When black streaks and stains form on an asphalt shingle roof it is usually a build up of blue-green algae. This usually occurs in warm climates or after very humid summers, it does no damage to the roofing, but obviously does not look pretty. Here are a couple ways we recommend getting rid of roof algae and ways to prevent it from coming back.
The least expensive way to rid your roof of an algae problem is to spray it down with a 50-50 mixture of water and bleach. A couple things to remember if you choose to use this process 1) Wet down plants with clean water that may be hit by your bleach mixture. Bleach and plants do not get along and wetting them down first will help protect them. 2) Do not use a power washer, the pressure can damage your roof and shingles. 3) Be sure to rinse your roof off after you are done.
Another option is that there are some companies make a special cleaner especially for removing the algae from your roof. If you choose this method, we recommend taking the same 3 precautions that we listed in the first step, just to be safe.
The easiest way to prevent roof algae from coming back again is to install strips zinc or copper underneath the shingles closest to the peak of the roof leaving a couple inches of the metal exposed to the elements. When it rains some of the metal molecules will run down the roof along with the rain killing and algae that may be trying to regrow on your roof.