What Causes Asphalt Shingles to StainMy Shingles have a black stain on them. We wanted to know what causes asphalt shingles to stain like this? It is very unsitely when you look at our roof from a distance. The shingles were advertised as 25 year shingles and this is only year 14! They appear to be wearing normally. There is no curling at the ends or abnormal loss of the grit. We also are well ventilated so the attic does not overheat in the summer. It is unsightly and since we live at the end of the street, everyone sees it when they drive down our street. We wanted to know what was causing this and what we could do about it?

What Causes Asphalt Shingles to Stain

Turns out that we live in an algae prone area. If you live in a humid area, there is a good chance that you will encounter this black algae on your roof. It is not harmful to the roof and will not cause your shingles to age prematurely.

The stains are not everywhere on the roof. Anytime there is a metal flashing or air vent, the area just below them are free of the black stain. This is due to the metal from these items being washed onto the roof in very small amounts.

You can wash the roof with chemicals to kill the algae, however it usually returns the following year. You may as well save your money. When it is time to replace your roof purchase algae resistant shingles to avoid this problem.

There are several products on the market specifically designed to remove algae stains from a roof. These include Wet & Forget and Moss Out. A mixture of trisodium phosphate (TSP), bleach, and water will also remove stains. Oxygen bleach lightens stains as well and is less harmful to the environment, but it doesn’t produce as immediate or dramatic an effect as chlorine bleach. Be careful that you do not damage the roof each time you walk across it while applying the chemicals.

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