How to Kill and Remove Moss From Your Roof

A closeup of moss on roof

Corey Mckown / EyeEm / Getty Images

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 will seriously damage 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.

How Moss Will Ruin a Roof

Northern areas that have less 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 area, 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. The moss that begins as a thin green layer on the top of the shingles will multiply and become wide, thick, and mat-like. The seams between the shingles and the shingles' edges also develop moss because they are even more shaded.

As moss thickens, it will work under the shingles, acting like a jack by raising the shingles. If you have wood shingles, the 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 tougher to remove than the relatively smoother planes of composite or asphalt shingles.

Once moss is a thick mat, it will become a sponge that soaks up and retains all moisture. This stored water will work its way under and between shingles and then onto the lower levels of roofing felt and structural members. This protected water will eventually lead to rot, and rot leads to further decay of roofing materials.

Few homeowners decide to replace a perfectly functional roof simply because the roof does not meet stylistic needs. In most cases, roofs are replaced because they have reached the end of their natural lifespan or they have not been properly maintained. If you maintain your roof well enough by killing and removing moss, you vastly improve the health of your roof and help the roof through its allotted warranty time span. With that kind of incentive, killing moss begins to look like a good idea.

Dry vs. Liquid Moss Killers

Application Process

Dry powder moss killers are applied by first climbing the roof and then sprinkling the application in lines parallel to the ridge of the house. Position each line about 5 feet from the adjacent line. Significant amounts of dry powder are required to cover a house. When it rains, the water will mix with the powder and run down toward the eaves. After about a week, the moss should clear.

Liquid moss killer containers attach to a garden hose and mix on a metered basis as the water flows. You can remain on the ground, as long as you have sufficient water pressure to reach the ridge (top) of the roof. Because it can be difficult to keep track of your progress, it helps to mentally section out portions of the roof and spray each section before moving onto the next one.

Pros

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 later removal process.

Liquid moss killers more evenly cover the roof than do dry powders. If you have a healthy fear of climbing roofs, liquid moss killers let you stay on the ground. If the water will not reach the peak of the roof, one fix is to mount a ladder about halfway and spray from that point. This allows you to better direct the spray without climbing onto the roof.

Cons

Dry moss killers can be difficult to dispense evenly, especially with brands that tend to clump up. Dry powders' white streaks sometimes do remain on the roof until several hard showers of rain later. Even then, the chunked or caked powder may remain for several more weeks. Conversely, sprinkled dry powder that is only lightly settled can blow off if a strong wind comes before the rain.

One frustrating downside of liquid moss-killers is that they are difficult to follow visually. It is hard to keep track of where you have been and where you are going. One way to deal with this is to make sure that you start with a perfectly dry roof. Liquid moss killers tend to be more expensive on a foot-by-foot basis than powdered moss killers.

Best Moss Killer Products

Bayer 2-in-1 Algae and Moss Killer 

Bayer 2-in-1 Algae and Moss Killer is potassium soap of fatty acids and inert ingredients in liquid form. Unlike zinc-based moss-killers, this product claims to be non-corrosive to metals. Roofs do have a lot of metal in the form of vents, gutters, drains, nails, and cables, so this can be a plus. Bayer 2-in-1 Algae and Moss Killer'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 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 to 5-10 gallons of water, killing moss over an area that is 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.

How to Remove Moss From a Roof

After you have killed the moss on your roof, the project is only halfway finished. The dead moss must be removed. Otherwise, the dead moss will act like a sponge and soak up water, thus promoting decay. Dry moss will not flake and blow away on its own; you must do it.

Moss removal is labor-intensive. Take it slow and break up this large project into smaller projects that ideally span a long weekend. Remove moss only when conditions are perfectly dry since you must stand on the roof.

  1. Gather an extension ladder, a scraping tool such as a putty knife or painter's 5-in-1 tool, and a hose hooked up to a water source.
  2. Lean the ladder against the edge of the roof. Mount the ladder with the scraping tool.
  3. Begin scraping from the bottom, then work up to the ridge of the house. Use the scraping 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.
  4. At the end of a session and before you take your break, 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.