Rafter Length Calculator
Accurate rafter length is fundamental to roof framing. Too short and the rafter won't reach the ridge; too long means waste and recutting. This calculator determines the total rafter length from building span, roof pitch, and eave overhang, accounting for the ridge board thickness.
The rafter forms the hypotenuse of a right triangle where the run (horizontal distance from wall to ridge) and rise (vertical height gained) are the two legs. The formula is: Rafter Length = √(run² + rise²) + overhang rafter.
The run is half the building span minus half the ridge board thickness (the ridge board sits between opposing rafters). The rise equals the run multiplied by the pitch factor (pitch ÷ 12). The overhang extends the rafter past the wall, calculated the same way using the overhang distance as additional run.
The calculator also provides the plumb cut angle (for cuts at the ridge and tail) and the seat cut angle (for the birdsmouth where the rafter sits on the wall plate). These angles are complements; they add up to 90 degrees. A speed square or framing square set to the pitch ratio produces these same angles.