Asphalt does not hold up to direct UV exposure. Modified membrane use small white colored stone chips called granules to protect the asphalt from environmental elements and to provide an esthetically pleasing look. Over time these shingle granules wear off. When this occurs the asphalt surfacing starts to break down causing a downward turn in the roof’s life. This is the reason why it is not recommended to pressure clean a modified roof. Pressure dislodges the granules from their asphalt embedment. It is also important to keep overhanging trees trimmed so they don’t brush off granules.