About the Rafter Length Calculator
Calculating rafter length accurately is essential for ordering the right lumber and cutting rafters to fit. The rafter length depends on the building width, the roof pitch, the ridge board thickness, and any overhang (eave) you want to add. This calculator accounts for all of these variables to give you the precise measurement from the ridge to the eave tip.
The calculation starts by determining the horizontal run of the rafter, which is half the building width minus half the ridge board thickness (since each rafter meets the ridge board at its center). The run is then multiplied by the pitch multiplier, a factor derived from the Pythagorean theorem that converts horizontal distance to the actual sloped length along the roof surface. The overhang rafter length is calculated separately using the same pitch multiplier.
When ordering lumber, round up to the next standard length (8', 10', 12', 14', 16', 18', 20') and account for the birdsmouth cut at the wall plate. For steep pitches above 8/12, consider using collar ties or rafter ties to prevent the walls from spreading under load. Always consult your local building code for specific rafter sizing requirements based on your snow load, wind zone, and rafter spacing.