How to Calculate Roof Pitch

The pitch of a roof, roughly speaking, defines how “pointy” it is. The more it appears to penetrate the sky, the more pitched it is. 

Calculating roof pitch is easier than you might think. In principle, it is the ratio between how much the roof rises vertically as you move horizontally. It is the same as what you learned in school: the “rise over the run.”

Measuring Roof Pitch

In this section, we run through the step-by-step process you’ll need to follow to measure the pitch of your roof. 

Measuring roof pitch is helpful in many situations, including when you want to cut new rake boards or install skylights. It allows you to determine the structural characteristics of your roof ahead of time. 

Step 1: Collect Your Materials

Before you do any measurements, you’ll need to collect some tools. Requirements include an 18 or 24-inch level, a pencil and a steel tape measure. 

Step 2: Measure The Horizontal Distance

Next, head on up to your attic. (You can measure the pitch of your roof from the inside which is much safer than using a step ladder to measure your roof externally). 

Start by measuring twelve inches from the end of the level and mark with the pencil

Step 3: Measure The Vertical Distance

Then measure the vertical distance by measuring directly up from the 12-inch mark to the rafter. This will give you the rise over the run. 

Worked Example Of Roof Pitch

Let’s say that you want to measure the pitch of your roof. Following the above process, you will get two measurements: 

  1. The fixed 12-inch horizontal distance from the rafter
  2. The variable vertical distance to the rafter. 

When expressing pitch, contractors will typically express it as “X-in-12 pitch” where “X” is the vertical distance. So, for instance, for a shallow pitch roof, you might have 6-in-12 pitch. For a steep roof, you might have an 18-in-12 pitch. In the former cases, this just means that the roof rises six inches over the run. In the latter, it means that it rises 18 inches over the run. 

Using A Roof Pitch Calculator

You can use a roof contractor pitch calculator to provide you with additional information about the pitch of your roof. For example, on entering the rise and the run, you can use software to calculate: 

  • The slope per 12 inches
  • The angle in degrees
  • The length in inches
  • The pitch or span

In some cases, it will be helpful to measure the rafter length. This tells you how much rafter you need to complete a roof project. Usually, rafters will have some overhang for tiles. 

The angle of the roof is the same as the slope per 12 inches, except measured in degrees. If you understand trigonometry, this can be extremely helpful. You can use lengths and angles to calculate other lengths and angles without having to measure them. Pythagoras’s theorem, sine, cosine, tangent and other operators allow you to complete triangles.

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