CORVALLIS, Ore. — In the moist Pacific Northwest, moss on roofs is inevitable, and when it clings to shingles it can cause damage leading to costly repairs or replacements.
There’s no changing the rain, but moss also depends on shade to thrive. The most effective long-term solution is to trim nearby trees so more sunlight reaches the roof.
“You can clean all you want but to keep moss at bay, the environment needs to change. You’ve got to let the sun in or it will return.”
“You can clean all you want but to keep moss at bay, the environment needs to change,” said Jay Pscheidt, a plant pathologist with the Oregon State University Extension Service and co-author of The Care and Maintenance of Wood Shingle and Shake Roofs. “You’ve got to let the sun in or it will return.”
Mosses are primitive, flowerless plants with an estimated 12,000 species worldwide. On home roofs, common culprits include Dicranoweisia cirrata and Bryum capillare.
The moss itself doesn’t directly damage roof materials. Instead, it traps soil and debris, holding moisture and extending the time wood roofs remain wet. This invites fungi, speeds decomposition and shortens roof life.
Moss also attaches to asphalt and metal roofs, where it can lift shingles and allow water to penetrate and damage underlying layers. Regardless of roof type, keeping surfaces clean is essential.
Control moss safely
Many people appreciate moss aesthetically, but it should be removed from roofs, decks and steps where moisture increases slip risk. If you enjoy moss, let it grow in garden areas where it is safe to view and manage.
Before applying any moss-control treatment, clean the roof. Experienced users may pressure-wash, but improper technique can damage shingles. When in doubt, hire a professional or use a long-handled scrub brush instead.
Once the surface is cleared, apply one of the following with a pump sprayer:
Zinc sulfate (monohydrate): Mix 3 pounds of powder in 9 gallons of water to treat 600–1,000 square feet, depending on wood condition. For heavy infestations, mix 1 pound per 3 gallons to cover about 200 square feet. Do not use if your roof has copper gutters, downspouts or flashing — zinc corrodes copper.
Potassium salts of fatty acids: These biodegradable, soap-based products kill moss by damaging cell membranes. They are noncorrosive with minimal risk to people and pets.
With either product, avoid spraying surrounding plants and rinse any accidentally exposed foliage. Clean application tools thoroughly after use.
Safety first
Roof moss removal is not for everyone. If you choose to do the work yourself, follow these precautions:
- Wear shoes with strong traction.
- Place the ladder on stable ground; extend at least 3 feet above the roofline and secure when possible.
- Use a bucket and rope to move tools — never carry them up the ladder.
- Use safety belts and lines on steep roofs, near edges or where eaves exceed 16 feet.
- Follow all product label safety instructions.
For moss-control guidance beyond the roof, visit:
Previously titled Maintaining a moss-free roof takes some effort
