Thick moss growth with its shallow root system keeps the roofing materials damp for extended periods of time. This moisture promotes wood rot and causes erosion of the shingles. Moss typically starts to grow at the top of the vertical slots between the shingles. These areas of asphalt shingles are the last place water evaporates from a roof. Moss, like lichen colonies, is quite damaging to shingles and some of the granules underneath the moss may already be gone as a result of this organism.