Roof Rafter Length & Pitch Calculator
Compute common rafter lengths, roof slope pitch angles, and overhang board lengths.
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How to Use the Rafter Calculator: Roof Pitch, Length & Overhang
- 1Enter the total building span (exterior wall to exterior wall width) in feet.
- 2Input your roof pitch ratio (inches of vertical rise per 12 inches of horizontal run, e.g. 6/12 pitch).
- 3Specify your desired eave overhang length in inches.
- 4Review your calculated rafter line length, total cut length with overhang, and recommended lumber board size.
Formula & How It Works
Rafter Length Equations
Uses Pythagorean theorem across half-span run and pitch rise.
Worked Examples
24-Foot Span with a 6/12 Roof Pitch
Input: Building Span: 24 ft (Run: 12 ft), Pitch: 6/12, Overhang: 12 in
Explanation: A 12-foot horizontal run rising 6 feet creates a 13' 5" rafter line length; adding a 12" eave overhang requires standard 16-foot 2x6 or 2x8 lumber.
Frequently Asked Questions
What is the difference between roof span and roof run?▼
Span is the total exterior width of the building from wall to wall; run is the horizontal distance a single rafter covers from the wall to the center ridge (Span ÷ 2).
What is a birdsmouth cut in rafter framing?▼
A birdsmouth is a triangular notch cut into the rafter consisting of a horizontal seat cut (resting on the wall top plate) and a vertical plumb cut.
How deep should a birdsmouth notch be?▼
Building codes generally specify that the birdsmouth notch should not exceed one-third of the rafter's total lumber depth.
What is the most popular roof pitch for residential homes?▼
Pitches between 4/12 and 8/12 are the most common in modern residential architecture, offering great water shedding and moderate framing difficulty.
Quick Reference & Conversion Table
| Pitch Ratio | Slope Angle (°) | Line Length per Foot of Run | Rise on 12-ft Run |
|---|---|---|---|
| 4 / 12 | 18.4° | 12.65 inches | 4.00 feet |
| 6 / 12 | 26.6° | 13.42 inches | 6.00 feet |
| 8 / 12 | 33.7° | 14.42 inches | 8.00 feet |
| 12 / 12 | 45.0° | 16.97 inches | 12.00 feet |
⚠️ Common Pitfalls to Avoid
- •Forgetting to deduct half the thickness of the center ridge board (typically 3/4" for a 2x ridge) from the rafter line length.
- •Cutting the birdsmouth seat notch too deep, which weakens the rafter tail and violates building code.
💡 Practical Real-World Uses
- •Framing residential roof trusses, detached garage roofs, and backyard storage sheds.
- •Laying out plumb cuts and seat cuts on framing squares with stair gauges.
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