Being on rooftops during Atlanta home inspections, the inspector gets the opportunity to see many issues. One issue that you may have seen while driving down the road, is black streaks running down the shingles on a roof. This is actually the build-up of algae on shingles. The algae is attracted to the limestone additive that some manufacturers use in shingles to weight them. This practice of adding weight to shingles started around 18 to 20 years ago or so.
Over time the algae grows, transforming from a blue/green tint which actually is its protective defense against ultraviolent rays, into black. The black means the algae is starting to decay. Algae will thrive in high humidity. Also, areas that are shaded promote growth because those areas remain moist.
If the algae are detected in the early stages, it can be washed away. However, if it has been growing for any length of time, power washing may cause more damage than it does good. After algae have grown for a while they begin to intermingle with other life forms such as fungus. In turn lichen can begin growing on your roof, in between the shingles, and even underneath them. If lichen is washed it does not go away, it can instead, continue to be detrimental to the roof. 
Another reason that power washing may not be the best solution for your roof woes is that the high intensity of water can actually wash off the granules which coat the shingles. In turn, this weakens the shingles and prevents them from performing their job. Other times, the washing is not the issue, but the solutions used in the process are the problem. Certain chemicals can saturate through the shingle coating as well as poison the lawn or landscape as it runs off the structure. 
Remember to research before making the final decision about pressure washing a home’s roof. Find out what the company suggests is the best alternative, what chemicals may be used in the process, and how long the treatment should last.
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