 If you go up on your roof, and if you cover the top ridge, that's that sort of peak of the roof from end to end, with a piece of copper flashing, maybe one piece on one side, one piece on the other, what you'll find is as rain hits that, it releases some copper, which acts as a mildew side, and also helps to keep the roof very clean. So a little trick of the trade there. Use a roof cleaner.