Rafter Calculator

How the Rafter Calculator Sizes a Stick-Framed Roof

This rafter calculator handles the three numbers every framer needs before cutting: rafter line length, total rafter length with the eave overhang, and the count of common rafters required for the building. It works for any standard gable roof — enter the wall-to-wall building width, the building length along the ridge, the roof pitch as rise per 12 in run, the desired eave overhang in inches, and on-center spacing of 12, 16, 19.2, or 24 inches.

Run, rise, and line length — the three rafter geometry inputs

Run is the horizontal distance from the outside of the wall plate to the centerline of the ridge — half the building width. Rise = run × (pitch ÷ 12). Line length = √(run² + rise²) and represents the centerline-to-plumb measurement of the rafter. A small deduction for half the ridge board thickness is built in so the rafter does not come up long when it butts the side of the ridge instead of meeting at its centerline.

Cut allowances: heel cut, seat cut, plumb cut, and tail cut

Line length is not the cut length. From the line length you add the slope distance of the overhang (overhang × √(1 + (pitch/12)²)) to reach the tail, then add a few inches of pencil for the heel cut at the bird's mouth and the plumb cut at the ridge. Most framers cut the first rafter, dry-fit it, and use it as a template — that single pattern rafter is the most important piece on the build.

Choosing Rafter Spacing: 12, 16, 19.2, or 24 inches On Center

• 16 in OC is the residential default for 2×8 to 2×12 rafters at moderate spans under shingle or metal roof finishes.
• 12 in OC is used where IRC span tables demand it — heavy ground snow load (verify with the roof snow load calculator), tile or slate dead load, or rafter runs longer than 16 ft.
• 24 in OC is common with engineered I-joists and deeper LVL ridges, especially under light roof finishes.
• 19.2 in OC matches the 96-inch sheet length of structural sheathing — five rafter spaces per 8-ft sheet.

Rafter vs truss: when site-built rafters still win

Stick framing pays off when the roof has complex geometry the truss plant cannot girder economically, when site access blocks a 60-ft truss delivery, or when an owner wants a cathedral or exposed-beam ceiling. For straightforward rectangular plans, an engineered truss package from the roof truss span calculator is usually faster and cheaper. Both paths can carry the same loads; the choice is logistics and architecture.

Common Rafter Layout Mistakes

• Forgetting the ridge board deduction. Rafters butt the side of a 2× ridge, not its centerline. If you skip the 3/4-inch deduction, the first rafter is 3/4 inch long and the whole roof racks.
• Using pitch “in degrees” in the rise-over-run formula. The formula expects rise per 12 in run (e.g. 6 for a 6:12 roof), not the angle in degrees. Convert with the pitch to degrees calculator if your plans only give an angle.
• Skipping a span check. Rafter length tells you cut length, not whether the lumber section is adequate. Always validate against IRC Table R802.5.1 or an engineered design.

Ridge board, ridge beam, or ridge LVL — what to order?

A ridge board is a 1× or 2× nailer that aligns rafters but carries no load — used on roofs with collar ties or ceiling joists tying the bottom of opposing rafters. A structural ridge beam (typically an LVL or PSL) carries the rafter loads where there is no bottom tie — common in cathedral ceilings. The rafter calculator returns ridge length, but the ridge member type and depth come from the engineered design.