Roof pitch

Roof pitch

Roof pitch measures a roof’s steepness through the ratio of vertical rise per horizontal foot, creating the fundamental angle that defines a roof’s character. A 3-inch rise per foot translates to a “3 in 12” pitch, while a flat roof maintains zero pitch.

Basic Measurements

The pitch calculation combines vertical ‘rise’ with horizontal ‘run’ to determine slope. US measurements express this as rise:12 (like 4:12 or 5:12), while metric systems use degrees or a “1 in X” ratio. A 1:1 slope equals 45 degrees, creating a perfect diagonal.

Climate Adaptations

Different regions demand specific pitches based on weather patterns. The Papua New Guinea longhouses feature steep, ground-sweeping roofs for tropical rains. Northern European buildings use high-pitched gables to handle heavy snow loads. Building codes in Buffalo and Montreal mandate minimum 6:12 pitches (26.6 degrees) for snow management.

Classification Systems

Flat Roofs

These include pitches from 1⁄2:12 to 2:12, requiring special waterproofing due to minimal water shedding capability.

Low-Slope Systems

Pitches up to 4:12 need specialized materials and installation techniques to prevent leaks.

Conventional Designs

Standard pitches range from 4:12 to 9:12, offering optimal balance between function and construction ease.

Steep Configurations

Anything above 9:12 qualifies as steep, demanding additional fasteners and specialized installation methods.

Historical Categories

Ancient builders named pitches based on cultural styles:

• Greek: 12.5° to 16° (1⁄9 to 1⁄7 ridge-to-span ratio)
• Roman: 24° to 34° (2⁄9 to 1⁄3 ridge-to-span ratio)
• Gothic: 60° (rafters equal to span)
• Elizabethan: Beyond 60° (rafters exceed span)

Construction Considerations

Lower pitches reduce material costs but demand superior waterproofing. Higher pitches shed water and snow effectively but increase construction complexity and material usage. The exact slope in degrees comes from the arctangent – a 3:12 pitch equals 14.0 degrees.

https://en.wikipedia.org/wiki/Roof_pitch