Roof pitch is a measurement of a roof's slope expressed as a ratio. Although there is no single standard roof pitch used on all sloped roofs, factors such as roofing materials and local climate help to determine the appropriate range of pitches for a given building.
Roof pitch is expressed as a ratio of the amount of the roof's vertical "rise" over a corresponding horizontal distance, called the "run." Pitch is written assuming a run of a foot, or 12 inches, so the ratio describes how much the roof rises for every foot of its run.
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For example, a roof that rises 4 inches for every 12 inches of run has a 4/12 pitch. The same pitch is also sometimes written as "4:12," "4 in 12" or "4 over 12."
Common Roof Pitches
The most commonly used roof pitches fall in a range between 4/12 and 9/12. Pitches lower than 4/12 have a slight angle, and they are defined as low-slope roofs. Pitches of less than 2/12 are considered flat roofs, even though they may be very slightly angled. Pitches above 9/12 are very highly angled and are designated steep-slope roofs.
Roofing Material Limitations
Roofs with extreme pitches, either low or steep, are inappropriate for certain roofing materials. Low-slope or flat roofs should not be roofed with asphalt shingles because it is much easier for water to infiltrate the roofing material on a roof with a shallow pitch. Asphalt shingles are appropriate on roofs with conventional pitches between 4/12 and 9/12.
On the other hand, roofing materials that work well on low-slope roofs, such as tar-and-gravel or rubber membranes, should not be used on conventional or steep-slope roofs because installation is problematic and the unattractive materials are very visible on a highly angled roof. Both tile and shingles work well for steep roofs.
The climate in which a home is built may also rule out certain roof pitches. In areas where snow accumulation is heavy, low-slope roofs can be problematic because excessive buildup of snow can become so heavy that the roof fails. Steep-slope roofs can also be a problem, however, because snow is likely to slide down to the eaves, a situation that can cause damage to the roof and present a danger to people below from falling ice and snow.