Lush and verdant, thick moss overtaking a roof can be picturesque. While it does give a house a fairy tale, Hobbit-like look, moss contributes nothing else to a roof. In fact, if moss is left unchecked, it can lead to serious damage to your home. When roof moss gets thick enough that it crowds out the shingles and creates decay, there is no turning back. At that point, the roof must be replaced.
But there are easy ways to kill moss on your roof with either dry or wet applications. Less easy but of far greater importance is the next step of removing the dead moss from the roof. Both tasks work hand-in-hand to ensure the longest possible lifespan for your roof. After you have killed the moss on your roof, you must remove the dead moss manually, or it will act like a sponge and soak up water, further promoting decay. Dry moss will not flake and blow away on its own.
How Moss Will Ruin a Roof
North-facing roof areas that receive little sunlight are prone to moss. Within these areas, though, moss does not grow in equal quantities everywhere. Moss develops in perennially shady areas and in places that tend to be cool and damp. So, even if you do live in a cool, damp climate, sections of the roof that get strong sunlight every day may not develop moss.
Moss on a roof begins with a thin dusting of green that you may notice only from low angles. This thin layer on the top of the shingles expands and becomes wide, thick, and mat-like. The seams between the shingles and the shingles' edges also develop moss because they tend to be especially shaded.
As moss thickens, it works its way under the shingles and raises them up. If the shingles are wood, this process can happen at an alarming rate. Wood's porous surface is prime real estate for moss growth. Once moss has adhered to wood shingles, the moss can be harder to remove than it is from the relatively smoother planes of composite or asphalt shingles.
Once the moss is a thick mat, it becomes a sponge that soaks up and retains all moisture. This stored water works its way under and between shingles and then onto the lower levels of roofing felt and the structural elements of the roof. This eventually leads to rot, and rot leads to further decay of roofing materials.
Working With Moss Killers
Moss-killing products are generally grouped into to main types: dry powder and liquid. Dry powder moss killers are applied by sprinkling the powder in multiple lines parallel to the ridge of the house, spacing the lines about 2 to 4 feet apart. When it rains, the water mixes with the powder and runs down toward the eaves. After about a week, the moss should clear.
Dry powder moss killers, being hands-on products, force you to go onto the roof and take stock of the moss problem. This allows you to view the moss in advance of the removal process that follows. However, dry moss killers can be difficult to dispense evenly, and they can leave white streaks that sometimes remain on the roof until several hard rain showers wash them away. At the same time, dry powder that has just settled can blow off if a strong wind comes before the rain.
Liquid moss killer comes in a container that attaches to a garden hose and mixes on a metered basis as the water flows. You can apply these from the ground, as long as you have sufficient water pressure to reach the ridge of the roof.
Liquid moss killers cover the roof more evenly than dry powders, and they let you stay on the ground for the application. If the water will not reach the peak of the roof from the ground, you can spray while standing on a ladder. Even so, you will always need to mount the roof later on in order to remove the dead moss.
Because it can be difficult to keep track of your progress when spraying the roof, it helps to mentally section out portions of the roof and spray each section before moving onto the next one. It also helps to start with a dry roof.
Popular Moss Killer Products
There are several widely available moss killer products on the market. They are commonly sold at home centers and through online retailers like Amazon.com.
Bayer 2-in-1 Algae and Moss Killer
Bayer 2-in-1 Moss & Algae Killer & Cleaner is a potassium soap of fatty acids and inert ingredients in liquid form. Unlike zinc-based moss killers, this product is advertised as being non-corrosive to metals. Roofs generally have a lot of metal in the form of vents, gutters, drains, nails, and cables, so this can be a plus for most types of roof.
Bayer 2-in-1 Moss & Algae's spray nozzle creates a flat stream that is easy to apply. Be careful not to deplete the bottle long before you have covered all of the mossy areas. The solution runs through quickly, so hit the target area once and then move on. If you want to double-spray, you must wait until after the initial run-through.
Moss B Ware
Moss B Ware is 99-percent zinc sulfate monohydrate, a proven moss killer and moss deterrent. In fact, one way to prevent moss from growing in the first place is to attach zinc strips to the ridge of your roof. Over time, as rain falls, zinc residue coats the roof to prevent moss growth.
Low cost and availability are strong advantages to this product. Moss B Ware can cake up in the container, but the product can be broken up by hitting the container against a hard surface or tapping it gently with a hammer. If you like liquid applications, this dry product can be mixed with water at the rate of 3 pounds per 5 to 10 gallons of water, which makes enough to kill moss over an area of 600 square feet.
Lilly Miller Moss Out!
Like Moss B Ware, Moss Out! from Lilly Miller is 99-percent zinc sulfate monohydrate. The active ingredients in Moss Out! are the same as Moss B Ware, but the texture and delivery system are different. Moss Out! is more granular and less powdery than Moss B Ware, and this makes it easier to shake out onto the roof.
Some communities may have restrictions about flushing zinc into stormwater discharge areas. Check with your local water quality agency about zinc runoff restrictions.
Equipment / Tools
- Tall extension ladder
- Garden hose
- Putty knife or painter's 5-in-1 tool
- Moss killer
Apply the Moss Killer
To apply dry powder moss killer, place an extension ladder against the house and climb to the roof. Dispense the moss killer in lines about 2 feet apart, parallel to the roof ridge. Maintain a consistent flow of moss killer down the line.
To apply wet moss killer, attach the garden hose to the container. Run the water for a moment until the moss killer begins to flow out. Then spray the roof, beginning at the ridge and working downward.
Moss killer works at any time of the year. Typically, it is applied in the fall, when sunlight is diminishing and moss begins to develop. Moss removal must occur after the moss is fully dead. You may need to wait at least a month or two for the moss to turn brown and die.
Wait for Precipitation (Dry Moss Killer Only)
Dry powder moss killer is activated and dispersed by precipitation. A light rain on the roof will cause the moss killer to flow downward and cover all areas of the roof. Lightly spraying the roof with water is another way to accelerate this process.
Scrape Off the Moss
Climb up to the roof and begin scraping off dead moss from the shingles, using a putty knife or painter's 5-in-1 tool and working from the bottom up. Use the tool to gently lift away matted moss that has gathered on the flat sections of the shingles. In the seams, use the sharp edge of the scraper like a dental pick to force out the moss. Work your way up to the roof ridge. If there are multiple roof planes with moss, repeat the same process with each section, starting at the bottom and working up.
Remove moss only when the conditions are perfectly dry. Never work on a wet roof, which can be slippery.
Spray Down the Roof
At the end of a session and before you take any long breaks, spray down the roof to move the moss debris down to the gutters. By the time you resume your next session, the roof should be dry again. Doing this allows you to better see the areas that you have not yet scraped away.
Maintaining a Moss-Free Roof Takes Some Effort. Oregon State University.