 maybe after the roof is getting clean, is at the top peak of the roof, put a piece of copper flashing across the entire top of the roof, maybe under the cap shingles. Because what happens is as the rainwater hits the copper, it releases some of the metal into the rainwater, and that acts as a mildew site as well. Have you ever noticed that around a chimney, sometimes you see streaking underneath it? That's because the water is connecting with the flashing and that water