In the Pacific Northwest, we cherish every shade of green. But during the rainy season as the landscape turns verdant again, so does the moss on our rooftops. 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 shingles 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.

Need expert advice for a tough problem? Check out our sliding scale, tailored services!

3 Key Tips for Moss on Roofs

1.   Preventing Moss: Sweep & Blow When Dry

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 ridge line 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.

2.   Treating Moss: Baking Soda

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.

3.   Caution: Toxic to Aquatic Life

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.

For expert advice on your pest or weed issues, check out our sliding scale Pest and Weed Management Consultation services.

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

Click to enlarge

By Gina Gervase, 2018; updated 2021

If you have problems with moss in your lawn, visit this page.