Calculating the length of a rafter is a fundamental task in carpentry and roof construction. A rafter is a structural beam that forms the sloping side of a roof. The length of a rafter depends on two primary measurements: the Horizontal Run and the Vertical Rise.
The Math Behind the Calculation
The relationship between the horizontal run, the vertical rise, and the rafter length forms a right-angled triangle. The horizontal run is one leg, the vertical rise is the other leg, and the rafter itself is the hypotenuse. Therefore, we can use the Pythagorean theorem to find the rafter length:
a² + b² = c²
Where:
a = Horizontal Run
b = Vertical Rise
c = Rafter Length (the hypotenuse we want to find)
Horizontal Run: Measure the horizontal distance from the ridge (the peak of the roof) down to the point where the rafter meets the exterior wall (the top of the wall plate). This is often half the width of the building for a symmetrical roof.
Vertical Rise: Measure the vertical height from the top of the wall plate up to the ridge. This is the total "rise" of the roof.
Click the "Calculate Rafter Length" button.
The calculator will output the precise length of the rafter. Remember that this calculation provides the theoretical length. In practice, you may need to add extra for overhangs, birdsmouth cuts, or other roof complexities, depending on your specific building plans. Always consult with a qualified builder or engineer for critical structural decisions.