Question answered by expert roofers Long Island Roofing:My roof has algae on it. Is this bad?
If your shingles and other roofing materials are taking on a greenish hue, algae is most likely to blame. Although algae doesn't appear to have an impact on the durability or quality of shingles and other materials, it's pretty unsightly. There are special types of shingles that help to ward off this problem. When you have a new roof installed, check into copper granules that can be used to keep algae growth at bay.