According to Owens Corning, a new shingled roof should last about 20 years. A roof well under this age will probably not need to be totally replaced, though some shingles or small areas might need repair, particularly if the cause of the damage was an external event such as a hail or wind storm.

On the Other: Signs of Deterioration

If your roof is around the 20-year mark, there are signs of deterioration you can look for that indicate it's time for a replacement. Shingles that show signs of rotting, buckling, cracking or blistering my mean the end for your roof. Discoloration and spots can be indications of algae growth or mold. Widespread split or missing shingles happen over time and get worse at the end of a roof's life. Infestation from termites can lead to not only shingle damage but damage on the underlying structure.

Bottom Line

A thorough visual inspection can tell you a lot about the condition of your roof, and the time factor is a good estimate of when a total roof replacement will be called for. Other factors to consider are proper installation, ventilation and climate. A reputable contractor can tell if repair or replacement is required.

Source:

Owens Corning: Do You Need a New Roof?