Understanding roofing squares
Roofing materials are sold by the square, where one square covers 100 square feet of roof area. To find the number of squares you need, divide the total roof area by 100. Because roof area is measured along the slope rather than the horizontal footprint, steeper roofs require more material per footprint square foot.
A typical bundle of three-tab shingles covers one third of a square, so three bundles equal one square. Architectural shingles may need four or five bundles per square depending on the product.
Roof area = Footprint area x Slope factor
The slope factor converts flat footprint area to sloped surface area based on roof pitch.
Squares = Roof area / 100
One roofing square equals 100 square feet of roof surface.