Things You'll Need
Notepad and pen
When building a home or other structure such as a shed or pool house, you'll need to know how many materials will be needed to complete the project. If you are planning to use plywood, you'll have to get the square footage of the roof's surface and use those figures to determine how many plywood panels you will need. As with any construction project, you should add a bit extra to compensate for mistakes.
Measure the length of the roof area with a tape measure and record the measurement on a notepad.
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Extend the tape measure across the roof perpendicular to the last measurement to get the width of the roof area; write this number down as well.
Multiply the length by the width with a calculator to get the total square footage of the roof area. For instance, if the roof is 40 feet long and 30 feet wide, you would enter 40 x 30 into your calculator to get a total square footage of 1,200.
Figure the square footage of the plywood sheets. Normally, plywood comes cut in 4-foot by 8-foot sheets. Multiply 4 feet by 8 feet to get 32 square feet.
Divide the total square footage of the roof by the square footage of the plywood sheets. Using the example figures, divide 1,200 square feet by 32 square feet to yield the number of sheets needed to cover the roof, which in this example is 37.5 sheets.
Add extra for waste. Because construction projects incur errors, you should include a 5-percent waste factor. Multiply 37.5 sheets by 1.05 percent to get 39.38, and then round up to the next whole number, or 40 plywood sheets to cover a 1,200-square-foot roof.