How to Determine Roofing Squares

Calculate the number of squares on a roof by measuring it with a tape measure. Get on the roof to measure from the peak to the outer edge, including any eave or overhang, and the length from front to back; if the roof is too long for the tape, measure it in sections and add the lengths. Multiply those numbers. A roof 20 feet wide and 36 feet long will have 720 square feet; divided by 100, that equals 7.2 squares.

Use roof sheathing to figure the squares on a house under construction. Count the number of panels of plywood or oriented strand board (OSB) decking on each side of the roof. Multiply that by 32; panels are 4 by 8 feet or 32 square feet. Multiply the number of full panels by 32, then add figures for partial panels to get square footage. Divide by 100 to calculate the squares. Perform the same calculations for both sides of a roof.

Determine squares from ground level by measuring the width and length of the house and using a roof pitch table to get a multiplier to adjust for the extra length of the slope. A typical multiplier for a roof with a 5/12 pitch -- rising 5 inches per foot from wall to peak -- is 1.15; a house 20 by 36 feet with a 5/12 pitch will have roof square footage of 828 per side or 8.28 squares.

Add square footages for any dormers or extensions off the main house, like a porch or carport. Calculate those square footages like you did with the main roof and combine the figures. A 20-by-36-foot roof with a 10-by-10-foot extension over a porch, for instance, will have 820 square feet or 8.2 squares.