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

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!

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.

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!