Lush and verdant, thick moss overtaking a roof can be picturesque, giving the home a fairy tale, Hobbit-like feel. But moss does nothing else good for a roof, and if left unchecked, it can lead to serious damage to your home. When roof moss gets thick enough to crowd out the shingles and create 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 before the situation gets too serious. There are both liquid and dry products that will easily kill roof moss, but the more important stage is removing the dead moss from the roof after it is killed. Unless removed, the dead moss will act as a sponge to soak up further water and accelerate the decay of the roof structure. You can't expect the dead, dry moss to flake off and blow away on its own.
How Moss Ruins a Roof
In general, it is north-facing roof areas that receive little sunlight that are prone to moss—especially when the overall climate is cool and damp. But even in damp climates, roof areas that receive even a little strong sunlight each day may not develop moss. You are most likely to find it on areas of the roof that are shady nearly all the time, such as roof areas beneath heavy tree canopies
Roof moss begins with a thin dusting of green that you may notice only from low angles. This thin layer on the top of the shingles gradually 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, it is much harder to remove than 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 quickly 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. The accelerating cycle can even destroy a roof in a matter of a few months.
Removing moss in the early stages, then, is crucial to protecting your roof and the rest of the home's structure.
Before You Begin
Moss-killing products are generally grouped into two 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 be dead.
Dry powder moss killers require that you go onto the roof and take stock of the moss problem during application. This allows you to determine the extent of the problem and plan the removal process that will follow. But 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. And if a strong wind comes along before the rain, the dry powder can blow off before it has a chance to work on the moss.
Liquid moss killer comes in a container that attaches to a garden hose and is blended in as the water flows. 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 spray doesn't reach the peak of the roof from the ground, you can spray while standing on a ladder. While the application can be easier than with dry powders, you will still 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 on to the next one. It also helps to start with a dry roof.
There are several widely available moss killer products on the market. They are commonly sold at home centers and through online retailers.
- Bioadvanced Moss & Algae Killer & Cleaner, originally sold under the Bayer brand, is made from potassium salts of fatty acids and inert ingredients in liquid form. Unlike zinc-based moss killers, this product is advertised as being non-corrosive to metal vents and gutters. The spray nozzle creates a flat stream that is easy to apply.
- Moss B Ware is 99-percent zinc sulfate monohydrate, a proven moss killer and moss deterrent. Low cost and wide availability are strong advantages to this product. 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.
- 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.
- Scott's Moss-Ex is another liquid product consisting of potassium salts of fatty acids and other inert ingredients. Marketed primarily for use on lawn mosses, it will also do the job on roof moss. A single bottle will treat about 600 square feet.
Moss killer works at any time of the year, but it is typically applied in the early fall when sunlight is diminishing and moss begins to develop. Moss removal must occur after the moss is fully dead, which can take a month or two.
Some communities may have restrictions about flushing zinc into stormwater discharge areas. Check with your local water quality agency about zinc runoff restrictions.
In most instances, you will be climbing onto the roof to remove the dead moss, and perhaps to apply dry moss killer. This is inherently dangerous work, especially if your roof is high or has a steep pitch. Make sure you are confident about working on a roof or on tall extension ladders before tackling this project. It's a good idea to learn how to use a roof harness for any projects involving climbing onto a steep or high roof. Roof harnesses include brackets that are secured to the roof, with straps designed to arrest your fall should you slip while working on the roof.
Roof harnesses can be rented at home improvement centers and tool rental outlets, but if you expect to have regular roof work projects, investing in a harness kit (about $100) might be a good idea.
If you are unsure of your skills or nervous about heights, this is a project best left to a professional service.
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Equipment / Tools
- Roof harness (if needed)
- Tall extension ladder
- Garden hose
- Putty knife or painter's 5-in-1 tool
- Long-handled, soft-bristle scrub brush
- Moss killer
Apply the Moss Killer
To apply dry powder moss killer, place an extension ladder against the house and climb onto the roof. Dispense the moss killer in uninterrupted horizontal lines about 2 to 4 feet apart, parallel to the roof ridge. Dry moss killer is best applied when a slow, steady rainfall is predicted for the near future.
To apply wet moss killer, attach a 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. You may need several bottles of moss killer for expansive roofs.
Wait for Rain (Dry Moss Killer Only)
Dry powder moss killer is activated and dispersed by precipitation. 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 if the weather doesn't cooperate.
Remove the Dead Moss
When all the moss has turned visibly brown, it is dead and can be removed. This can take a month or more after application of the moss killer. Climb onto the roof and begin removing the moss, working from the bottom up. The best tools are a long-handled, soft-bristle scrub brush, and a putty knife or 5-in-1 painter's tool. Use these tools to gently lift up mats of moss and brush them away. In the seams, use the sharp edge of the scraper like a dental pick to force out the moss.
Work from the eave line up to the roof ridge, carefully brushing away loose moss as you go (loose moss can pose a slipping hazard). If there are multiple roof planes with moss, repeat the same process with each section, starting at the bottom and working up.
This is a slow, steady process that may take several days if your roof is large. Pace yourself and take breaks whenever you become tired.
Remove moss only when the conditions are perfectly dry. Never work on a wet roof, which can be slippery.
Rinse Off 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. Don't resume your work until the roof is completely dry. Doing this allows you to better see the areas that you have not yet scraped away.