Black mold stains, streaks, and clumps of moss growing on the roof look unsightly. You may also worry that mold growth will harm your asphalt shingles and reduce the roof’s lifespan. No one likes having to buy a new roof much sooner than anticipated. In hot and humid climates like Tampa, algae growth on asphalt shingles can be a real problem. Why do algae grow on the roof, and what are the safest ways to clean asphalt shingles? We’ll show you how to clean mold off the roof the right way, and how to prevent algae from growing in the first place.
Are those black stains on the roof mold or algae?
Mold, algae, and moss are very different organisms. Each requires a different method of safe removal and prevention to keep your roof healthy and looking good. Black stains or streaks on the roof will appear anywhere in the U.S., where it’s hot and humid. In Florida, roof discoloration is a common problem. Most people will refer to these stains as “mold,” but algae actually cause them.
What’s the difference between mold and algae seen on a roof? Mold is a fungus that eats a variety of nutrients to grow and thrive. In contrast, algae is technically a plant organism that requires all the things other plants need to grow—namely, sunlight and moisture. Mold won’t form on a roof because it’s allergic to sunlight, and as soon as the ambient temperature surrounding the mold becomes dry, the mold dies.
Moss, those green, feathery clumps you may see forming on the roof, is a different type of plant and isn’t the same thing as algae or mold. Algae is a single-celled organism, while moss is not. Moss also looks like a plant with its tiny stems and leaves. Algae do not have plant-like characteristics. Instead, it spreads out as a clump of single cells and will be slimy to the touch.
No matter which two of these nasty things you’ve got growing on the roof, all require moisture to survive. Moisture tends to accumulate on the roof’s surface, especially in humid and coastal climates. Usually, these organisms will take several months or even years to grow before they become visible from the ground.
Why do algae and moss grow on asphalt shingles?
The wind and small animals will carry algae spores, and it’s one of the reasons why you may see algae-covered roofs clustered in a single neighborhood. Moss, on the other hand, likes moist, shady environments and often grows on the north-facing side of trees. Overhanging tree branches and debris left to accumulate on a roof is why you may find clumps of this green stuff gathering on asphalt shingles.
Will algae and moss growth damage asphalt shingles?
Algae will not damage asphalt shingles. It just looks terrible. But moss is a different animal—it’s sponge-like and collects moisture. That moisture then sits on the surface of the roof for an extended period, keeping the roof wet. When this happens, shingles will curl and pull away from the underlayment, which weakens them. In a windstorm, the shingles can get torn off the roof and shorten its lifespan if you don’t repair the roof.
How can homeowners discourage algae and moss from growing on the roof?
Trees overhanging the roof can encourage algae and moss growth. So trim back any overhanging branches. Twigs, leaves, and sticks that accumulate on the roof should be promptly removed with a leaf blower. Always direct the airflow down the slope of the roof, so debris doesn’t get stuck under the shingles.
Keep the gutters clean and clear. If a higher roof is draining onto a lower roof, that’s a recipe for algae and moss growth. A simple fix is to direct the downspout of the upper roof into the lower roof gutter.
If you’re due for a roof replacement, then installing algae-resistant shingles is an excellent way to prevent unsightly algae growth. Algae hate copper, and you can also put strips of copper (or zinc) under the shingles nearest the roof peak. This will prevent future algae cells from making your roof their home. You can also replace your old asphalt shingles with new ones in a darker color to hide streaks and stains.
What’s the safest way to clean algae and moss from asphalt shingle roofs?
You might think that taking a power washer to the roof is the quickest and most effective way to remove algae and moss from the shingles. But power washers are too strong for the shingle granules and will wear them down. Asphalt shingles are designed to withstand rain, not the force of a power washer. Power washing your roof will reduce its lifespan.
Instead, gently pray the roof down with a mix of half bleach and half water. Let the mixture sit on the roof for 20 minutes before you rinse it off. But be sure to wet your plants around the foundation down with water first. This will protect them from the bleach mixture run-off. Then, rinse your foundation plants down with clean water again, so they aren’t damaged.
Rinsing the roof after bleaching it won’t make it look pristine. Your roof will actually look a bit worse before it gets better. The algae will slowly disappear with each rainfall after you bleach it. Moss, however, will look yellow right after bleaching. Gradually, it will loosen from the roof shingles. Once it does, then you can safely remove it with a leaf blower. It can take several weeks to several months for the moss to pull away from the shingles.
Removing Moss and Algae from Asphalt Shingles: The Bottom Line
Preventing algae and moss from growing on your roof is easier than trying to remove it. Removal can take several months before the moss can be blown away, revealing clean shingles underneath. Always keep your roof clear of overhanging branches, and clean your gutters regularly. If your roof in Tampa needs repaired thanks to unchecked moss growth, contact Code Engineered Systems today for a free, no-obligation quote.