Asphalt shingles are composed of mats made from organic or fiberglass material impregnated with asphalt, in which are embedded colored mineral granules. Organic-base asphalt shingles, also called composition shingles, have a felt mat made from wood and paper fibers. Fiberglass-base asphalt shingles, commonly called fiberglass shingles, have a fiberglass mat. Asphalt roofing is also manufactured in the form of roll roofing (page 33).

Though shapes vary, many asphalt shingles are notched at intervals to form tabs, giving the appearance of smaller units. They usually last from 15 to 25 years, depending on the climate and type of mat. Asphalt shingles that are aging may show bald spots; another clue to aging is a heavy accumulation of granules in the gutters, indicating crumbling shingles.

Check your roof's condition on a warm day when the shingles are flexible. Remove a tiny piece of the corner from one or two shingles on each roof plane; the core of the shingle should be black. Gently bend several shingles back to see if they're flexible. If a number of shingles appear gray and bloated, if the material crumbles easily or if you see large bare spots or damaged areas, consider replacing the roof.

Cracked, torn, or curled shingles can be repaired, as shown below; replace any loose or missing nails. If some of the shingles are badly worn or damaged, replace them (see below). Use shingles that remain from the original roof installation. If you don't have any leftover shingles, you'll have to buy new ones—identical in brand, color, and size, if possible. Fasten the shingles with galvanized roofing nails long enough to penetrate all roofing layers (at least 11/2 inches long).

Don't remove a damaged shingle that's on a ridge or along a hip; instead, nail each corner in place. Then apply roofing cement to the bottom of a new shingle and place it over the defective one. Nail each corner, then cover the nail heads with roofing cement.

When you repair asphalt shingles, do the work on a warm day when the shingles are more pliable; cold shingles are brittle and can break easily Also, have roofing cement at room temperature so it will spread more easily.