The surface of wood, exposed to the effects of sunlight and moisture, begins to change. This physical and chemical change is weathering.  The initial color change from reddish brown to silvery gray is the result of the ultraviolet rays from the sun.  This usually happens within the first six months to a year.

Gradually the roof will take on a darker gray color.  This indicates that your roof is being colonized by micro fungi.  The surface edges will erode through continued exposure to heat, cold and abrasive particles carried by wind and water.  Wood destroying organisms continue to slowly degrade the shingle's surface and edges.  Debris from trees may begin to accumulate on the roof valleys and between the shakes.

After each rain, these areas will remain moist longer and because of the moist conditions, other plants will begin to grow.  These are primarily lichens and mosses.  Your roof's deterioration will the accelerate!

When it rains, the moss and fungus will retard the shedding of water, which promotes leaks and rot.  After the rain stops, it prevents the surface from drying, which further encourages the growth of additional wood rotting organisms.