Depending on the location you live, your siding should be cleaned every 2 to 4 years. The more humid the climate, the more often it should be washed. Why? Siding tends to have mold/algae rapidly grow on the surface, and it begins on the narrow underside of each run. One it gets started it overtakes one side, and then works its way all over the house. It can grow about anywhere on your house but most prevalent on the North side of your home. Maintenance is required to keep this algae at bay!