First, you need to know the total roof area in square feet (ft^2). You must take into account the roof slopes, if any, in determining this value.
Secondly, you need to know the square footage of each panel (ft^2) and deduct the appropriate area for overlaps with adjacent sheets to figure out the NET AREA......then you must divide the total roof area by the sheet Net Area....presto whalla the minimum required number of roofing sheets. Don't forget to include a "bump up" figure to the total number of sheets to account for material waste, because the roofing sheets must be cut in the field....typically this can range from 3% to 10%, depending on the layout and the accuracy of the roofing guys.
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