Removing Moss and Algae From Roofs
A layer of green moss might look cozy and rustic atop your house, but it can drastically shorten your roof’s lifespan. Its time for a roof cleaning.
A green, moss-covered roof may make you think you’ve wandered into a fairy tale, complete with a quaint little woodcutter’s cottage. But, in the real world, moss is much less a fantasy than it is a nightmare. Left untreated, the clumpy greenery can cause virtually any roofing material to degrade.
Moss thrives in a damp, shady environment. For this reason it often occurs on the north side of a roof — since it receives the least amount of sun — or under overhanging trees that provide shade.
Do you often ask yourself?
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?
Hundreds of thousands of roofs on houses and other buildings suffer from the stains you’re plagued with. It’s a harmless algae that’s gorging itself on the powdered limestone filler that’s often added to the liquid asphalt in shingles to add weight to them.
You could replace all the roofing with new shingles dark enough to disguise the staining, or with shingles laced with copper granules, which are lethal to algae. But that would only make sense if the shingles were worn out.
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.
If you’ve had experience with mold and mildew in the home, you know the best mildew remover is chlorine bleach. But outdoor, heavy duty cleaning usually calls for more heavy-duty mixtures such as TSP and chlorine bleach.
Just wait for the next cloudy day before you head out to the roof with your cleanser of choice — you don’t want the solution to evaporate too quickly. Keep in mind that both commercial and homemade spray cleansers can damage sensitive plants and discolor siding, decks, or pathways, so you may want to spread plastic sheeting below your work area before you get started.
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.
Once the roof is clean, you can prevent the algae from coming back by installing a three-inch strip of copper at the top of the roof. Every time it rains, a tiny amount of copper will wash down the roof. Copper is a natural biocide, and the algae will not grow in its presence. Zinc works, too, as does lead, but copper is more powerful.
