Rafter Calculator
Calculate rafter length and angle for roof framing
Width of the building
Rise per 12" run
On-center spacing
Rafter Length
—
ft
Plumb Cut Angle
—
degrees
Number of Rafters
—
rafters
Ridge Board Length
—
ft
Lumber Cost
—
USD
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Our rafter calculator determines rafter length, plumb cut angles, and material quantities for your roof framing project.
- Measure building span
- Determine roof pitch
- Select rafter spacing
- Enter specifications
- Add 10% for waste and cuts
Expert Insights
Smart analysis for your Framing & Structure project
Stud Spacing Optimization
16" OC for load-bearing, 24" OC allowed for non-bearing. AI optimizes stud count based on wall type and building code requirements.
Header Size Calculator
AI cross-references span and load against IRC Table R602.7 to recommend proper header sizes for your openings.
Structural Tie Requirements
Rafter ties prevent roof spread. Double top plates at wall intersections. AI flags critical structural connections per IRC code.
Calculation Formula
Rafter length = (Span / 2) / cos(arctan(pitch/12))
Plumb cut angle = arctan(pitch/12) x (180/pi)
Rafters = (Ridge length / Spacing) + 1
Frequently Asked Questions
How do I calculate rafter length?
For a 6/12 pitch on a 24 ft span: rafter run = 12 ft, rise = 6 ft, rafter length = sqrt(12^2 + 6^2) = 13.42 ft per side.
What size lumber for rafters?
2x6 for spans up to 10 ft, 2x8 for 10-15 ft, 2x10 for 15-20 ft, 2x12 for 20+ ft. Check IRC span tables.