How can something grow on your roof? Quite simply, all it takes is food, water and air. The food, depending on your roofing material is: for asphalt roofs, the limestone filler used to thicken the shingles, for wood shingle roofs it is just the wood. As you probably noticed, your roof is clean on some sides but dirty on others, this because the shaded parts or north facing roofs hold moisture longer because of lack of direct sunshine providing water for this growth problem. If you notice long black streaks or stains, your roof is infected with black algae, if you can see flowering growths that could be either lichen, moss or a combination of both. Is it damaging my roof? Yes, I have seen moss eat through two layers of shingles right into the wood underlayment, black algae is a surface feeder, eating the shingle granules, while spreading over the roof. Just look in your gutter of the infected side, I guarantee it is full of discarded shingle granules. So, yes it can cut a 40 year shingles life expectancy in half or worse. There are three options when it comes to roof cleaning. You could have the entire roof replaced with new shingles, buy a specialized roof cleaning product and tackle the job yourself or seek out a roof cleaning company.