Share this

I should be a large image.

Moss on Roofs & Walkways

Several species in the Pacific Northwest
Updated Apr 20, 2023
 
1

Make a Positive Identification

Common mosses found on structures in the Pacific Northwest include Dicranoweisia cirrata and Bryum capillare, among others.

Species: Moss
Moss growing on wall

"Oxford Moss" by lunaman is marked with CC BY-NC-SA 2.0 (cropped).

Moss grows on surfaces in shady areas. It is rarely a problem when growing on rock walls and landscape features, as in the photo above.

Species: Moss
Moss growing on seams of asphalt shingle roof

Inga, iStock

Moss grows on roofs. It can damage shingles and lead to water damage. Removal of moss on composite shingles as shown in the photo helps to prolong the life of the roof.

Species:
Moss growing on sidewalk is a slipping hazard

Weston Miller, Oregon State University

Moss growing on sidewalks is a slipping hazard. Removal of moss from high-traffic areas is advised.

LOOK-ALIKES: LIVERWORTS AND LICHENS
Species: Algae
Algae on deck

Scharvik, iStock

Algae grows on walls, sidewalks, and patios in shady locations. Algae on walkways is a slipping hazard.


Different risks or methods

Pressure-wash algae growing on patios. Preventive measures for moss will help for algae. To minimize regrowth, you must also eliminate or lessen the factors that lead to algae growth.

Species: Liverworts
Liverwort plants

"2009.07.03-11.04.41_IMG_5548" by Andrey Zharkikh is marked with CC BY 2.0 (cropped).

Liverworts grow in moist environments with mosses. They rarely require any management.


Helpful

Liverworts are usually not a problem and can be left alone.

Species: Lichens
Light-colored lichen growing on a dark surface

"Lichen close-up" by Linda, Fortuna future is marked with CC BY-NC 2.0 (cropped).

Lichens look similar to mosses, but are not plants. Lichens tend to grow more slowly than mosses. Unlike moss, they don’t contribute to wood or composite shingles damage.


Helpful

Lichens are usually not a problem and can be left alone.

Free help Identifying Weeds, Insects & Pests
Get expert pest management info & advice online from OSU's Ask Extension.
Get Help
 
2

Moss on Roofs & Walkways Benefits

  • Moss grows throughout the Pacific Northwest.
  • It covers surfaces and provides habitat.
  • Moss absorbs water, and has a role in nutrient cycling and erosion control.
  • It’s used as an indicator of air pollution.
  • Moss adds an attractive element to landscapes.
 

Moss on Roofs & Walkways Risks

  • It becomes a problem when it grows on roofs, patios, decks, and walkways.
  • Moist, shady rooftops or shaded patios are ideal for moss growth.
  • Over time, moss growth slows or prevents water flow off the roof. It can contribute to shingle damage.
  • Moss on patios, decks, and walkways can be slippery. Remove it when needed.
Risk Card
Does it cause harm?
Adults & Children
Some
Property
High
Pets
None
Annoyance
High
Environment
Helpful
Action Recommended
 
3

TAKE ACTION?

Moss is common. Different areas and amounts of moss growth require different levels of management.

Do I need to take action?

  • Consult with a roofing professional about moss on your roof. Determine if you should take action to control moss on your roof. Weigh the benefits and risks of taking action and not taking action.
  • Remove moss from walkways if it is a slipping hazard.

What if I do nothing?

  • Moss growth may reduce the lifespan of your roof.
  • The sooner you act, the cheaper and easier it is to manage. If you ignore moss, you may have additional water damage to repair when you replace the roof.
 
4

Prevent Moss

Broom sweeping debris on roof

Weston Miller, Oregon State University

Keep the Roof Clean
  • Annual maintenance is essential to minimize moss growth on roofs and other structures.
  • Remove dead leaves, conifer needles, and debris by sweeping, leaf blowing, or washing. Remove debris before rain.
Conifer branch shading shed roof

Weston Miller, Oregon State University

Minimize Shade Where Possible
  • Remove or thin overhanging tree branches and vegetation that shade the surfaces you are maintaining.
  • The conifer branch in the photo sheds needles and shades the roof. These factors lead to moss problems.
Metal roof with no moss growth

Weston Miller, Oregon State University

Invest in a Metal Roof
  • Consider replacing a shingled roof with a metal roof. Moss doesn’t grow easily on metal roofs. Metal is too slippery for moss to take root.
  • The initial cost for a metal roof is higher than for a shingle roof. But they require minimal ongoing care and have a long life span.
Zinc strip on roof with no moss growing below it

ROTban Ultra Cleaning Corp of Greater Vancouver, BC Canada.

Zinc Strips Are Not Recommended
  • Installation of metallic zinc or copper-coated strip products on your roof is not effective to eliminate moss.
  • Moss won’t grow on the strip itself. Plus, rain water washes metal particles down the roof. This prevents moss from growing immediately below the strip as shown in the photo.
  • Also, zinc strips are typically nailed to the roof. This could lead to roof leaks if not installed correctly.
  • Zinc and copper are harmful to fish and other aquatic organisms.
 
