Rafter Length Calculator
Cutting rafters for a stick-framed roof? Input your roof pitch and run to calculate the exact length of each common rafter, including overhang and ridge board deduction.
Rafter Geometry: Run, Rise, and Pitch Relationship
Every common rafter forms the hypotenuse of a right triangle. The run is the horizontal leg, measured from the building's outside wall to the centerline of the ridge. The rise is the vertical leg, measuring from the wall's top plate to the peak. The rafter itself is the diagonal connecting those two points.
Roof pitch expresses the ratio of rise to run. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. Convert this to a multiplier by dividing: 6/12 = 0.5. Multiply your run in feet by 0.5 to get the rise in feet. A 15-foot run with 6:12 pitch rises 7.5 feet (15 × 0.5).
Once you know run and rise, the Pythagorean theorem gives you rafter length: √(run² + rise²). For our example, √(15² + 7.5²) = √(225 + 56.25) = √281.25 = 16.77 feet. This is the theoretical length before adjusting for overhang and ridge board.
Overhang, Ridge Deduction, and Cutting Details
Overhang extends the rafter beyond the outside wall, creating eaves that protect siding and windows from rain. Measure overhang horizontally from the wall's outer face to the desired fascia location. Common overhangs range from 12 to 24 inches, though architectural styles and climate influence this choice.
The rafter tail (overhang portion) runs at the same slope as the main rafter. To find its length, use the pitch multiplier again. A 12-inch horizontal overhang on a 6:12 roof adds √(1² + 0.5²) = 1.118 feet, or about 13.4 inches of rafter length. Many carpenters use rafter tables printed on framing squares to speed this lookup.
Ridge board deduction accounts for the board's thickness at the peak. A standard 2× ridge board measures 1.5 inches thick, so each rafter stops 0.75 inches short of the ridge centerline. Otherwise, rafters overshoot and prevent proper seating. Measure this deduction perpendicular to the rafter face at the top plumb cut, not horizontally along the building span.
Rafter Size Selection and Span Tables
Rafter size depends on species, grade, spacing, and span. Southern pine 2×8 rafters at 16 inches on center can span roughly 16 feet in a typical residential application with 30 psf live load and 10 psf dead load. Increase spacing to 24 inches and that span drops to about 13 feet. Move to 2×10 and you gain another 4-5 feet of span.
Building codes reference span tables published by lumber grading agencies. These tables account for wood species (Douglas fir, spruce-pine-fir, hemlock) and grade (#1, #2, select structural). Higher grades cost more but allow longer spans with smaller lumber. Many framers default to #2 grade southern pine or SPF as the cost-performance sweet spot.
Cathedral ceilings and vaulted designs eliminate the ceiling joist that normally ties rafter pairs together. Without that tension tie, rafters push outward at the walls under load. Solve this with structural ridge beams, collar ties, or engineered solutions like scissors trusses. Never remove ceiling joists from a rafter roof without engineering input—the walls will spread and the ridge will sag.
Frequently Asked Questions
How do you calculate rafter length from pitch and run?
Use the Pythagorean theorem: rafter length = √(run² + rise²). First find the rise by multiplying the run by the pitch ratio (pitch/12). For a 15-foot run with 6:12 pitch, rise = 15 × 0.5 = 7.5 feet, so rafter length = √(15² + 7.5²) = 16.77 feet.
What is rafter run versus span?
Run is the horizontal distance from the outside wall to the ridge board centerline, typically half the building span. A 30-foot span building has a 15-foot run on each side (assuming centered ridge).
Why do you deduct for ridge board thickness?
The rafter doesn't run all the way to the ridge centerline—it stops at the face of the ridge board. For a 1.5-inch ridge board, deduct 0.75 inches (half the thickness) from each rafter to ensure they meet flush at the ridge.
What size lumber for rafters?
Rafter size depends on span, spacing, and load. Common residential rafters use 2×6 for spans up to 10 feet at 16-inch spacing, 2×8 for 12-14 feet, 2×10 for 16-18 feet, and 2×12 for longer spans. Always check local span tables.
How much overhang should a rafter have?
Typical eave overhangs run 12 to 24 inches. Shorter overhangs (6-12 inches) work in tight urban settings, while 24-36 inches protect walls in rainy climates. Overhangs beyond 24 inches may require additional support brackets.