- 1). Measure the length and width of each individual flat surface, or "plane," on your roof in feet and inches. Some roofs are composed of one flat area, and thus have only one plane, and other roofs have two or more flat areas. For a roof area that is an irregular shape (i.e., not a simple square or rectangle), visualize a combination of squares, rectangles, and triangles that covers the irregular plane.
- 2). Multiply the width by the length of each simple square or rectangular plane. For irregularly shaped planes, multiply the width by the length of each square or rectangle within the plane. To calculate the area of triangle-shaped areas within an irregular plane, multiply the width of the triangle base by the triangle's height, then divide the result by 2. When you have calculated all the individual areas within each plane, add all the figures to get the total area of your roof in square feet and inches.
- 3). Multiply the figure for the total area of your roof by 0.1, then add the result to the total area figure. This extra 10 percent allows for trimming during installation.
- 4). Divide the total amount you calculated in Step 3 by 100 to determine the number of "squares" of each kind of roofing material you need to purchase.
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