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The black growth covering roofs is a fungus.  This fungus, because it lacks chlorophyll, is unable to manufacture food from raw materials as other plants do.  It must therefore get nutrition from some form of organic matter.  When fungus receives its nutrition from dead organic matter, it is called a saprophyte.  This particular fungus also requires a warm humid environment to thrive in.

Now that we know what this growth is and what it needs to survive, how does this apply to a roof?  Saprophytic fungus, in this instance, normally begins a life cycle as airborne spores that settle on asphalt/shingle roofs.  The spores that settle on the northern exposure of the roof typically stand a better chance of survival because it is normally the last part of the roof to dry after rain or morning dew.  Once the sun heats the roof, the moisture trapped around the base of the ceramic granules begins evaporating.  This raises the humidity on the surface of the shingle, thus creating a perfect breeding environment.

The tar used in the manufacture of asphalt is fossilized, dead organic matter – the specific food source needed to support saprophytic fungus.  The fungus secretes enzymes into the asphalt on which it grows.  The enzymes digest the material, which is then absorbed through the walls of the hyphae.  Asphalt, at the ceramic granule base, is normally consumed first.

Once these granules dislodge, accelerated deterioration will occur.  Ceramic granules represent the outer hard shell that protects against hail and other falling debris.  Ceramic granules protect against damaging UV radiation and insulate the roof against extreme heat.  

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