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Materials that cover a broad flat area, such as a roof, are determined by the AREA of coverage - the length of the surface multiplied by the width of the surface.

Roofing shingles are often estimated by the "square" - which represents a 10 foot by 10 foot (100 square foot) area. For example, asphalt shingle estimators use 3 bundles of shingles per 'square' of roof. So, a roof that is 2000 square feet will require 60 bundles of asphalt shingles (2000 divided by 100, multiplied by 3 = 60). A roof this size would be called a '20 square roof'. Extra shingles would be added to account for ridges, hips and perimeters.

Flashing materials are calculated differently. A contractor who installs flashing will measure the LINEAR amount required - a measurement of a straight line length. Flashing is often sold in standard lengths of 10 or 12 feet, or it may be bent / shaped using a metal brake and a roll of flat stock metal.

Some roofers may have the equipment to bend flashing on the job site, while some roofing systems (eg. steel roofing) may have flashing components that are specific to their product and are formed in a factory.