A: There are a number of indicators that one's roof has lived past its proper lifetime. Exposure to weather and the sun's radiation cause the shingles to degrade, resulting in the loss of the granules that cover the shingles. This can be difficult to spot from the ground, but exposed areas of the shingle where there are no granules is an indication of serious age requiring immediate replacement. Other indicators are the corners of the individual shingles start to curl up (often breaking their seal). Black streaks, usually the result of algae growth, does not generally indicate a roof needs replacement. But excessive amounts, along with lichen and moss colonies (the green or white patches), are commonly associated with aged roofs in need of replacement. Your best option is to have a reputable roofing contractor examine your roof. Most will do this for no charge.