Overtime roofs get dirtier when the sun doesn’t shine on it or if there are loads of trees around black stains will appear and heavy moss will start to grow on the shingle eating away and damaging them. The only thing you need to do is spray the chemical on the roof load it needs a lot of it if it’s full of moss.! You will see that the moss will go from green to white this means the moss is dead. If you can take solid brushes and brush off the big pieces and clean up what falls on the ground. And that’s it mother nature does the rest with the rain it will slowly run off and the black streaks will disappear.