Draw the layout of the roof onto a piece of paper. Include dormers or areas where different sections of the roof meet at an angle.
Measure the roof dimensions and record the measurements onto the sketch.
Calculate the area of the roof by multiplying the length and width together. If the roof has oddly-shaped areas, such as dormers or angles, break them down into different sections, then add up the areas of each section to get the total surface area of the roof.
Multiply the length and width of the roof sheets to determine their surface area.
Divide the total area of the roof by the area of one sheet to calculate how many sheets you need to cover the roof.