The most common cause of staining on shingles is from blue - green algae known as Gloeocapsa Magma, which is prevalent in humid areas of the country. While algae, dirt or mold are certainly unpleasant to look at, the most harm that they’ll most likely do to your home is hurt the resale value. Be aware that attempting to remove the stains incorrectly can damage your roof. The best removal method is by chemical cleaning, since scrubbing or pressure washing can remove particles of the shingles, thus shortening the life of your roof.
The good news is, this can be done with chemicals and equipment that you may already own. All you’ll need is:
It is best to pick a cloudy day to do this, so the bleach solution doesn’t evaporate too quickly when applied. First, lay the plastic sheeting over any susceptible plants that may be harmed by the bleach solution. Next, use the ladder and remove any leaves or debris that have accumulated on the roof or in the gutters. Once that is done, prepare the roof for cleaning by soaking it thoroughly with the hose. This can be done from the ground –no need to climb the ladder. Next, put on your goggles and gloves and mix a solution of 1 part bleach, 3 parts water and 1/16 part laundry soap (for example, 1 gallon bleach, 3 gallons water and 1 cup of laundry soap). Make sure the laundry soap does not contain ammonia, since that will produce a toxic gas when mixed with bleach.
Put the solution in your pump sprayer and, using the ladder, apply generously to the roof. Allow the solution to set for about 20 minutes. Next, rinse your roof with the garden hose (from the ground). Be sure to keep your goggles on during this step, since there is a chance of getting solution in your eye as you are rinsing. Once finished, allow the roof to dry. If there are still a few stubborn stains, repeat treatment on those specific areas.