Although these dark spots and ugly black streaks on your roof may look like dirt, mildew, soot or mold, what they really are is algae. The most common type is known as gloeocapsa magma—also known as blue green algae. This algae growth usually begins as small spots and then eventually becomes visible streaks. By the time this algae is noticed by the homeowner, it has usually been on the roof at least two or more months. It can be cleaned safely though with the proper chemicals and equipment.
If and when you want to have pressure washing done at your residence or business, please call a professional. Although you can find cheaper prices, sometimes you can get what you pay for, so be careful and make sure you hire a professional that has the knowledge and experience to get the job done correctly.