If you're planning to put a new roof on your house, it's important to purchase enough matching shingles to complete the job. To do that, you need to accurately calculate the area of your roof. Calculating area is easy in two dimensions, but nearly all roofs slope up to a peak, which adds a third dimension. Fortunately, you don't have to know complicated geometry to factor in the slope of your roof. You need only a level and a tape measure to find the "rise" of your roof. After that, just plug in the corresponding factor. You'll find the instructions below, along with the appropriate factors and formulas.
Determine the dimensions of the roof. Multiply the length times the width to calculate the square footage.
Determine the slope of the roof. This is expressed in terms of how many inches the roof rises for every foot of horizontal distance. Mark a level 12 inches from one end. Place the other end of the level at the peak of the roof and hold it level. Measure the distance from the surface of the roof to the bottom of the level at the 12-inch mark. This distance is the rise. For example, if your measurement is seven inches, the rise is expressed as 7:12 in roofing terminology.
Find your "pitch" factor based on the rise you calculated in Step 2. Choose one of the following: If your rise is 3:12, the pitch factor is 1.035. If your rise is 4:12, the pitch factor is 1.055. If your rise is 5:12, the pitch factor is 1.085. For a rise of 6:12, the pitch factor is 1.12. Finally, if the rise is 7:12, use a pitch factor of 1.16.
Multiply the square footage you calculated in Step 1 by the pitch factor you determined in Step 3. The answer is the area of your roof.
In the construction industry, roofs are measured in "squares." One square equals 100 square feet. Divide the area you calculated in Step 4 by 100. The answer to this is the total number of squares on your roof for which you need to purchase shingles.