Have you noticed black streaking on your roof? You may be surprised to hear the streaks are from algae colonies, which many would expect to be green. Regardless of their cause, the streaks are ugly. They can sometimes be removed by lightly spraying your roof with certain chemicals (protect that landscaping!), but algae streaks do not harm the functionality of your roof. So if you can live with the appearance, algae will not interfere with your roof’s ability to shed water.
This cannot be said, however, of two other organisms that feed on roofs: Lichen and Moss. Moss is the most harmful. Moss colonies feed on the minerals found in shingles, and their roots can crumble shingles or weaken the seals holding the shingle flaps in place. They appear as soft green patches, usually in localized areas where there is an abundance of moisture, such as where branches overhang the roof. Lichen is also harmful to roofs, though their colonies tend to be less extensive. They are often associated with moss colonies, and appear as white or sometimes orange scaly patches on the roof.
Moss is fairly easy to remove by merely brushing it off. Lichen is less easy. But in either case, if their colonies are extensive, they indicate an advanced stage of roof deterioration, and the roof has neared the end of its serviceable life.