Q&A / 

Removing Moss and Algae From Roofs

DEAR TIM: Part of my roof gets very little direct sun and is shaded. Both moss and green and black algae grow on the shingles. Are these organisms causing damage to my roof? What is the best way to remove them. I am very hesitant to use chlorine bleach as it may damage the roofing material. Once clean, is there a way to permanently keep the roof free of the moss and algae? Scott W., Toledo, OH

DEAR SCOTT: People who live in all parts of the nation and have roofs made from all sorts of materials, wood, metal, asphalt, clay or concrete tiles, etc. can have moss and algae accumulate on their roofs. Those places that receive more rainfall and have sustained high levels of humidity have more moss and algae growth as the moss and algae need water to grow and multiply.

This moss may give a roof a fairy-tale look, but it can shorten the roof's lifespan at the same time. PHOTO CREDIT: Tim Carter

This moss may give a roof a fairy-tale look, but it can shorten the roof's lifespan at the same time. PHOTO CREDIT: Tim Carter

The growth is often more pronounced on the north-facing portions of roofs since they tend to stay wetter longer. Large trees that produce lots of shade on a roof can also foster the growth of moss and algae as evidenced by my own west-facing roof. It used to get covered with moss and algae. Morning dew and rainfall tend to evaporate slower on these surfaces because of the minimal amount of direct sunlight these roof surfaces receive.

Moss is of great concern to me as it absolutely can shorten the life span of untreated wood and asphalt roofing materials. The thick moss growth with its shallow root system keeps the roofing materials damp for extended periods of time and this moisture promotes wood rot and can and does erode the asphalt in shingles.

Fortunately, the moss is quite easy to remove. Use a standard bristle scrub brush on a pole and push the brush down the roof to tear the moss's shallow root system from the shingles. Do not brush up the roof as you might break the bond between shingle layers.

Note how the moss typically starts to grow at the top of the vertical slots between shingles. These areas of asphalt shingles are the last place water evaporates from a roof. Once the moss is removed, inspect these slots to ensure the colored ceramic granules are still in place. If the granules are gone and you see just the interior fiberglass mat of the shingle, your roof is well on its way to being replaced.

Oxygen Bleach was just sprayed on this roof. You can see it quickly foam up while it is cleaning the algae and dirt all on its own!

Oxygen Bleach was just sprayed on this roof. You can see it quickly foam up while it is cleaning the algae and dirt all on its own!

The green and black algae, in my opinion, are not harmful to the actual roofing material because they do not develop roots. They are more of an aesthetic issue since they make the roof look horrible. Both of these algae can be cleaned from the roof, however the green algae is far easier to remove than the black algae.

It is wise to avoid chlorine bleach as a roof cleaner. Chlorine bleach can remove the natural color from wood roofing shakes, it can kill vegetation on the ground if it runs onto the ground as it is rinsed from the roof and it can accelerate corrosion of metal gutter and downspout systems.

A better roof cleaner might be non-toxic oxygen bleach. This widely available powder is mixed with water and applied to a cool roof surface, preferably on an overcast day. If you keep the roof surface wet with the solution for just 20 minutes, and then lightly scrub the roof surface, the algae almost always comes off. Severely stained roofs may require multiple applications of the oxygen bleach solution. The solution also helps to loosen the grip of the shallow moss roots.

Here is an easy way to keep your roof shingles free from moss, algae and mildew. Watch this video to discover the natural wonders of copper. You'll never have to clean green scum off your roof.

Once the roof is clean you can hinder moss and algae growth by introducing copper onto the roof surface.Visit a roofing supply company that sells tin, copper and galvanized metal products to residential roofers. Often these companies sell rolls of copper that can be cut into long strips. I like to cut strips that are 5 or 6 inches wide and are ten feet long. Using a metal brake tool, I put a 20 degree bend one-half inch in from the edge on one of the long sides of the copper strip. This bend eliminates unsightly waviness that often develops as you cut the copper with a tin snips.

Place these strips of copper near the top of the roof. Slide the unbent edge up under a row of shingles so that the bent edge and 4 inches of copper is exposed to the weather. Each time it rains some copper molecules wash down onto the roof and create a poisoned environment that both the moss and algae dislike. Be patient as the copper eventually will turn a distinctive dark brown and then the classic green color. If you getting ready to install a new asphalt shingle roof, be sure to buy shingles that have copper hidden in the colored ceramic granules.

