If you live in a humid climate, you have probably noticed those nasty dark streaks on your (and your neighbors?) asphalt shingle roofs. While most people assume that the dark streaks are a result of dirt accumulation, defective shingles, mold, or mildew, what is really is may surprise you.

The most common cause of dark streaks is a blue-green colored algae called Gloeocapsa Magma. It is spread through the air by spores. Algae is not harmful to your roof, but the stains can be unsightly and may lower the value of your home.

Have you noticed that these algae stains are never found directly below the metal flashing surrounding chimneys and roof vents? That is because the zinc coating on galvanized sheet metal is toxic to algae, so whenever it rains, trace amounts of zinc are spread down the roof, making it impossible for algae to grow there.

One simple way to prevent algae growth is by installing algae resistant shingles. These shingles have trace amounts of copper, which like zinc, is toxic to algae. You can also prevent algae growth by installing a strip of copper or zinc coated metal on each side of the roof.

Algae stains can be removed, though if preventative measures are not taken, they can easily return. Here is what you will need if you are taking on roof cleaning as a DIY project:

• Cleaner (bleach, water, and trisodium phosphate)
• Pump sprayer
• Garden hose
• Safety goggles
• Rubber gloves
• Old clothes
• Slip resistant shoes
• Ladder

Cleaning on the roof can be very dangerous, particularly as the roof becomes wet. Be sure to take adequate safety precautions. Also, remember to protect your skin and eyes from the harsh chemicals.

Bleach can kill other plants in addition to the algae, and may cause harm to other aspects of the home so make sure you are rinsing any areas that may receive runoff such as grass and shrubs.

Methods used to remove and prevent algae can also be applied to moss growth on your roof and other exterior parts of your home.

For the best results, you will want to hire a roof cleaning company or exterior house washing service. Hiring professionals will maximize efficiency and lower your risk of personal injury.