Roof washing has become a common practice in the past few years, in order to remove the black streaks (which are a type of mold) from asphalt shingle roofs. This mold feeds on the limestone granules on the surface of the shingles and over time can shorten the life of the roof. However, the cleaning must be done gently and carefully, so as not to damage the shingles. High pressure should never be used for this. Again, we use cleaners that are specific to the task of removing the stains, using low pressure and often without needing to get on the roof surface.