Hip Rafter Calculator - Regular & Irregular Hips in 3D
Explore the capabilities of our Online 3D Hip Rafter Calculator, expertly designed for professionals requiring comprehensive rafter solutions. This tool enables precise calculations for both regular and irregular hip rafters, allowing the generation of detailed 3D models and comprehensive cut lists. Download your customized rafter designs as PDFs for seamless integration into your projects. Ideal for builders, architects, and roofing professionals seeking accurate and efficient rafter planning.
How to Use This Tool
Enter the run and pitch - regular or irregular
Type the common run and roof pitch for a regular hip, or set a different pitch for each roof plane for an irregular hip. The calculator solves the hip geometry either way - including corners where the hip no longer bisects the plan at 45 degrees.
Review the hip in 3D
Orbit the 3D model to check the hip rafter, its backing angles, and how the jack rafters land on each side before anything gets cut.
Check the cut list
Every member lands on the cut list: the hip with its plumb cut and cheek cuts, and the jack rafters with their lengths and side cuts, stepped down at your spacing.
Export the blueprint PDF
Download or print the dimensioned drawings with the angles labeled, and save the design to re-cut the same roof later.
The math behind the hip rafter calculator
A hip rafter travels diagonally across the plan, so its unit run is not 12 inches but 16.97 inches - 12 times the square root of 2 - for every 12 inches of common run. That is why framing squares are used with 17 on the blade for hips: the pitch stays the same rise, but over a 17-inch run. The hip length follows from one formula: common run times the square root of (2 + (pitch/12) squared).
Jack rafters step down evenly along the hip: each one is shorter than its neighbor by the spacing times the common pitch multiplier - the common difference. On an irregular hip the two roof planes have different pitches, the hip leaves the corner at an angle other than 45 degrees in plan, and the cheek cut angles differ side to side; the calculator solves that geometry, which is the part that is genuinely tedious by hand.
hip unit run = 16.97 in per 12 in of common run (regular hip)
hip rafter length = common run x sqrt(2 + (pitch/12)^2)
hip plumb cut = pitch over 17 on the framing square
jack common difference = spacing x sqrt(1 + (pitch/12)^2)
cheek cuts = 45 degrees in plan (regular hip); unequal on irregular hips
Worked example: 24 ft wide hip roof at 6/12
- Building 24 ft wide, full hip at 6/12. Common run = 24 / 2 = 12 ft.
- Hip length: 12 x sqrt(2 + 0.25) = 12 x 1.5 = exactly 18 ft of line length - 6/12 is the rare clean case.
- Hip plumb cut: 6 in of rise over 16.97 in of run = 19.5 degrees. Set the framing square at 6 and 17.
- Cheek cuts at the ridge: a double 45-degree side cut on a regular hip, so the hip noses into the corner between the ridge and the last commons.
- Jack rafters at 16 in on center step down by 16 x 1.118 = 17-7/8 in each - cut one, and each next jack is one common difference shorter.
- The calculator draws all of it, applies the ridge and hip deductions, and prints each rafter with its angles on the blueprint.
Frequently Asked Questions
How do you calculate hip rafter length?
Multiply the common run by sqrt(2 + (pitch/12)^2). For a 12 ft run at 6/12 that is 12 x 1.5 = 18 ft; at 8/12 it is 12 x sqrt(2.444) = 18.76 ft. The formula works because the hip runs diagonally: its run is the common run times the square root of 2, and its rise is the same as the commons. Deduct for the ridge and add the tail the same way you would for a common rafter, measured along the hip.
Why do framers use 17 on the square for hips?
Because the hip's unit run is 16.97 inches - the diagonal of a 12 x 12 inch square - for every foot of common run. Rounding to 17 keeps the same rise number usable: a 6/12 roof means 6-in-12 for commons and 6-in-17 for the hip. That one substitution converts every common-rafter layout technique to hips, which is why the pair of numbers is stamped into a century of framing squares.
What is the difference between a regular and irregular hip?
A regular hip joins two roof planes of the same pitch, so the hip bisects the corner at 45 degrees in plan and both cheek cuts match. An irregular hip joins planes of different pitches - say 8/12 meeting 4/12 - so the hip shifts toward the shallower side, the cheek cut angles differ, and the jack lengths differ side to side. This calculator handles both; irregular is where hand layout usually goes wrong.
What is hip backing, and do I need to drop the hip?
The hip's square top edge sits proud of the two roof planes it joins, so sheathing would rock on the corner. You either back the hip - bevel its top edges to match both planes - or drop it, cutting the birdsmouth slightly deeper so the edges land flush in plane. Backing angles depend on both pitches; the calculator reports them, which matters most on irregular hips where the two bevels differ.
How much shorter is each jack rafter?
By one common difference: the on-center spacing times the common pitch multiplier. At 16 in on center on a 6/12 roof, each jack is 16 x 1.118 = 17-7/8 in shorter than its neighbor, measured along the rafter. Cut the longest jack, then use it to mark the rest. Every jack also gets a cheek cut where it meets the hip - 45 degrees in plan on a regular hip.
How do you calculate a hip roof?
Work from the plan: commons come from half the building width times the pitch multiplier, hips from the diagonal formula (run x sqrt(2 + (pitch/12)^2)), and the ridge of a simple rectangular hip roof is the building length minus its width. Roof area for sheathing and shingles is the plan area times the pitch multiplier - a 24 x 32 building at 6/12 carries about 859 sq ft of roof surface.
What size lumber do hip rafters need?
Hips are typically one size deeper than the commons they collect - 2x10 hips over 2x8 commons is a common pairing - because jack loads accumulate along them, and the deeper stock also gives the jacks' cheek cuts full bearing. Spans and loads still govern: check your species and grade against the span tables and your snow load, and verify with your building department like any structural member.