Guidelines for Roofing Pitches

Pitch Numbers

• Roof pitch is expressed in inches per foot, as in 5/12 or 6/12 for roofs that rise at 5 or 6 inches per foot of run -- the space each rafter must support from wall to peak. There are three main categories of pitch: flat or low-slope, at 4/12 or less, medium 5/12 to 9/12 and steep 9/12 and above. Even a flat roof must have some slope, to drain water, usually at least 2 inches per foot.

Roof Loads

• Pitch is affected by roof loads. Dead load is the weight of the roof itself plus the framing and roof covering, which will vary with the slope. Live load is accumulation of snow and ice, which bears more heavily on lower pitches. Wind load is the force of winds on a roof, which is more of a factor with a steep and more vertical roof.

Locales

• Areas with heavy winter snows require steeper pitches to prevent snow and ice from building up and adding weight. Warmer, drier regions can use lower pitches and may be more affected by wind loads. Areas subject to tropical storms are most vulnerable to these winds, which are less of a factor with low-slope roofs.

Costs

• Roof pitch can affect costs and type of roofing material. Steep roofs require more framing material and more labor, but they shed snow better. Flat roofs require more complex roofing materials and more expert labor, and they often are more subject to leaks. Metal roofing is often used on steeper roofs in snow areas, because its slick surface sheds snow better than the rougher texture of composition shingles or wood shakes.

Typical Pitches

• Residential roofs in most areas of the country are medium pitch, from 5/12 or 22 1/2 degrees to 9/12 or almost 37 degrees of slope. These angles shed rain, snow and ice effectively and provide some interior attic space. Homes in mountain areas tend toward 45-degree angles, 12/12 slope, or higher, and A-frame type roofs go up to slopes of about 60 degrees.