A roofing square is 100 square feet of materials. Manufacturers package normal asphalt roofing shingles so that three packages are equal to a square. They package 3-foot wide rolled roofing in 36-foot lengths, so each roll is approximately one square.
Roofers estimate the number of squares they need to cover a roof by calculating the square footage of the roof and dividing by 100. With sloping roofs, the roofer must account for the additional square footage the slope adds to the area of the roof. Some roofers make the calculation based on the slope of the roof. Others estimate the size of the roof based on the square footage of the home. Several satellite services offer estimates of the amount of roofing materials a home requires using satellite photographs.
Roofers often use squares because they deal with large numbers. Instead of dealing with a 2,000- to 3,000-square foot roof, they speak of a 20- to 30-square roof, simplifying the math when purchasing materials or calculating quotes for homeowners.
Regardless of how a person estimates the roof size, it is important to remember that the results are not exact. Most roofing professionals order approximately 15 percent over the estimate to accommodate waste.