By Zencalcy Editorial TeamLast reviewed About us
A common rafter runs from the outside of the wall plate to the ridge, plus the overhang. Its length is the rafter run divided by the cosine of the roof angle — the same slope multiplier used for roof area. Getting it right the first time saves a trip to the lumber yard.
Rafter length, ridge height and overhang for a gable roof.
rafter = (width ÷ 2 + overhang) ÷ cos(atan(pitch ÷ 12))
Measured along the top edge from the plumb cut at the ridge to the tail. The ridge height is measured from the top of the wall plate. Deduct half the ridge thickness if you want the finished length, and allow for the birdsmouth seat cut.
| Symbol | Meaning |
|---|---|
| W | Building width outside to outside. |
| p | Rise per 12 inches of run. |
| o | Overhang past the wall. |
Half span plus overhang is 13.5 ft; at 26.57° the rafter is 15.1 ft, and the ridge sits 6 ft above the plate.
Continue with the Roof Pitch Calculator, Roof Truss Calculator, Roofing Calculator, or the Roof Shingle Calculator — or browse more roofing and all construction calculators.
Along the top edge, from the ridge plumb cut to the tail cut. The bottom edge is shorter by the depth of the notch.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026