Professionally answered below!
Most of these dark stains are algae. Much of this is caused by trees hanging over your roof. This also can also be caused by leaves left on your roof. I suggest having your trees cut back if they hang over your roof, also have your roof and gutters cleaned annually. Some companies may pressure wash your roof, however this may take life off your roof.
The dark stains on your asphalt roof are probably algae known as Gloecapsa Magma. Dark stains will generally appear on the north side (which receives less light) or heavily shaded areas of the roof first, as they thrive in wet environments caused by dew or shade. Algae travel through air so once the spores take hold on one roof infection throughout an entire neighborhood is highly likely.
Dark staining caused by algae growth is a relatively new phenomenon, resulting from shingle manufactures replacing oil based binders in their shingles with less expensive calcium carbonate binders, on which algae thrive.
While algae growth and the resulting dark staining is mostly an aesthetic nuisance, the moisture they retain can cause premature aging of the shingles.
Never attempt to clean the stains with a high-pressure power washer, as this will adversely affect the performance and integrity of the shingles. Next time your shingles need to be replaced, consider using an algae resistant shingle.
If you have green stains instead of black or dark stains, it’s probably moss resulting from overhanging trees.
I hope this helps!
Sometimes the dark stains are oil leaching out of the shingles. This can not be helped and it might be best to contact the shingle manufacturer to get there advise. If the stains are Gloecapsa Magma dark stains will generally appear on the north side of the roof so if the staining is on other sides then North it might be oil leaching out of shingles. Never attempt to clean the stains with a high-pressure power washer, Some times just running water on the roof will remove the leaching oil. You will need to water the ground and areas around the house where water is running off roof as well.
Best of luck with the problem
The dark stains or better known as black streaks are not caused by trees overhanging the roof nor do they come from leaves on the roof. It is an algae known as Gloeocapsa Magma. This is an algae that spreads thru the air and can and will land on your roof. While the algae itself will not harm the roof it will hold moisture which will then allow the growth of mosand lichens, which will harm your roof. Black streaks, moss and lichen can be cleaned off shingles without any damage to the roofing material as long as it is done the correct way. We never use any type of pressure or power washer equipment to clean a roof, our SoftWash System is recommended procedure for cleaning roofs.The dark streaks are a type of algae that lives off the minerals in some types of roof shingles. Algae thrives in moisture, so it’s seen most often on shaded or north-Âfacing roof slopes that don’t get a lot of direct sun.
While it's true that algae doesn't damage roofing, it sure looks bad. To get rid of it, use a pump sprayer to wet down the roof with a mix of one part bleach to two parts water. Bleach will damage foundation plantings, so rinse them well with fresh water before you start and after you're done.Then scrub the surface gently with a soft window—washing brush mounted on a telescoping extension pole. Don't use a pressure washer, which could damage the shingles. Also, do this work from the safety of a ladder or from the ground, not by climbing on the roof.
To stop the algae from returning, mount zinc or copper strips near the roof ridge. Then, every time it rains, the water that washes over the strips' exposed edges will pick up ions from the metal and inhibit the algae from regaining a foothold. And when it's time for a new roof, look for shingles with algae—resistant granules and the best warranty you can find. courtesy of Tom SilvaDark stains are caused by couple of things, minimul son exposure and algae, it can not be stoped as it travel in the air. After a while you will notice it turnes to green. Do not attempt to power wash it, that damage the shingles & will not solve the problem. Now in the market there are algae proof shingles. The manufatures have replaced there formula to cheeper matarials, That cause this problem.