If you have a roof of a certain age you may have begun to see stains and streaks appear over time—especially if you’ve got light-colored asphalt shingles. What may appear to be black mold-like stains and streaks are actually a type of blue-green algae called Gloeocapsa magma.
Will it damage your roof? No—but it definitely looks bad.
So how do you get rid of it?
How to eliminate, prevent, and hide algae on your roof
You can eliminate algae by spray washing your roof with a 50 percent mix of bleach and water. Avoid using a pressure washer to do this, though, because it’s likely to damage your shingles. Before spray washing it’s important to wet any plants you have around the foundation of your home first—plants don’t tolerate bleach well, and wetting them with plain water first can protect them. When you’re done spray washing, rinse your plants clean with water.
To prevent the growth of algae you can re-shingle your roof with shingles that are laced with copper granules, which kill algae. This option, however, would only make sense if your shingles were worn and needed to be replaced.
Another way to prevent algae growth is to use zinc or copper inserts. Have 6-inch-wide strips of zinc or copper inserted beneath the row of shingling that’s closest to the peak of your roof, keeping an inch or two of the lower edge of the metal exposed. When it rains, molecules of the exposed zinc or copper will wash down the roof and kill off any algae trying to grow on your shingles.
If you look throughout your neighborhood you may see this concept used on different roofs in the way of chimneys with copper flashing. You’ll notice that the areas directly beneath the flashing are free of algae stains.
The strips can also benefit roofs that experience moss buildup. Scrub off the moss, spray wash with the bleach/water combo we mentioned earlier, and have the zinc or copper inserts installed.
Another way to handle algae is to hide it using the art of camouflage. Replacing your roofing with new shingles that are dark enough can hide algae’s staining affect. But doing so would only make sense if your roof already needed to be replaced, and it won’t prevent algae growth.