Rafter Length Calculator - Calculate Roof Rafter Measurements
Free calculator to determine precise rafter measurements for roof construction including rise, run, pitch, and overhang requirements
Rafter Length Calculator
Results
What is a Rafter Length Calculator?
A Rafter Length Calculator is a free construction tool that helps you determine the exact measurements needed for roof rafters. It calculates rafter length based on span, rise, overhang, and pitch measurements for accurate roof framing and material planning.
This calculator works for:
- New Roof Construction - Calculate rafters for new building projects
- Roof Replacement - Determine measurements for roof renovations
- DIY Roofing Projects - Perfect for homeowners doing their own roofing
- Construction Planning - Accurate material estimates and cost planning
Need to determine your roof pitch first? Use our Roof Pitch Calculator to calculate exact pitch ratios and angles.
Planning roofing materials? Our Roof Shingle Calculator helps estimate shingles, underlayment, and fasteners needed.
For lumber material planning, try our Lumber Calculator to calculate board feet and costs.
How Rafter Length Calculation Works
The calculation uses the Pythagorean theorem:
Where:
- Rise = Vertical height from wall to ridge
- Run = Half the total span (horizontal distance)
- Overhang = Distance rafter extends beyond wall
- Angle = arctan(Rise ÷ Run) in degrees
Key Rafter Concepts Explained
Roof Span
The total horizontal distance from one wall to the opposite wall. Span = Run × 2. Measure inside-to-inside of walls.
Roof Pitch
Steepness expressed as rise per 12 inches of run. Example: 6/12 pitch = 6" rise per foot of horizontal run.
Rafter Overhang
Eave projection beyond the wall. Typical range: 12-24 inches for weather protection and fascia support.
Common Rafter Sizes
2×6 for spans up to 10'. 2×8 for 10-14'. 2×10 for 14-18'. 2×12 for longer spans or heavy loads.
Birdsmouth Cut
Notch where rafter sits on wall plate. Seat cut = level, plumb cut = vertical. Don't exceed 1/3 rafter depth.
Roof Pitch Reference Table
| Pitch | Angle | Multiplier | Common Use |
|---|---|---|---|
| 4/12 | 18.4° | 1.054 | Low slope, warm climates |
| 6/12 | 26.6° | 1.118 | Most common residential |
| 8/12 | 33.7° | 1.202 | Colonial, Cape Cod |
| 10/12 | 39.8° | 1.302 | Snow country |
| 12/12 | 45.0° | 1.414 | A-frame, steep roofs |
Multiplier: Multiply run by this value to get rafter length (excluding overhang).
Common Rafter Calculations
🏠 24' Span, 6/12 pitch, 12" overhang
Run: 12'. Rafter length: √(12² + 6²) + 1 = 13.4' + 1' = 14.4 feet
🛖 16' Shed, 4/12 pitch, 18" overhang
Run: 8'. Rafter length: √(8² + 2.67²) + 1.5 = 8.4' + 1.5' = 9.9 feet
🏔️ 30' Cabin, 10/12 pitch, 24" overhang
Run: 15'. Rafter length: √(15² + 12.5²) + 2 = 19.5' + 2' = 21.5 feet
How to Use This Rafter Calculator
Enter Span
Measure total width from wall to wall
Enter Rise
Measure vertical height to ridge
Add Overhang
Specify desired eave length
Calculate Results
Get exact rafter measurements
Benefits of Using This Calculator
- • Precise Measurements: Get exact rafter lengths to prevent costly cutting mistakes.
- • Material Planning: Calculate exactly how much lumber you need for your roof project.
- • Cost Estimation: Determine total project costs based on lumber prices and quantity.
- • Structural Accuracy: Ensure proper roof angles and load distribution.
Factors That Affect Your Results
1. Roof Span
Longer spans require longer rafters and may need structural reinforcement depending on local codes.
2. Roof Pitch
Steeper pitches create longer rafters for the same span and affect snow load and wind resistance.
3. Overhang Length
Longer overhangs extend rafter length and create more eave space but may require additional support.
Frequently Asked Questions (FAQ)
Q: How do I calculate the length of a rafter?
A: Rafter length is calculated using the Pythagorean theorem: length = √(rise² + run²) + overhang. Where rise is the vertical height, run is half the span, and overhang is how far the rafter extends beyond the wall.
Q: What is the difference between run and span?
A: Span is the total distance from wall to wall that the roof covers. Run is half the span - the horizontal distance from the center ridge to the wall plate.
Q: Why do I need to calculate rafter length?
A: Knowing the exact rafter length helps you purchase the right amount of lumber, ensures proper roof framing, prevents material waste, and helps with project cost estimation.
Q: How does roof pitch affect rafter length?
A: Roof pitch directly affects rafter length. Steeper pitches (higher rise per foot) result in longer rafters for the same span. The pitch determines how much vertical rise you get for each horizontal foot of run.