How to Choose a Moss Killer 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 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.

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 multiplies and becomes 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 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 tougher to remove than from the relatively smoother planes of composite or asphalt shingles.

Once 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.

Dry vs. Liquid Moss Killers

Dry powder moss killers are applied by sprinkling the powder in multiple lines parallel to the ridge of the house, spacing the lines about 5 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. 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.

Liquid moss killers cover the roof more evenly than dry powders, and they let you stay on the ground for the application (however, you will likely need to get on the roof later to remove the dead moss, in any case). If the water will not reach the peak of the roof from the ground, you can spray while standing on a ladder.

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.

Moss Killer Products

Bayer 2-in-1 Algae and Moss Killer 

Bayer 2-in-1 Algae and Moss Killer is a 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 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.

Tools and Supplies You Will Need

  • Tall extension ladder
  • Putty knife or painter's 5-in-1 tool
  • Garden hose

Instructions

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 remove it manually.

Wait for Dry Weather

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.

Gather Your Supplies

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.

Set Up the Ladder

Set up the ladder safely. It should lean against the roof edge at a 15-degree angle and extend above the roof edge by at least three ladder rungs. Ideally, the ladder should be tied off to the roof on both sides.

Scrape Off the Moss

Climb up to the roof and begin scraping off dead moss from the shingles, working from the bottom up. 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. 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.

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.