Standard three-tab asphalt shingles typically require three bundles to cover one square of roof area. Therefore, 120 bundles of shingles would be needed to cover a 40-square roof. Each bundle covers approximately 33 1/3 square feet because a “roof square” is equal to 100 square feet.
According to the Owens Corning website, it is typical to add 10% to a shingle estimate to account for trimming. Three bundles of shingles would only cover one square of a roof if shingles were to be trimmed at the edge, bottom, or top. Therefore, 132 bundles would be required for a 40-square project, plus an additional 12 bundles for trimming allowance.