Areas susceptible to algae stain: In many cases, you will not see algae anywhere directly below the metal flashings that surround roof vents or chimneys. The reason for this is that the zinc and copper coatings that are found on the galvanized sheets do not tolerate the growth of algae. For people who did their roofing not too long ago, you must have realized that roof manufacturers are now adding copper granules into their roofing products so as to make them resistant to algae. You may want to try these new products if you live in an area that is susceptible to algae growth. You may want to compare the cost of these new roofing products with the cost of roof cleaning.
Chemical roof cleaning: There is a better way of dealing with algae on the roof even though they could return later. An occasional roof cleaning using chemicals will not do any harm to the roof of your house. However, trouble may be when you use some harsh chemicals; some of these chemicals have a corrosive effect especially when they are used with high pressure washing machines which will definitely damage or shorten the lifespan of your asphalt shingles. You need to be careful when choosing your roof wash contractor so you can be sure they use only those chemicals that are environmentally friendly.