How to Calculate the Slope of Your Roof

Brady Roofing offers free roofing estimates that will include the slope of your roof in the bid. However, you may be interested in learning how to measure the pitch of your roof on your own. This can be useful information to give to contractors in order to get more information about your roof before an estimate. The good news is, slope is relatively easy to configure. You will need the following supplies:

2 rulers (or any measuring tool with the same units)
a level

Roof slopes are measured in units of twelves. You can measure with twelve feet or twelve yards, but in this article we will use twelve inches. For example, when a roof has a slope of 5/12, the roof is 5 inches high for every 12 inches long. Another way of stating this is that the roof has a rise of 5 and a run of 12. Figuring this is simple.

First, hold the ruler horizontal with the bottom edge against the bottom of the roof. Use a level to make sure it is perfectly horizontal. Then, hold another ruler vertically from the twelfth unit (12 inches or 12 cm). Again, use the level to ensure that the ruler is perfectly vertical. The number where the ruler intersects with the bottom of the roof is the top number of slope. For example, let’s say the vertical ruler hit the bottom of the roof at the 6 inch marker. This roof would have a slope of 6/12.

When you know your roof slope, you can more successfully categorize your roof. There are flat roofs or low-slope, and steep-slope roofs. Flat roofs are actually not flat. If you build a roof to be flat, it will not have a way to drain and can cause serious damage to your roof and home. Instead, flat roofs are categorized by having a slope between 2/12 and 4/12. A steep-slope is one above 4/12.

If you have a flat roof, or a low-slope roof, you will want to use a membrane roofing system such as TPO or EPDM. Steep-slope roofs require a pitched roofing system such as architectural shingles, tiles or metal roofing.