Covering a roof with three-tab shingles requires three bundles for every 100 square feet of roof to be covered. Retailers sell shingles by the bundle and by the square, a unit equal to 100 square feet. Each standard bundle of shingles is equal to one-third of a square, but manufacturers sometimes package heavier roofing material so the bundle is one-fourth or one-fifth of a square.
A common mistake of homeowners purchasing roofing material is to buy based on the square footage of the home. The roof surface is typically a larger area than the footprint of the house, as the surfaces slope and have overhangs. Accuracy in calculating the number of bundles of shingles requires measuring the dimensions of each roof surface from edge to edge and along the slope. The area should also include architectural features, such as dormers. After determining dimensions, calculate the area of each surface by multiplying length by width. Add all of the areas and divide by 100 to determine the number of squares of roofing to purchase. Manufacturers recommend adding 10 percent to the total to account for waste.
Shingles are bundled in packages to make them easier to carry. The standard bundle contains 29 shingles, each 12 inches by 36 inches. When installed to the manufacturer's specification, they cover the stated square feet.