Degrees of Roof Slope

Degrees vs. Slope

• In math terms, a degree is a gradient of a circle and slope is a gradient of a triangle. This means that the degrees in a roof's slope will not equal the pitch of a roof and instead have to be calculated using the Pythagorean theorem. For this reason, roof slopes are expressed in two numbers (between a dash) that represent the number of rising inches on a roof for every inch of roof run, or length. For example, a 12-12 roof rises 12 inches for every 12 inches of roof run, which equals an angle of 45 degrees.

Common Roof Slopes

• In America, the most common roof slope and the standard in most home construction is 4-12, or 18.43 degree angle. A 4-12 roof rises 4 inches for every 12 inches of roof run until the roof ends at the uppermost level of the roof, or ridge. In the south, where ice and snow accumulation are not an issue, roofs are often constructed in a 3-12, or 14.04 degree angle slope.

Slope Categories and Degrees

• A roof is considered low slope if the pitch is in the 1-12 to 4-12 range, or a 4.76 to 18.43 degree angle. Steep roofs range from 4-12 to 21-12 pitch, or a 60.26 degree angle. To get an idea of how steep this is, consider that any roof over 8-12, or 33.69 degrees, is too steep to walk. Extremely sloped roofs with a pitch of 21-12 to 60-12, or 72 degrees, is called a Mansard roof. A Mansard is a traditional roof style with steep, nearly vertical slopes on each side of the structure that rise to meet a flat connecting roof.

Calculating Roof Pitch

• To calculate a roof's pitch, you will need a ladder, tape measure, 12-inch carpenter's level and access to at least 1 foot of roof run. Once you are atop the ladder, hold out the carpenter's level so that it is parallel to the ground below and touching one point of the roof's slope. Make sure that the center bubble is in place. Next, run the tape measure from the end of the level (in your hand) down to the roof, making sure to keep the tape measure perpendicular to the level. The number on the tape measure, in inches, is your roof pitch. Add a dash and the number "12" to represent the second number in the pitch equation, which is the number of inches on the carpenter's level.