A few tips: To figure the size of your roof, see our square footage calculator. In roofing terminology, a square is 100 square feet. Composition shingles (asphalt or fiberglass) come in bundles that cover 1/3 square. Roofing comes in rolls 3 feet wide and 36 feet long, for a coverage of about 1 square. When installing roofing over bare wood, roofing felt -- often called tar paper -- is laid down first. A roll of 15-pound felt covers about 4 squares; a roll of thicker 30-pound felt covers 2 squares.