-3/4 inches before shortening for the girder connection.
- The hip pitch is 8/16.97 = 5.66/12, so cut the hip plumb cuts at arctan(8/16.97) = 25.2 degrees, not the 33.7-degree common plumb cut.
- Jack rafters at 24 inches on center: common difference = 24 x sqrt(1 + (8/12)^2) = 24 x 1.2019 = 28.85 inches. Each jack is 28-7/8 inches shorter than its neighbor, with a 45-degree cheek cut where it meets the hip.
- Print the plan and cut list, frame the corner from the longest jack down, and the stick-framed hip end planes in flush with the prebuilt truss tops.
Frequently Asked Questions
Why is the hip end of a truss roof stick-framed at all?
Truss packages often stop the full-profile trusses at a girder truss set back from the end wall, then use progressively shorter flat-top step-down trusses or leave the corner to be field-framed with hip and jack rafters. Field framing the corner is common on smaller jobs because it avoids a series of one-off truss profiles. This calculator gives you the hip and jack dimensions so the field-framed corner matches the truss planes exactly.
Why is the hip rafter cut at a different angle than the commons?
The hip runs diagonally in plan, covering 16.97 inches horizontally for every 12 inches a common covers, while rising the same amount. Same rise over a longer run means a shallower angle: an 8/12 common is 33.7 degrees, but the hip on that roof sits at arctan(8/16.97) = 25.2 degrees. Cut the hip plumb and seat cuts at the hip angle, or the birds mouth will not seat and the top edge will not plane in.
What is a jack rafter common difference?
It is the fixed amount each jack rafter shortens compared to the next as they march toward the corner, equal to the on-center spacing times the pitch multiplier. At 16 inches on center on an 8/12 roof, that is 16 x 1.2019 = 19.23 inches, about 19-1/4 inches. Cut the longest jack, then keep subtracting the difference. Every jack also carries a 45-degree cheek cut where it lands on the hip - the calculator lists the whole ladder.
Do I need to drop or back the hip rafter?
Yes, one or the other, so the sheathing planes cross the hip without a hump. Dropping means deepening the birds mouth so the hip top edge sits below the theoretical line - for a 1.5-inch thick hip, the drop is 0.75 x pitch / 16.97, about 3/8 inch on an 8/12 roof. Backing means beveling both top edges instead. Production framers nearly always drop because it is one deeper seat cut instead of two long rips.
How does the hip connect to the girder truss?
The hip typically lands on the girder face or on a corner of the flat-top region with an appropriate hanger, and the girder is designed to carry that concentrated load - which is why girders are often multi-ply. Use manufacturer-rated hangers, not toenails alone, and never notch a truss chord to seat the hip. IRC R802.10.4 bars unapproved truss alterations, so if the layout requires modifying the girder, get the truss designer involved. Verify connection details locally.
Can I use this calculator for a full stick-framed hip roof too?
The hip and jack math is identical whether the hip meets a girder truss or a ridge board, so the lengths, plumb cut angles, and jack ladder all transfer directly. For a complete stick-framed hip roof you will get more out of the full roof builder, which draws the entire plan from your wall perimeter and exports blueprints for every rafter, valleys included. This calculator is the focused option when only the hip corner is being field-framed.