Been meaning to post this for some time. The shingles on my roof were turning black, getting worse each year. Figured it was just dirty, but it wouldn't go away, so I was planning to get the roof re-done. But after some web searching, I found out it was most likely an algae (Gloeocapsa magma). Searched around and found BlackTip Cleaning, and Preston knew what was happening and gave a completely reasonable quote to fix it, probably 1/10th the cost of replacing the roof! I was travelling when he did the work (in 2017), and he managed it all himself and did a beautiful job. Now two years later and it's still in great condition. See before and after pics of a section. Wholly recommend BlackTip!