Draw a simple sketch of your roof. Divide the drawing into rectangles and, if necessary, triangles.
Climb onto your roof and measure its dimensions with a tape measure. You'll need to measure the length and width of the rectangular portions of the roof, as well as the base and height of any triangular sections. Write these measurements onto your sketch.
Find the square footage of each section of the roof. For rectangular sections, multiply length times width. For triangular sections, multiply base times height and divide by 2. Add the square footage of each shape to find the total square footage.
Determine the square footage of a piece of sheet metal by multiplying its length times its width. For example, if the sheet is 20 feet long by 2 feet wide, multiply 20 times 2 to get 40 square feet.
Divide the total square footage of the roof by the square footage of a single piece of sheet metal. For example, assume that a roof contains 1,800 square feet of space and a piece of sheet metal has an area of 40 square feet. In this case, you would divide 1,800 by 40 to get 45. This means that you would need 45 pieces of sheet metal to cover the roof.
Multiply the total amount of sheet metal by 0.08 to 0.10 and add the product to the total amount to account for waste. In this example, you could multiply 45 times 0.10 to get 4.5. Then add 4.5 to 45 to get 49.5 pieces of sheet metal.