5
SOLUTIONS FOR MOSS ON ROOFS AND WALKWAYS
  • Assess the problem. Decide if you want to take on the problem yourself. Is the roof safe for you to walk on and repair? How will you gain access to the roof?
  • Should you consult with a roofing professional or a moss removal specialist?
  • Remove the bulk of the growth.
  • Moss killer (mossicide) works to kill moss. However, you must still remove the dead material. Moss killers don’t prevent moss from regrowing.
  • After removing moss, you should commit to an annual maintenance routine. Follow up to minimize regrowth on roofs and other structures.

NEED HELP?

Consider a licensed pest control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon  and other states  can suggest other methods.

Jump To

Method Does it work? Is it safe? Recommendation
A
Physically Remove
Somewhat effective
High risk
B
Moss Killer (Mossicide) Products
Somewhat effective
High risk
Use if Necessary
C
If Using Moss Killer, Protect Yourself and Minimize Risks
 
A

Physically Remove

Non-Chemical Method

Weston Miller, Oregon State University

Physically Remove

Remove moss by scraping or scrubbing.

Does it work?
Somewhat effective

Use preventive measures for best results. To minimize regrowth, you must also eliminate or lessen the factors that lead to moss growth.

How much effort?
High effort

Scraping and scrubbing a patio or walkway takes effort. Accessing your roof and removing moss is a major chore.

What's the risk?
High risk
Possible risk of exposure or harm from chemicals
NONE
  • Remove moss by scraping with a dull blade such as a putty knife. Scrub it with a stiff bristle brush. Or sweep it off with a broom with stiff bristles.
  • Dead moss is removed the same way as living moss.
Moss growing in asphalt shingle seam

"Free Insulation" by Luke Miiliron is marked with CC BY-NC-ND 2.0 (cropped).

Scrape off the moss with a putty knife. Or use a brush. Asphalt shingles damage easily, so be gentle.

Pressure washing wood deck

BanksPhotos, iStock

Pressure washing is effective for decks, patios, and sidewalks. It’s not advised for roofs. Pressure washing damages shingles.

Moss Removal Tips

  • Following moss removal, clean roofs of all dead leaves, conifer needles, and debris. Sweep or use a leaf blower to clean the surface before rain.
  • Remove or thin overhanging tree branches that shade the surfaces you want to maintain.
 
B

Moss Killer (Mossicide) Products

Chemical Method: Use with caution

Bill Oxford, iStock

Moss Killer (Mossicide) Products

Use if Necessary

Moss killer (mossicide) works to control moss. However, you must still remove the dead material.

Does it work?
Somewhat effective
  • Moss killer treatments are short acting. They wash off surfaces during rainfall.
  • Moss killers don’t provide long-term moss control.
  • Use preventive measures for best results.
  • To minimize regrowth, you must also eliminate or lessen the factors that lead to moss growth.
How much effort?
High effort
  • Using moss killer on a patio or walkway takes effort. Getting access to your roof and spraying is a major chore.
  • You still need to scrape off the moss after treatment with mossicide.
What's the risk?
High risk

Fall Safety Considerations

Moss Killer

  • Moss killer products (mossicides) come with real risks. ALWAYS read the entire label front to back. Use a magnifying glass.
  • These products can run off your site into waterways and may harm wildlife, especially aquatic organisms if runoff enters waterways. See How to Keep Pesticides Out of Waterways.
Possible risk of exposure or harm from chemicals
Using moss killer (mossicides) includes some amount of risk. The lowest risk comes with using alternative methods.

You may be exposed to a mossicide if you:

  • Get it on your skin
  • Breathe it in
  • Eat or smoke afterward without washing hands
  • Touch or eat plants that are wet with spray (you, pets, or children)
  • Bring it inside on your shoes or clothes

Follow directions closely to reduce risk.

Moss killer (mossicide) products work to kill moss when used according to the label instructions. To minimize regrowth, you must also eliminate or reduce the factors that lead to moss growth.

Mossicide product for use on lawns, decks & roofs

Weston Miller, Oregon State University

Moss-Killing Soap and Citrus Oil

  • The label states the product is for use on lawns, decks, and roofs.
  • Moss-killing soaps that contain ammonium and potassium salts are effective for killing moss. Also, products containing citrus oil (d-limonene) work. Both types of products are biodegradable and non-corrosive.
  • Avoid overspraying on adjacent plants. Rinse any plants that are accidentally covered.
  • Don’t allow runoff that contains moss-control products into storm drains or waterways.
Mossicide product for roofs and structures

Weston Miller, Oregon State University

Copper, Iron, Zinc & Ammonium Products

  • Zinc sulfate corrodes copper gutters, downspouts, and flashing.
  • All these products are corrosive. They are toxic to humans, pets, landscape plants, and aquatic animals.
  • Products that contain zinc or copper are not biodegradable.
  • Some products stain concrete.
  • Don’t allow runoff that contains moss control products into storm drains or waterways. The chemicals can harm aquatic life.

