Dark, stained roofs are what most homeowners imagine when they think of an old roof. Today’s shingles are algae resistant to a degree against blue-green algae. According the CertainTeed’s Shingle Technology Manual, there is no proven effect that algae has on the life of a roof. (pg. 22, 10th edition) In the manufacturing process of AR, or algae resistant, shingles there is a mix of granules that are wrapped in Copper before entering the Ceramic coating process. These copper-containing granules are mixed throughout the shingles intermingling with standard granules. Thus, when it rains, the copper produces a wash that helps prohibit the growth of airborne algae and fungus. There is only so much copper in any given shingle so the warranty against certain airborne algae and fungus is 5, 10 and 15 years depending on the type of shingle. Reading the fine print is always recommended. One thing is for sure in real life and fine print, no amount of copper will ever prevent the growth of moss on a roof. Moss is a result of a compost bed that produces an environment for growth. Once moss begins to grow, a shingle will begin to age rapidly and it may very well be too late to consider basic maintenance and cleaning.
There are services available to clean a stained roof but you have to be certain that the mixture will not harm the shingle during the cleaning process. Most manufacturers recommend 1 part laundry detergent and 3 parts water with a pinch of TSP. Keep in mind when cleaning a roof surface that 1. You never use a pressure washer to remove the solution and 2. The shingles will become slippery and dangerous to walk on. As in every situation with your roof, it’s always better to hire a certified and knowledgeable roofing contractor to handle this type of work.