Understanding Roof Pitch
Roof pitch is the measurement of a roof's vertical rise divided by its horizontal span (run). It's typically expressed as a ratio, such as "4:12" or "6:12".
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Common Roof Pitches
Most residential roofs have pitches between 4:12 and 9:12. Lower pitches are common in commercial buildings.
- Low Slope: 2:12 to 4:12
- Medium Slope: 4:12 to 8:12
- Steep Slope: 8:12 and above
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Pitch vs. Slope
Pitch is expressed as a ratio (rise:run), while slope can be shown as:
- Degrees (angle from horizontal)
- Percentage (rise/run × 100)
- Ratio (x:12 format)
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Climate Considerations
Roof pitch selection should consider local weather conditions:
- Snow Regions: Steeper pitches (6:12 or greater)
- Rain Regions: Minimum 4:12 pitch
- Hot Climates: Lower pitches acceptable
Technical Specifications
Minimum Pitches by Material
- Asphalt Shingles: 2:12 minimum
- Metal: 1:12 minimum
- Tile: 2.5:12 minimum
- Slate: 4:12 minimum
- Wood Shakes: 3:12 minimum
Load Considerations
- Snow Load: 40-70 psf
- Dead Load: 15-20 psf
- Wind Load: Varies by region
Material Weights
- Asphalt: 2.5-4.0 psf
- Metal: 1.0-2.0 psf
- Tile: 9.0-12.0 psf
- Slate: 8.0-15.0 psf
Advanced Calculations
Pitch to Angle Conversion
Angle (degrees) = arctan(rise/run) × (180/π)
Where:
- Rise = Vertical height
- Run = Horizontal span
- π = 3.14159
Rafter Length
Rafter Length = √(rise² + run²)
Add for overhangs and ridge thickness
Roof Area
Area = (Roof Length × Run) × √(1 + (Rise/Run)²)
For both sides of gable roof: multiply by 2
Hip/Valley Length
Length = √(rise² + (run² + (span/2)²))
Where span is the building width
Material Coverage Guide
Coverage Rates
- Asphalt Shingles: 100 sq ft per 3 bundles
- Metal Panels: 36" width coverage
- Concrete Tiles: 90 tiles per 100 sq ft
- Slate: 115-130 pieces per square
Waste Factors
- Simple Gable: 10-15%
- Hip Roof: 15-20%
- Complex Roof: 20-25%
- Valleys: Add 5-10%