Application Tips

  • Accessing the roof to apply moss-control products involves risk of falling.
  • If you kill moss with a chemical treatment, you will then have to clean the dead moss off the surface.
  • Preventive chemical treatments for moss are short-acting. To minimize regrowth, you must also eliminate or reduce the factors that lead to moss growth.
  • Mossicide products wash off the roof, patio, and sidewalk with rainfall. They don’t provide long-term moss growth control.

ExampleS of Products With Copper, Iron, Zinc & Ammonium Active Ingredients:

  • Ammonium sulfate
  • Copper sulfate
  • Ferric and ferrous (iron) sulfates
  • Sodium pentachlorophenate
  • Zinc chloride
  • Zinc sulfate
 

If Using Moss Killer, Protect Yourself and Minimize Risks

Chemical Method: Use with Caution

Mossicides can travel into waterways and harm aquatic life. They can damage nearby plants.

Why is it important to read moss-killer labels?

  • You have detailed information on how to use the product correctly and legally.
  • They contain information on potential hazards of the product.
  • They offer instructions you should follow for poisonings and spills.
  • Following label instructions helps you to minimize the risks and maximize the benefits.

Key Moss-Killer Safety Tips

  • Read the entire label front to back. Follow the instructions.
  • Review the instructions even for brands you know.
  • Moss-control products often require goggles. Glasses are not sufficient.
  • Only apply the product where the label says it may be applied.
  • Be precise in your application. More is not better.

The Label is the Law

ALWAYS read the label before using mossicide products. The label is a legal document that provides information on how to safely use the mossicide. This helps avoid harm to human health and the environment. Using a mossicide in off-label ways is illegal. It can result in legal enforcement actions.

READ THE LABEL & Follow Instructions
It has instructions to protect you and the environment.

  • Labels are different for every product and they often change over time.
  • Use a magnifying glass for small print.
  • Pay attention to CAUTION, WARNING, and DANGER statements.
  • Pay attention to the PRECAUTIONARY STATEMENTS.
  • The law states you must read and follow mossicide instructions.

Avoid Wet, Windy or Hot Weather
Use during favorable weather for best results.

  • Don’t use when it’s raining or when rain is expected in the next 24 hours. Mossicides can wash into waterways and harm aquatic life.
  • Wind causes spray and dust to drift that can get on you and desired plants.
  • Don’t let the spray or dust contact plants that you want to keep.

Protect Yourself
Eye, skin & lung irritants

  • Wear the right protective gear. This often includes chemical-resistant gloves, safety goggles (safety glasses often not enough for mossicides), a long sleeve-shirt, pants, socks, and shoes.
  • Take care not to inhale the dust.
  • Wash hands after applying and before eating or smoking.
  • Take a shower immediately after handling mossicides.
  • Wash clothes worn while mixing or applying separately from other laundry.

Protect Children & Pets
Children and pets are risk if they eat or touch treated areas before mossicides dry.

  • Keep them away during and after applying mossicides (read label for how long).
  • Remove toys and pet dishes from yard before applying.
  • Don’t track mossicide products into home on shoes or clothes.

Storage & Disposal

  • Store in a secure storage area away from children.
  • Don’t put unused mossicides products in the trash. Take them to a hazardous waste facility.
  • Never pour product down any drain or waterway.

Call  1-800-CLEANUP (1-800-253-2687) to find out where to dispose of mossicides.

For the Portland metro region in Oregon, contact Metro’s Recycling Information. Call  503-234-3000, email   or visit Metro’s website  

More about:

About Using Pesticides on School Grounds in Oregon

If using pesticides on school grounds, there are special rules in Oregon. See School Integrated Pest Management  (Oregon Department of Agriculture).

NEED HELP?

The National Pesticide Information Center (NPIC)  can answer questions about pest control chemicals.
 1-800-858-7378 or npic@ace.orst.edu  

Consider using a licensed pest or weed control company. Learn How to Hire a Pest Control Company.

Your local Extension Specialist in Oregon  and other states  can suggest other methods.


Content provided by editor Weston Miller and writer J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.

 Peer reviewed by OSU Department of Horticulture.

Photo of Weston Miller

Weston Miller

Project Founder and Content Writer

Weston Miller served as Community and Urban Horticulture faculty for Oregon State University Extension Service for Clackamas, Multnomah, and Washington Counties. Weston is an author for content for this website. He developed funding partnerships with Portland area agencies to initiate and build out the Solve Pest Problems website focused on this goals:

J. Jeremiah Mann

J. Jeremiah Mann

J. Jeremiah Mann completed a Physical Science undergraduate degree at Humboldt State University, and M.S, Ph.D focusing on plant science topics at UC Davis. He went on to work for the Natural Resources Conservation Service and in a leadership position serving a private agricultural technology company. He currently lives in Sacramento California where he consults on pest and property management topics.

Photo of Kaci Buhl

Kaci Buhl

At the state level, I lead the Pesticide Safety Education Program (PSEP). The program hosts live recertification events around the state, serving over 1,000 licensed pesticide applicators each year. We also produce web-based training modules and license-preparation study manuals. Special training for unlicensed pesticide applicators is also available through a grant from the USDA Specialty Crop Block Grant Program. The PSEP at OSU works closely with the Oregon Department of Agriculture's Pesticides Division.