 for a while. It's not an instant thing. What it's doing is killing the moss, and then Mother Nature releases it and washes it away. The other thing that you can do, 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.