I need to measure my roof to find out how many squares of shingles I need to buy. From what I can say … In the terminology of ceilings, a "square" is 100 square feet. Composition shingles (asphalt or fiberglass) come in packs covering third. Roofing comes in rolls 3 feet wide and 36 feet long, for a coverage of about 1 square. When installing on bare wood ceilings, roofing felt – often called tar paper – set first. A roll of felt covers about 15 pounds for 4; thicker roll 30 lb. Felt covers 2 squares. When ordering any of these materials, add 15 percent of the waste.