There are several methods of cleaning asphalt roofing. From bleach solutions to special chemicals. Depending upon the current condition and age of your roof, one product may work better then another. Do have the ability to post a photo of the roof in question and or explain exactly what is going on as far as condition of roof, Is it just stained, or is there moss growth on it as well? How old is the roof, how steep etc.

Home Depot is selling a new product in their roofing section...it might be work a go for a homeowner.....the product screws onto a garden hose. You can also add zinc strips along the roofline which will form zinc oxide whenever it rains. This will kill any mold as a permanent solution. Obviously, stay off the roof while doing this....or be extremely careful....a friend of mine fell about 10' while doing a repair and now has a broken back after 4 days in the hospital...she's facing surgery now