 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 and then a piece of copper flashing installed into that ridge will give you some ongoing protection against future streaking.