Rafter Length Calculator
Calculate rafter length accurately for your build. Get material quantities, waste allowances, and project cost breakdowns.
Formula
Rafter = sqrt(Run² + Rise²) - Shortening + Tail
The run is half the building width. The rise equals run times pitch divided by 12. The rafter line length is the hypotenuse of the run-rise right triangle. Subtract the ridge shortening (half ridge thickness along the slope). Add the tail length (overhang divided by cosine of roof angle) to get the total rafter length including overhang.
Frequently Asked Questions
How do I calculate rafter length from building width and pitch?
Divide the building width by 2 to get the run (horizontal distance from the outside wall to the center of the ridge). Multiply the run by the pitch ratio (rise per 12 inches of run divided by 12) to get the total rise. The rafter line length is the hypotenuse: square root of (run squared plus rise squared). Then subtract the shortening at the ridge (half the ridge board thickness divided by cosine of the roof angle) and add the tail length for any overhang. The tail length equals the overhang distance divided by cosine of the roof angle.
What is the rafter shortening at the ridge?
When rafters meet at a ridge board, they do not extend all the way to the centerline of the building. Each rafter must be shortened by half the thickness of the ridge board, measured horizontally. For a standard 2x ridge board (1.5 inches actual), each rafter is shortened by 0.75 inches measured horizontally. This horizontal dimension must be converted along the rafter slope by dividing by the cosine of the roof angle. For a 6/12 pitch, the shortening along the rafter is 0.75 / cos(26.57) = 0.839 inches.
How do I determine what lumber length to buy for my rafters?
Calculate the total rafter length including the tail (overhang) and round up to the next standard lumber length. Standard dimensional lumber comes in 8, 10, 12, 14, 16, 18, and 20 foot lengths. Add 6 to 12 inches to your calculated length for trimming and cutting waste. For example, if your total rafter length calculates to 13 feet 4 inches, purchase 14-foot boards. For longer rafters, some lumberyards stock 22 and 24 foot lengths, but these may need to be special ordered and are more expensive per board foot.
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Is Rafter Length Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.