Roof Pitch Calculation

Pitch Terminology

• In the construction industry, roof pitch is written as two numbers separated by a dash, such as 4-12. On most homes, the first number usually ranges from 1 to 12, with the lowest number (1) representing the lowest slope and the highest number (12), the steepest. The first number is the number of rising inches for every inch of roof run, and the second number is always 12, or the number of inches or roof run. For example, a 4-12 roof rises four inches for every 12 inches of roof run.

Materials

• To calculate roof pitch, all you need is a 12-inch carpenter's level, a tape measure
  and a pen and pencil. You will also need access to at least 12 inches of roof length, or run, which usually means climbing a ladder. You do not need to walk on the roof to measure the pitch, especially if conditions are unsafe or the pitch is steep.

Measurement

• Position a ladder against the home or building. Try to find the most level ground possible and only use a ladder that is higher than the lip of the roof. Gain footing at the top of the ladder and climb atop the roof. You also have the option to remain on the ladder, as you will only need to measure 12 inches of roof run. Hold out a 12-inch carpenter's level so that the far end touches a point on the roof. The rest of the level should be horizontal to the ground. Raise or lower the level until the bubble is centered. Run the tape measure from the end of the level (by your hand) to the roof. Once the tape is unfolded, the tape measure must be perpendicular to the carpenter's level, which will look like a triangle.

Calculation

• Climb down from the ladder and record the tape measurement number. To find the roof pitch, write down the number of inches found on the tape measure, which will usually range from 1 to 12, followed by the number 12, which is the number of inches on your carpenter's level. For example, if your tape measurement was 8 inches, you have an 8-12 pitch roof.