Prevent and Treat Moss on Roofs

(by Gina Gervase, Communications Assistant)

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In the Pacific Northwest, we cherish every shade of green. From our lush, fir tree forests and endless grassy pastures to our sustainable practices ethos, we are green. But there are times when greenery can be a nuisance and even destructive.

During the rainy season, as the landscape turns verdant again, so does the moss residing atop our roofs. Revived with the winter rains, the resilient Bryophyta begins to soak up and store excess rainwater as it has no roots. The stored water begins to seep under to the shingles and can seep through the roof underlayment and sheathing, the protective material between the singles and the roof deck. Waterlogged roofing material can lead to unhealthy growths of mold and costly repairs from rot, sometimes compromising the structural integrity of your roof.

Beautiful as it may be, moss belongs on trees, in between rocks, and on forest floors, not on your roof. The best way to avoid costly repairs is to maintain a moss-free roof which can be accomplished through cleaning as little as once or twice a year. Treating a mossy roof can be more time consuming but is doable with the right tools.

So what is an environmentally-conscious Pacific Northwesterner to do? We’re here to help you figure out which method will best prevent moss from growing on your roof or treat it if you already have a mossy roof.

Preventing Moss

Since moss likes shade, moisture and a certain pH, amending any of these can help. Moss also needs a substrate to attach to. Dirt or particles on roofs can create places for moss to start growing, so the primary preventative each year is to clean your roof. This can be done by having it swept and blown once or twice a year (when dry) to remove leaves, twigs and any dirt particles. After blowing, apply baking soda at the ridgeline in late summer. Baking soda will raise the pH, which moss doesn’t like. Baking soda washes off easily, so more than one application may be needed each year.

Treating Moss

For treatment, use baking soda when rain is minimal and allow several weeks for the moss to die. Products containing d-limonene (citrus oil) are also effective at killing existing moss. Results are usually seen in 2-3 days. With either treatment method, some moss will slough off naturally as it dies. Gently remove the rest with a wire or nylon brush.

Among the products to use with caution are zinc, potassium salts of fatty acids and oxidizing bleaching agents that do not contain chlorine.  Zinc strips can be installed at the top of your roof to prevent moss from growing. As rain runs over the strips, zinc drips down your roof. Zinc is known to be toxic to fish, and NCAP does not recommend this as a first option.

Potassium salts of fatty acids can be an effective option for moss treatment, but are known to be toxic to aquatic invertebrates. Oxidizing bleaching agents that don’t contain chlorine are usually based on peroxides such as hydrogen peroxide, sodium percarbonate and sodium perborate. Tests appear incomplete on some of these products, but they are considered slightly hazardous to water. If you choose either of these, we recommend disconnecting your downspouts during the application and cleaning process, to allow the wash a chance to infiltrate into the soil more before reaching streams.  In addition, it is probably best to use these early during the dry season, to reduce the likelihood of chemical runoff.

See our Moss on Roofs flowchart to help you choose the option that's right for you:

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Click to enlarge

Showing 4 reactions

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  • commented 2018-03-05 10:30:02 -0800
    Thanks for the question Lon! Copper impairs the sense of smell for salmon, at fairly low concentrations. Smelling is how they imprint on their natal streams and find their way home after their ocean migrations, so we don’t recommend the use of copper on roofs.
  • commented 2018-03-01 14:38:03 -0800
    How about copper metal at the ridge just as zinc is suggested? Copper is toxic to conifers, but how much copper can be tolerated in runoff and how much to be effective aganst moss? Copper sulfate solution toxicity? What is down side to copper sulfate?
  • commented 2018-02-14 14:42:25 -0800
    I’m so glad you didn’t recommend using tide! I work in the moss removal business and loved your post. Here’s a link to more information: https://www.nanaimomossremoval.com/moss-removal—demossing.html

    Sincerely, Tim Alan
  • commented 2018-02-10 21:32:41 -0800
    Just switched to a metal roof after constant issues with my roof over the past 15 years. Great article, I wish I would have read this before spending thousands on roof repair.