Understanding Roof Pitch
Roof pitch describes how steep a roof is. In the United States, it is commonly expressed as the amount of vertical rise for every 12 inches of horizontal run. For example, a 6 in 12 roof pitch rises 6 inches for every 12 inches of horizontal distance.
A larger first number means a steeper roof. A 4 in 12 roof is relatively moderate, while a 12 in 12 roof rises as much as it runs and is considerably steeper.
Roof Pitch vs. Roof Angle
Roof pitch and roof angle describe the same slope in different ways, but they are not the same measurement.
Pitch is commonly written as a rise over a 12-unit run, such as 4 in 12, 6 in 12, or 8 in 12. Roof angle expresses that slope in degrees measured upward from horizontal.
For example, a 6 in 12 pitch corresponds to an angle of about 26.6°. A 12 in 12 pitch corresponds to an angle of 45°.
What Are Rise and Run?
The rise is the vertical distance the roof increases in height. The run is the horizontal distance covered over that rise.
When describing standard roof pitch, the run is normally converted to 12 units. This is why a roof that rises 5 inches for every 12 inches horizontally is called a 5 in 12 roof.
Rise and run can also be measured in feet, centimeters, or other units when calculating an actual roof section, as long as both measurements use the same unit.
Roof Pitch and Rafter Length
A steeper roof requires a longer sloped distance to cover the same horizontal run. This sloped distance is sometimes called the roof length or rafter length.
For example, a roof with a 12-foot horizontal run will have a sloped length greater than 12 feet. As the pitch becomes steeper, that difference increases.
The calculated sloped length is useful for estimating and layout, but actual rafter cuts may also need to account for overhangs, ridge details, birdsmouth cuts, and other construction requirements.
Measuring the Pitch of an Existing Roof
One way to determine an existing roof's pitch is to measure a horizontal run and then measure the vertical rise over that distance.
For example, place a level horizontally and measure 12 inches from one end. Measure vertically from that point to the roof surface. If the vertical measurement is 6 inches, the roof is approximately a 6 in 12 pitch.
The same calculation can be made using larger measurements when convenient, because the calculator can convert any rise-to-run ratio into the equivalent pitch per 12 units.
Why Roof Pitch Matters
Roof pitch affects more than the appearance of a building. It influences roof surface area, rafter length, drainage, snow shedding, material requirements, and how easily or safely the roof can be worked on.
Roofing materials can also have minimum slope requirements, so always check the manufacturer's installation instructions and applicable building codes before choosing or installing a roofing system.