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Free rafter length calculator for roof construction. Calculate rafter length, cutting angles, birdsmouth dimensions, and material requirements with professional accuracy. Our calculator provides precise measurements for safe and efficient roof construction.
Last updated: February 2, 2026
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Common pitches: 4/12 (18.4°), 6/12 (26.6°), 8/12 (33.7°), 12/12 (45°)
Common spacing: 12", 16", 19.2", 24" on center
Rafter Length
133.33"
Rafter Angle
26.57°
Includes 12" extra for cuts and waste
Construction Guidelines:
Application
Gable Roofs
Most common type, runs from ridge beam to wall plate
Application
Hip Roofs
Runs from ridge to corner, requires special cutting angles
Application
Valley Roofs
Supports roof intersection, requires precise measurements
Application
Hip/Valley Systems
Graduated lengths, requires careful layout and cutting
For a 240" building width with 6/12 pitch and 16" spacing:
Rafter Length
133.33"
Rafter Angle
26.57°
Our rafter length calculator uses geometric principles and construction standards to provide accurate measurements for roof construction. The calculation considers building width, roof pitch, rafter spacing, and overhangto determine the optimal rafter dimensions and cutting angles for safe, efficient roof construction.
Run = (Building Width - Ridge Thickness) ÷ 2Rise = Run × (Pitch ÷ 12)Rafter Length = √(Run² + Rise²)Total Length = Rafter Length + OverhangThis formula uses the Pythagorean theorem to calculate the actual rafter length needed, accounts for ridge thickness, and includes overhang to determine total material requirements. The calculation ensures structural integrity and code compliance.
Shows run, rise, and rafter length relationship
Rafter construction must comply with local building codes for safety and structural integrity. Key requirements include proper spacing, adequate lumber sizing, correct connections, and appropriate load calculations. Our calculator helps ensure compliance while providing optimal material efficiency and construction safety.
Need help with other construction projects? Check out our stair stringer calculator and baluster calculator.
Get Custom Calculator for Your PlatformResult: 133.3" rafter length, 26.6° cutting angle, 16 rafters needed
Material needed: 145.3" (12' 1.3") per rafter including overhang and waste factor.
8/12 pitch for same building width
Result: 147.2" rafter length, 33.7° angle
360" building width with 6/12 pitch
Result: 200.0" rafter length, 22 rafters needed
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