A: The tiny organisms that grow on the roofs and walls of houses can be moss, algae, lichen, or mildew or mold (two terms often used interchangeably). All can create greenish blotches on siding. Especially from a distance, they can look alike, especially to a casual observer.
They are strikingly different, however.
Only mosses are classified as plants. Like other plants, they use photosynthesis to turn sunlight and water into the food they need. Algae, once considered plants, are now in a kingdom of their own, but they also depend on photosynthesis. So do lichens, which are combination organisms that have algae or bacteria living within a structure made of fungus, with the fungus dependent on the food the algae or bacteria make via photosynthesis. Mildew are solely fungi, also an independent kingdom from a botanist’s perspective. They get energy by feeding on organic matter where they live. They don’t use photosynthesis, so they don’t need sunlight.
Close up, it’s usually possible to differentiate among them. Mosses and lichens have real structure, so you can feel them if you wipe your hand across the surface. Mosses generally pull up or wash off easily because they don’t have roots, although they do have root-like growths that hold them in place. Lichens are far more tenacious. Also, mosses have little leaflets. Lichens do not, although they can have a tangle of growth that resembles stems or leaves. Color is another clue. Mosses are usually green, although some are brown, yellow or black. Lichens are often green or light green when wet because green algae show through the fungal structure. Lichens when dry can take on a wide range of sometimes brilliant colors.
If there is no texture, you’re probably dealing with algae or mildew. It’s harder to distinguish between these. A green color usually points to algae, but not necessarily. Some algae are red, and although mildew are often black, they can also be green, yellow or brown. Because algae depend on photosynthesis, you’re more likely to find this type of growth on surfaces exposed to the sun — such as your house. Mildew thrive in the dark, which is why indoor surfaces are far more likely to harbor mildew than algae.
All four kinds are most likely to become established on damp, because they all need moisture to thrive. The abundance of rain this past year could help explain why you are seeing this growth for the first time. Also, if it’s been decades since you painted, there’s a good chance the paint is more porous than it was initially, keeping the surface moist longer and giving organisms an easier place to take hold. Differences in the porosity of the brick or previous paint layers might explain why the growth is only on the new section of your house. There could also be a difference in how quickly the wall sections dry after a rainstorm because of differences in insulation or other construction features.
Whatever the type of growth, you may be able to simply scrub it away, especially if it’s algae or mildew. Start with the gentlest of procedures, using mildly soapy water and the type of soft-bristle brush or sponge you might use to wash a car. If that’s not enough, buy a siding cleaner labeled as effective against the type of growth you have. A mildewcide won’t work against algae, and an algicide won’t work against mildew. However, some cleaning solutions are labeled as effective on all four kinds of growth. One example is Spray and Forget roof and exterior surface cleaner, $39.95 a gallon at Home Depot. If you aren’t sure what type of growth you’re dealing with, an all-purpose cleaner is the way to go. Lichens can be particularly difficult to remove without using a suitable cleaner.
Power-washing with plain water or with cleaner mixed into the tank is an option and will most likely be the tool a professional would use. But too high a pressure can blast off the paint, so discuss this concern and check references. Gentle scrubbing and rinsing with a hose is safer, although whether you want to do that yourself depends on how comfortable you are working up high on a ladder.
Often, it’s possible to wash away growth and be done. To keep the siding looking good, wash again when necessary.
But if it has been decades since you repainted, as your letter implies, it’s probably time for a fresh coat. In that case, paint manufacturers generally recommend washing the walls first with a bleach solution — 1 part bleach to 3 parts water — to kill any spores or remaining growth. If the paint is chalky or cracked, or if you are switching from old oil-based paint to water-based paint, apply primer before you repaint. For the top coat, buy water-based paint formulated to resist growth of mildew and algae. Although there are stir-in additives, it’s better to buy a high-quality paint that already incorporates the protection you need. With stir-in products, there’s always a risk of compromising the paint formula. Glossier paints have a slicker surface so they dry faster and tend to stay clean longer.
Always read the entire label before you use a cleaner or a paint fortified against mildew and algae, and follow safety precautions.
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