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Why Is There Moss on My Roof?

Posted December 2, 2021 by Jerry Becker
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While most people don’t like roof moss because it ruins the look of their home, moss can actually seriously damage a roof. Mainly because moss can start growing underneath the shingles and go unnoticed for a long time, at which point you’ll need professional moss removal to get rid of the annoying and unsightly growth.

What is Moss?

What most people refer to as “moss” growing on the roof is actually a mixture of 3 different organisms:

  1. Moss: A small, non-vascular flowerless plant that grows in dense clumps, which are springy to the touch. Moss needs a lot of water to grow, but can’t grow completely immersed in water.
  2. Algae: A simple plant that grows in clusters in water. Algae is also non-vascular, meaning it has no root, stem or leaves.
  3. Lichen: An organism that grows from a combination of fungus and algae. The fungus gives lichen the majority of its characteristics, while the rest comes from green or blue-green algae (sometimes both).

Why Does Moss Grow on My Roof?

Moss thrives in cool, damp environments with a lot of shade. These conditions prevent moisture from evaporating, which allows moss to start growing between the shingles.

Since moss needs a lot of shade and moisture, you’ll usually find it growing in areas with little sunlight, such as:

  • Areas shaded by trees
  • North-facing sections of the roof (since that direction gets the least amount of sunlight)

Once moss starts growing, it absorbs water and grows exponentially, eventually becoming several inches thick.

You may have heard about anti-moss shingles, but unfortunately, those don’t actually exist. Some shingles are anti-algae (only for certain types of algae), but the only thing that prevents moss from damaging your roof is regular cleanings and preventative agents.

Does Moss Damage My Roof?

While a little moss growth may seem harmless, it can cause serious damage and weaken the structure of your roof.

Moss growth can:

  • Loosen shingles. As moss underneath shingles grows thicker, it lifts the shingles and causes them to become loose. You’ll eventually need to replace the loose shingles to prevent leaks and water damage to the roof frame.
  • Strip protective oils. Asphalt shingles have oils that hold the top layer of granules in place. Moss strips these oils, which causes the granules to erode. These granules protect the shingles from ultraviolet rays (which break down asphalt), eventually ruining the shingle.
  • Cause wood rot. Moss absorbs and retains water, causing the wet moss underneath the shingles to rest on the roof’s wood frame. This constant contact with moisture causes the wood to grow mold and eventually rot.
  • Add additional weight. The water that moss retains when it rains can add a lot of additional weight to your roof—especially if you have significant moss growth. Your roof isn’t designed to hold that much additional weight and will eventually need reinforcing or repairing.
  • Block gutters. Like other debris, moss can grow thick enough to clog your gutters, preventing them from carrying water off your roof as they should. Removing moss helps clean out your gutters to get them flowing freely again.

Learn more about how moss damages roofs in our blog, “Ask a Pro: Is Moss on My Roof Harmful?

How To Prevent Roof Moss Growth

The best things you can do to prevent moss growth on your roof is:

  • Keep your roof and gutter clean and free of debris. Moss thrives where there’s a lot of moisture and shade. Debris like leaves and branches trap moisture when it rains, keeping your roof wet longer than necessary and enabling moss growth.
  • Have your roof treated to prevent moss growth. Professionals can apply agents to your roof to help prevent moss from growing again after it’s removed. However, some agents are corrosive and can damage your roof even more. At Roof Life of Oregon, we thoroughly inspect and clean your roof before treating it with a non-corrosive, non-toxic moss prevention concentrate that prevents moss growth for 6 months to a year.
  • Get regular roof cleanings. Regular roof maintenance keeps your roof free of moss and debris, which helps prevent expensive repairs. We recommend roof cleanings every 3–5 years, depending on the type of roof you have.

Contact Us for Moss Removal and Prevention in Portland, OR

Call us at (503) 925-0125 or fill out the form below to get a quote for removing moss from your roof and helping it stay off.

At Roof Life of Oregon, our professionals can go over why moss grows on your roof before inspecting and cleaning it. We genuinely care about your roof, so we make sure to only use safe, environmentally friendly moss removal and prevention products.

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