According to Florida contractor Ron Bortolini, if you can see mildew without a microscope, the problem is already serious. It takes about 2,500 spores in a line to make one inch. Oak trees, said one contractor, hold considerable moisture and may subject a building to mildew. There are many products on the market for treating mildew, but contractors should be sure the method they use is effective in both cleaning the surface and killing existing mildew, not just one or the other. Dirt is considered a delicacy among mildew spores, and they'll go to great lengths to protect their dinner.
Mildew often follows dirt, and the combination is difficult to wash off a building.
Detergent and water alone is almost hopeless, Stevens said. My preferred method is to apply it using a garden sprayer, because that allows you to move fairly slowly. If you start at the bottom and go up, you'll saturate the entire surface, because the rundown will cover anything you missed and won't leave streaks. After 15 minutes, take a garden hose with the strongest nozzle pressure and rinse it. The bleach will kill the mildew, Key explained, but won't get rid of the dirt.