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- 1). Draw a diagram of the roof layout before you start. Virtually all residential roofs are composed of several sections of rectangles or triangles. In some cases a roof section combines both. Divide up such sections into separate rectangles and triangles on the diagram.
- 2). Use a tape measure to determine the dimensions of each section on your diagram. For rectangles, simply measure the length and the width. Triangles are a bit more complicated, but not much. Measure the base of the triangle. This is the side opposite the largest angle (and also the longest side of the triangle). Next, measure the height. This is the length of the line from the largest angle that intersects the base at a 90 degree angle. Use a carpenter's square to make sure the line is perpendicular to the base.
- 3). Calculate the area in square feet of each section of the roof. For rectangles, multiply the length and width. For triangles, the area is equal to the length of the base multiplied by the height divided by 2. For example, if the base is 10 feet and the height is 8 feet, you have 10x8 ÷ 2 = 40 square feet.
- 4). Add up the areas of all sections to find the total area of the roof in square feet. Add 5 percent to allow for scrap due to cutting shingles and breakage.
- 5). Determine the amount of shingles needed to properly roof the building. Round the total area up to the next 100 square feet, and then divide by 100 to find the number of squares. For example, if the total roof plus the margin for waste is 1,850 square feet, round up to 1,900 and divide by 100, for a total of 19 squares. Since there are usually three bundles of shingles per square, you need 57 bundles (3x19 = 57).
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