You will need two measuring tools (like a ruler) and a level to get the job done right. Slopes are measured in twelves. When you are told that a roof has a slope of 7/12 the roof is seven inches high for every twelve inches it is long. This number can also be expressed as the roof having a rise on 7 and a run of 12. After you understand what the fraction, or rise and run mean, it is much easier to understand roof pitch measurements.
To measure your roof hold your measuring tool horizontal with the bottom pressed against the edge of the roof and then use a level to ensure that it is perfectly straight. Next, take the second measuring tool and hold it vertically from the end of the first ruler. Check the second ruler with the level to make sure it is exactly vertical. The number where the ruler meets with the bottom of the roof is the rise or top number of the fraction. Using a tape measure is easier than trying to do this with a ruler, because you’ll be able to hook the end of the measuring tape to the end of the roof. Putting this number over 12 will give you your roofs slope. If you live in a country that uses the metric system, pitches are expressed in degrees. Most roofs are built at a pitch of more that 20 degrees.
Knowing your roof’s slope with tell you whether it is a steep-slope, flat or low-slope. Flat roofs are almost perfectly flat; however, they need some pitch to allow for drainage. These coverings typically have a pitch of 1/12 – 2/12. Low-pitch usually have a pitch of 2/12 – 4/12. Steep-slope, sometimes found on houses located in snowy areas, have a slope of 4/12 or higher. Some roofs can even have a slope of 18/12. These can be dangerous spaces to work on and homeowners should take care when attempting DIY repairs to steep-slope roofs.