Click on a roof cleaning photo to see an enlarged before and after image. 
The streaks are caused by Black algae (usually gloeocapsa magma). There is debate over whether black algae degrades your roof. It is unsightly, but can be removed using a gentle washing technique and proper chemicals. As you can see by the photo, there is a dramatic difference. The spores for gloecapsa magma are in the air. Cleaning your roof will remove what is growing on it. The spores can eventually come back since they are airborne, however it would take many years for it to build up like this.
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