Builders measure the slope of a roof, which is stated as either a pitch or an angle. A steep slope drains water away quickly and allows snow to slide off the roof. A shallow slope is easier to construct and often costs less to build and maintain than a steeper slope. The choice often is based on the owner's preference and the design of the home.
A roof pitch is stated as the inches of vertical change in relation to a horizontal distance. A roof slope that changes 4 inches vertically for every 12 inches of horizontal run is said to have a pitch of 4 inches in 12 inches. This is written as 4:12. Any slope of roof can be stated as a pitch.
The roof angle states that same slope as an angle. A roof that stands straight up, basically a wall, would have a 90-degree slope. A slope of 12:12 is a 45-degree angle. The 4:12 slope stated in the earlier example is an 18.5-degree slope. Use a chart to determine the degree of slope.
Measuring the slope of a roof requires a carpenter’s level and tape measure. Place one end of the level on the roof and elevate the other until the bubble is centered. Measure 12 inches, along the level, from the roof. Measure from that point down to the roof. This is the pitch of the roof. For example, if it is 6 inches from the level to the roof, the pitch is 6:12 and the angle of the slope is 26.5 degrees.
Knowing the roof slope is critical to determining how to work safely on the roof. Roof pitches of 6:12, or less than 26.5 degrees, can be walked on with basic precautions. Roofs of 7:12, or 30.5 degrees, require additional caution. Roofs of 8:12, or 33.75 degrees, or steeper require special roof scaffolding for safe construction or maintenance work.