🪵 Rafter Length Calculator
Calculate rafter length from span and pitch. Includes overhang, birdsmouth cut depth, and total rafters needed with spacing.
How to Use This Calculator
Enter the building span (outside wall to outside wall) and select the roof pitch. Set the overhang length in inches and the ridge board thickness. Choose rafter spacing — 16" on center is standard for most residential construction. Enter the building length to calculate total rafter count.
Understanding Your Results
Rafter length is calculated from the ridge to the wall plate using the Pythagorean theorem. The tail (overhang) is added separately. Birdsmouth seat depth should not exceed one-third of the rafter depth to maintain structural integrity. Total rafter count includes both sides of a gable roof.
Frequently Asked Questions
How do I calculate rafter length?
Rafter length is calculated using the Pythagorean theorem. The run is half the building span (minus half the ridge board), and the rise is the run multiplied by the pitch. Rafter length = √(run² + rise²). Add overhang length separately.
What is a birdsmouth cut?
A birdsmouth is a notch cut in the rafter where it sits on the wall plate. It has two parts: the seat cut (horizontal, rests on the plate) and the plumb cut (vertical). The seat cut should not exceed 1/3 of the rafter depth.
Should rafters be 16 or 24 inches on center?
16 inches on center is standard for most residential roofs and provides better support. 24 inches on center is acceptable for lighter loads and reduces material costs. Check local codes and load requirements.
How many rafters do I need?
Divide the building length by the spacing (16" or 24") and add 1. Remember you need rafters on both sides of a gable roof, so double the count. Add extra for gable end rafters.