alCAUSES: Algae is a growth that occurs where water is readily available to the painted surface. Usually algae forms on the lower portion of the north exterior walls, often around faucets. Growth requires some indirect sunlight and is difficult to prevent. Many paints do not contain an algaecide. A paint which warrantees against mildew may not contain an algaecide.
SOLUTIONS: Clean the affected area with bleach solution as is recommended for mildew. Power wash after the application of bleach to remove all algae and biomaterial. Allow the surface to dry completely then apply a latex paint with an algaecide and mildewcide. Check with your local independent decorating products retailer as to the algae resistant nature of the paint.