The northwest, north and west side of your roof gets more shade early in the day providing moisture for moss, lichens and algae to grow. Especially if you have a low hanging tree branch to block the sun from drying the area out. The stains can range from discolorations, ugly black streaks, to scales or flakes. If left untreated, over time the algae and moss can cause damage and decay to your shingles.
Getting Your Hands Dirty
If you have shingles with copper coated granules, than you should not have to worry about the fungus growing on your roof. You can also apply a zinc or copper oxide strip to the top of your roof. The copper releases zinc oxide when it rains washing the roof and keeping the growth down.
Another option is to clean your roof. Be extremely careful as the moss, lichen and algae are very slippery especially when your rooftop is wet. It is wise to use a safety harness to prevent dangerous slips and falls.
What You Will Need
You will need safety equipment, a ladder, garden hose, and into a pump sprayer mix 1 gallon hot water, 1 cup liquid chlorine bleach, and 1 cup laundry detergent. Starting at the top of the roof spray the mixture downward onto a small section of the roof and let sit for 15 minutes, rinse with the garden hose. Spraying the water upward could cause water to go under your shingles and create leaks inside your home. If there is no damage to the shingles continue on to the rest of the roof. Avoid scrubbing as this could loosen the granules and void any warranties you may have on your roof.
If you want to use a pressure washer be sure to start with the smallest psi first and carefully test it on a small section to check for damage. The granules on your asphalt shingles can be washed away if the pressure is too much. You will be damaging your roof if you see patches of solid black or fiberglass mesh.
Think safety, wear safety gear and rubber soles shoes, and do not mix chemicals that can be dangerous (acid, bleach and ammonia, etc.) Remember, your roof will be slippery. If you are uncomfortable being on your roof be sure to hire someone who has plenty of experience and will not cause any damage to your roof or shingles or use unsafe practices.
