Houses which have roofs in very shaded areas are more prone to lichens, algae and moss growth. If you live in a heavily wooded area or if your home is surrounded by many tall trees, your roof may not be getting enough direct sunshine to dry out the moisture in the algae or moss. Trim back the branches of heavily over-hanging trees to let more sun light on to your roof. Stone roofs prove far more difficult to remove lichens from. Here is an example of a roof I inspected the other day. Lots of life left under all that vegetation. Maintenance was not done and here is the result.