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.
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.
Evan Gillespie grew up working in his family's hardware and home-improvement business and is an experienced gardener. He has been writing on home, garden and design topics since 1996. His work has appeared in the South Bend Tribune, the Fort Wayne Journal-Gazette, Arts Everywhere magazine and many other publications.