Hip Rafter Calculator

About the Hip Rafter Calculator

Hip rafters run diagonally from the wall corner to the ridge on a hip roof, creating the characteristic sloped edges at each end of the building. Calculating hip rafter length requires understanding that the hip travels at a 45-degree angle in plan view while also climbing the pitch of the roof.

This hip rafter calculator computes the line length of a hip rafter from the common rafter run and roof pitch. For equal-pitch roofs (where both roof planes have the same slope), the hip rafter run is the common rafter run times the square root of 2, and the hip pitch per foot is rise per 17 inches of run rather than per 12 inches.

The calculator also provides the backing angle (the bevel cut along the top edge of the hip rafter) and the compound miter angles for the plumb and cheek cuts at the ridge. These are the trickiest cuts in roof framing, and having accurate numbers prevents costly mistakes.

When This Page Helps

Hip rafter geometry involves compound angles that are difficult to calculate by hand. This calculator gives you the exact rafter length and cut angles, preventing the trial-and-error approach that wastes time and material on hip roofs.

How to Use the Inputs

1. Enter the common rafter run (horizontal distance from wall plate to ridge center).
2. Enter the roof pitch (rise per 12 inches of run).
3. Enter the overhang distance (measured horizontally).
4. Read the hip rafter line length, total length with overhang, and cut angles.
5. Use the backing angle to set your saw for the top-edge bevel.

Example Calculation

Tips & Best Practices

• Hip rafters are typically one size deeper than common rafters to maintain structural depth at the hip line.
• The hip rafter backing bevel is cut along the top edge so the roof sheathing lies flat across the hip.
• Use a framing square with the 17-inch mark (instead of 12) for hip rafter layout.
• For equal-pitch roofs, the hip always uses 17 as the run constant on the framing square.
• Pre-cut and test-fit one hip before cutting all four—compound angles leave no room for error.
• Some framers prefer to drop the hip rafter instead of backing it, lowering it by the backing height.

Hip Roof Geometry Basics

A hip roof has sloped ends instead of vertical gables. The hip rafter forms the diagonal line where two adjacent roof planes meet. For a rectangular building with equal pitches on all sides, the hip sits at exactly 45° in plan view, making the geometry predictable and the calculation straightforward.

Jack Rafter Layout

Jack rafters are shorter rafters that run from the wall plate to the hip rafter. They decrease in length at a constant increment (the common difference) as they approach the corner. The common difference equals the jack rafter spacing divided by cos(pitch angle) for each increment.

Irregular Hip Roofs

When the two roof planes meeting at a hip have different pitches, the hip rafter no longer sits at 45° in plan. The geometry requires solving the hip angle from the two different pitches and recalculating the hip length using trigonometry specific to unequal pitches.

Frequently Asked Questions

• Why use 17 instead of 12 for hip rafter layout?

  The hip rafter travels diagonally in plan at 45° to the common rafters. For every 12 inches the common rafter runs, the hip runs 12×√2 = 16.97″, rounded to 17 inches. So hip pitch is expressed as rise per 17 inches of run.

• What is the backing angle on a hip rafter?

  The backing angle is a bevel cut along the top edge of the hip rafter. Since the hip sits between two sloped roof planes, its top edge must be beveled so sheathing lies flat. The backing angle depends on the roof pitch.

• Can I drop the hip instead of backing it?

  Yes. Dropping the hip means lowering the birdsmouth cut by the backing height amount. This achieves the same result as beveling the top edge but is simpler to execute. The drop distance equals the hip backing height.

• How do I connect the hip rafter at the ridge?

  The hip rafter connects at the corner of the ridge, typically with a compound cheek cut that fits against both the ridge board end and the last common rafter. Some framers use a plumb cut with a short return.

• What size should a hip rafter be?

  Hip rafters should be at least one size deeper than common rafters because they carry a larger tributary area. For 2×8 commons, use a 2×10 or 2×12 hip. Always check with span tables or an engineer.

• Does overhang affect hip rafter length?

  Yes. The overhang adds length along the hip slope. The overhang on the hip is the common rafter overhang times √2 (about 1.414), adjusted for the pitch angle.