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What most people don’t know, is that those unsightly stains on their roof aren’t  usually dirt, mold or even mildew but actually a form of algae that spreads on airborne spores.  There are often streaks below metal flashing around chimneys and roof vents because the copper or zinc in this metal is toxic to the algae so as rain washes over it trace amounts are transferred to the shingles below. If you’re installing a new roof you can prevent this problem by choosing algae resistant shingles which have small amounts of copper mixed into the granules. If a new roof isn’t in your immediate future your best bet is cleaning and there are solutions made specifically for this purpose or you can make your own from bleach, tri sodium phosphate and water. Whatever you use, be sure to rinse the roof well to get rid of that algae and those unsightly stains.