Cleaning roofs can be dangerous work. Algae that becomes wet is often as slippery as wet ice. Try to work on dry parts of the roof and reach sideways to scrub and clean those parts of the roof that are wet with cleaning solutions. Once a roof is clean, they often are not too slippery if they are wet. Wear shoes that have excellent traction. Sitting and working on the roof instead of standing lowers your center of gravity and minimizes your chances of falling from the roof.

The copper material works far better to poison roofs than zinc. Zinc is indeed effective, but copper tends to do a better job for a longer period of time. The copper strips are easy to install and they tend to blend into the roof once the shiny copper begins to oxidize and turn the distinctive dark brown.

Dear Tim,

Instead of putting copper or zinc strips on a roof to inhibit moss growth, wouldn't it be cheaper to use pennies? Use two or three per linear foot, slipped under the edge of shingles near the top or the roof, or stuck in place with a dab of silicone caulk. It's more time consuming, but easier for someone who doesn't have the cutting or bending tools. Lee R., Portland, OR

Dear Lee,

It would work so long as you install *enough* pennies. You need to have the square inch area similar to the exposed copper strip. This is a great idea, by the way!

Message from Tim:

Years ago while researching a column about cleaning decks, I discovered the wonders of Oxygen Bleach. It is perhaps the 'greenest' cleaner I know of as it uses oxygen ions to break apart stains, dirt and odor molecules. There are no harsh chemicals, and it works on just about anything that is water washable.

I decided to create my own special blend using ingredients made in the USA. In fact, the raw materials in the active ingredient are food-grade quality registered with the FDA. I call my product Stain Solver. I urge you to use it to help with cleaning your roof moss and algae. You will be amazed at the results!

Column 539


19 Responses to Removing Moss and Algae From Roofs

  1. Could you tell me if any of the fungacide or bleach products used to remove moss from a tiled roof are harmful to animals once the moss has fallen to the ground?

  2. Hi.
    I have black algae forming on my east roof and as I have A light tan roof I would like it gone.
    I have not found copper strip for sale in my area but zink strip is pushed by many sales outlets. I do not care to use zink as I understand it does not work as well as copper.
    My father was an electrician for over 30 years before his retirement and upon his death I cleaned out his sheds and found several rolls of copper ground wire.I was wondering if I could use this wire instead of the copper strip? I thought 2 or 3 wires could be attached to the roof using copper plumbing straps or would this plan be A waste of time?
    I would have to hire this job out as I am in my 60's and I am unable to get on roof's anymore and I would hate to waste my money doing this job twice.

    Thanks for your time.
    G.R. Ronan

  3. Do you know if a solution of water and lemon juice would hurt/bleach the shingles? I would think the acid would help prevent more growth (and friendlier on my sewers)

  4. Forgot to mention, I've enjoyed your specifications for new house construction that I bought a while back, it's helped with our design of the house. I'm a detail-oriented person, so I appreciate your effort to include the nitty gritty.

    • Stain Solver WILL help you make the roof look like new. You will need to scrub the roof. To keep the roof FREE of algae, you need to install some copper strips along the ridge line of the roof.

  5. We have a 5-year-old high-end GAF roof. It's blue. On the north side, a downspout from the top gable drains onto a lower gable, and there's black visible there, which I'm assuming is mold. Since that's the only place it's appearing, I was wondering if I couldn't put a copper strip in the mouth of that downspout, rather than at the ridge. Shouldn't this work? Any downside if it doesn't?

  6. I have a cottage in Northern Wi near the UP of Michigan. I was informed by my roofer, the supplier and the manufacturer that I do not need to apply zinc strips to the new roof that I am installing. I am using a Landmark Pro asphalt AR shingle that carries a 15 year Algae Resistant warranty.

    I am told placing a zinc strip woul be redundant since there is zinc in the material. Can you please comment on the feasibility of putting zinc or copper strips on these particular shingles. What is the benefit of using copper over zinc? Is the higher cost for copper worth it? What is effective shelf life of zinc and what is effective shelf life to protect against algae for copper?
    Am I better off waiting 5 - 10 years to put on the zinc and or copper strips since new shingles have zinc and come with a 1( year AR warranty?
    Please respond ASAP as I need to inform contractor working on project.
    Thank You.

  7. I'm a roof repair tech and I am thinking about using Ajax Oxygen Bleach, since it seems to be the same thing, am I correct on this?

    • Well, you need to ask the Ajax people these questions:

      Do the raw materials used to make Ajax come from the USA or China?
      How much active ingredient is in Ajax?

      My Stain Solver oxygen bleach is Certified organic, the two ingredients are Made in the USA, and we have the most amount of active ingredient as allowed by law so we can ship via any method in the USA.

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