Rafter Length
Calculate rafter length, cutting angles, and lumber requirements for roof framing. Includes pitch conversions, span tables, and detailed cutting instructions.
Rafter Length Calculator
Length • Pitch • Angles • Plumb Cut • Seat Cut
Quick Examples — Click to Load
Small Shed
8 ft span, 4:12 pitch
Garage
20 ft span, 6:12 pitch
Ranch House
28 ft span, 6:12 pitch
Steep Roof
24 ft span, 8:12 pitch
Low Slope
16 ft span, 3:12 pitch
Large Span
36 ft span, 6:12 pitch
Total width of building (wall to wall)
Gable Roof
Simple triangular roof with two slopes
Pitch: 6:12
Angle: 26.57° | Standard residential
Typically 1.5" (2× lumber) or 3.5" (2×4)
2×6
Actual: 5.5" × 1.5" | Max Span: 13 ft
Standard for residential roofs
Planning estimates only. Verify with a licensed engineer or contractor before construction.
What is a Rafter Length Calculator?
Determines exact rafter length from ridge to wall plate, including overhang. Provides plumb cut and seat cut angles for framing.
Length
Exact with overhang
Angles
Plumb & seat cut
Pitch
Ratio to angle
Lumber
Requirements
How to Calculate
When to Use
Roof framing, lumber ordering, cut list generation.
Why It Matters
Correct lumber length, accurate cuts, proper roof construction.
Where Applied
Residential construction, sheds, additions, framing.
Formulas
L = √(Run² + Rise²). Plumb = arctan(Rise/Run). Seat = 90° - Plumb.
Tips
- • 4:12 = 18.4°; 6:12 = 26.6°; 8:12 = 33.7°
- • Add overhang to run length
Common Mistakes
- • Forgetting ridge board deduction
- • Wrong pitch units (per foot vs per inch)
FAQs
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