When mold and mildew -- which are the words for the same thing -- grow on the outside of a house, controlling them is usually more a matter of cosmetics than one of health. Mold interferes with paint adhesion, it looks bad, and it often signifies the presence of wood-damaging rot, which is also a fungal growth. A strategy for remediation should include physical removal, moisture control and containment through wood sealing and finishing.
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Mold Is Everywhere
When mold and mildew collect on your house siding and trim, or on the decking or porch, it's a sign that the area is moist. Mold needs water to survive, and wood near the ground, in the shade or in an area with poor ventilation or drainage is especially vulnerable. Mold species number in the hundreds of thousands, spores are everywhere, and those of wood-damaging species are already on your house. They will grow as soon as conditions are right. The hope of keeping them away from your house may be unrealistic, but you can effectively remove mold without much trouble, and you can take steps to protect wood from damage.
Exterior Mold-Cleaning Solutions
Government entities, such as the Environmental Protection Agency, have stopped recommending bleach for cleaning mold, but many home maintenance professionals continue to do so. Rather than using chlorine bleach, though, you can prevent wood and paint discoloration and damage to landscaping plants by using oxygen bleach, available at most hardware stores. Mix the bleach powder with warm water to make a mold-busting solution that's more environmentally friendly and more able to penetrate into the wood than chlorine bleach. You don't really even need bleach, though. The EPA, FEMA and other government agencies recommend a simple detergent-water solution. If you need extra cleaning power, use a strong detergent, such as trisodium phosphate or a TSP substitute.
The secret to mold removal isn't really a secret -- you have to scrub it off. One easy way to do this is to use a power washer; set the pressure to around 2,000 psi and work with a 45-degree nozzle to avoid damaging the surface you're spraying. This option is especially effective if you use a machine that accepts detergent; spray the detergent solution on the mold, let it sit for five minutes and blast it off. It isn't safe to power wash some surfaces, such as vinyl siding or shingles, because the spray can get underneath. In that case, wash with a bucket of cleaning solution and a scrub brush, then rinse with a garden hose.
Detailing and Prevention
Exterior mold can be especially problematic in areas where two pieces of wood join in an inside corner, such as on roof overhangs and between decking boards. Scrub these areas with a small wire brush or even a toothbrush. Do your mold cleanup on a clear, dry day so that all the moldy areas you clean can dry quickly. As soon as they do, consider sealing the wood with a coat of paint or varnish to keep the spores that are still in the wood isolated from the surrounding air and to prevent them from growing again.