 roof and saturate it and let it work for a while it's not an instant thing you know you have to because what it's doing is killing the moss and then sort of 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