Our professional detergent destroys the algae, moss, & lichen growing on the roof at the time of the application. However, the dead algae, moss, & lichen may remain on the roof for a few days, weeks, or even longer in some cases. This allows time for the roots to naturally disintegrate so that the growth will safely wash down in the rain without removing shingle granules in the process. If we were to remove moss & lichen before the roots disintegrate, we would remove some of the shingle granules with them (think about pulling a weed in a garden – you always get some dirt with it).