Though those ugly black streaks and stains on your shingles might appear to be some sort of roof mold, a more accurate description would be to call it roof algae. This is a hardy species of algae called Gloeocapsa Magma, and if you aren't using the right chemicals or methods it can be extremely difficult to remove. Call it roof mold, roof algae, roof mildew, or whatever else you want, but just be sure to clean it off in a timely and safe manner.
Just to be thorough, there are other types of organic growth that can form on a roof other than roof algae (or mold). There's also the possibility of lichen or moss growth. Lichen is usually a whitish color, but it can also have tones of green, yellow, and brown. It often forms in patches of individual circles, each averaging about the size of a dime. It's also noticeably raised from the surface, unlike the black algae stains.
Moss, on the other hand, almost always has a green color and if left untreated for several years will become so thick as to be noticeable from the ground. I've seen some situations where it's been ignored for so long that it literally looks like a roof is covered with green carpet. Honestly I don't know what people are thinking when they allow this to happen to their home.
So how does one go about treating a roof with lichen and/or moss? Well, in most cases it should be handled in the same manner as the black algae roof stains (or "roof mold"), which has already been described in length on this site. In a nutshell, a bleach or chlorine-based solution applied with non-pressure methods will kill lichen and moderate moss growth almost instantly. It should turn a whitish color within minutes, which tells you that it's dead. Unlike black algae stains which come off immediately with the rinse phase of the cleaning, dead lichen and moss will require a few heavy rains to finish rinsing off the roof. It could take awhile but rest assured that lichen and most moss situations, once dead, will eventually dislodge and rinse off the roof thanks to mother nature.
If you are the impatient type and want the lichen and moss removed immediately, you'll have to use a pressure washer, but as you should know by now I highly discourage the use of high pressure on a roof. In those rare cases where the moss is extremely thick and out of control (this is more common in the northwest) your only choice might be to use a pressure washer, but even in these extreme scenarios you should dial down the pressure and use extreme caution.
In summation, black roof "mold" stains (algae) should come off immediately if using the proper methods. Roof mold removal doesn't have to be rocket science, people. White or green lichen and moss, on the other hand, will be killed with the same chemicals but will need a few heavy rains to finish rinsing away. If this all just sounds like too much of a hassle and you'd rather keep your feet safely on the ground (good idea) then do take a look at our directory of non-pressure roof cleaning services. There's more than likely a company in your area that knows how to remove your roof algae in a professional and safe manner.
