Rafter Length Calculator
Calculate the length of a common rafter from building span, roof pitch, and overhang. Uses the Pythagorean theorem with standard ridge drop adjustment.
Roof Framing Inputs
Formulas
Rafter run
Run = (Span ÷ 2) − (Ridge_thickness ÷ 2 ÷ 12) Half the span, minus half the ridge board thickness (converted to feet). This is the horizontal distance the rafter must cover.
Rafter rise
Rise = Run × (pitch_rise ÷ 12) Vertical height from the plate to the top of the rafter at the ridge.
Structural rafter length (Pythagorean theorem)
Length = √(Run² + Rise²) The hypotenuse of the right triangle formed by run and rise. Equivalent to Run × pitch_factor.
Ridge drop
Ridge drop = (Ridge_thickness ÷ 2) × (Rise ÷ Run) How much lower the ridge board’s top edge sits relative to the calculated peak height, so rafter tops are flush. Required for hip roofs where ridge height must align with hip rafters.
Related Tools
- Roofing Calculator — full material estimator from footprint and pitch
- Roof Pitch Calculator — convert pitch to degrees
- Roof Area Calculator
Frequently Asked Questions
A common rafter is a structural roof member that runs from the wall plate (top of the wall) to the ridge board at the peak of the roof, perpendicular to both. Common rafters are the most frequent type in simple gable and hip roofs.
Rafter run is the horizontal distance the rafter covers — typically half the building width for a symmetric gable roof, measured from the outside wall to a point directly below the ridge.
Using the Pythagorean theorem: rafter length = √(run² + rise²). Since rise = run × (pitch rise ÷ 12), you can also write it as: rafter length = run × √(1 + (rise/12)²), which equals run × pitch_factor.
A birds mouth is a notch cut into the bottom edge of a rafter where it sits on top of the wall plate. It creates a flat bearing surface for the rafter to rest on the wall. The seat depth is typically the width of the top plate (1.5″ for a 2× wall).
The rafter's structural length typically ends at the outside of the wall. The overhang (tail) extends beyond the wall and is calculated separately. This calculator provides both the structural rafter length and the total length including a specified overhang.