With a whole roof cleaning, your shingles should remain algae-free for 2-3 years. Most roofs however, remain clean for at least four years before the algae reappears. Partial roof cleanings are NOT recommended. The algae is nearly invisible in its early stages and once it appears on some roof surfaces, it is most likely beginning to spread to other areas as well. It is less noticeable and spreads more slowly on roof surfaces that receive the most sunlight.