Rafter Calculator
Calculate precise rafter dimensions for your roof framing project. Our calculator determines rafter length, rise, run, and construction materials needed based on your building span, roof pitch, and specifications for accurate roof construction.
Last updated: December 15, 2024
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Quick Example Result
For a typical roof project (20ft span, 6/12 pitch, 1ft overhang):
12.2 feet rafter length needed
How This Calculator Works
Our rafter calculator uses fundamental geometry and construction principles to determine accurate rafter dimensions. The calculation considers building span, roof pitch, wall height, and overhang requirements to provide construction-ready measurements for your roof framing project.
The Formula
rafter_length = √(rise² + run²) + overhangThis Pythagorean theorem application calculates the hypotenuse (rafter length) using the rise and run measurements. The run equals half the building span minus half the ridge thickness.
Shows rise, run, pitch angle, and rafter length relationship
Construction Principles
Proper rafter calculation ensures structural integrity and code compliance. The calculator accounts for standard construction practices including ridge board thickness, birdsmouth cut specifications, and lumber sizing requirements based on span and loading conditions.
- Accounts for ridge board thickness in run calculations
- Calculates proper birdsmouth cut dimensions for secure wall plate connection
- Determines lumber requirements based on standard spacing (16" or 24" O.C.)
- Provides material cost estimates using current lumber pricing
- Supports multiple pitch input methods (ratio, degrees, rise/run)
Building Codes & References
- International Residential Code (IRC) - Standard requirements for residential roof framingChapter 8 covers rafter sizing, spacing, and connection requirements
- American Wood Council (AWC) - Span tables and load calculations for dimensional lumberTechnical resources for proper rafter sizing and spacing
- National Association of Home Builders (NAHB) - Construction best practices and guidelinesIndustry standards for residential roof construction
Planning other construction projects? Check out our stone calculator and electrical panel upgrade calculator.
Get Custom Calculator for Your BusinessExample Calculation
Project Specifications:
- Building Span: 20 feet
- Roof Pitch: 6/12 (6" rise per 12" run)
- Wall Height: 8 feet
- Overhang: 1 foot
- Ridge Board: 1.5" thick
Calculation Steps:
- Run = (20 ÷ 2) - (1.5 ÷ 2) = 9.25 feet
- Rise = (6 ÷ 12) × 9.25 = 4.625 feet
- Base length = √(9.25² + 4.625²) = 10.35 feet
- Total length = 10.35 + 1.0 = 11.35 feet
Result: Each rafter needs to be 11.35 feet long with a ridge height of 12.625 feet
This calculation provides the exact measurements needed for cutting and installing your rafters.
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