What is algae/fungus?
Roof algae, commonly known as Gloeocapsa Magma, is a type of algae that thrives in humid and warm climates and usually appears first on the North slope of the roof where shade is prevalent and moisture tends to accumulate.  The result is usually a black streaked roof. 

Why are algae black? The organism Gloeocapsa produces a dark-pigmented sheath to protect itself from ultra-violet rays. By the time the black streaks are noticeable, the algae usually have been growing for months or years - the result of generations of dead algae cells built up over time.

Roof algae, once attached to a shingle roof, feeds on the dead organic matter used in the manufacturing process of the shingle.  As the fungus breaks down the asphalt shingle, the ceramic granules protecting the surface of the shingle are loosened and begin to fall off.  This causes the roof to deteriorate prematurely and diminishes the roof's abilities to reflect heat rays.  If not treated, the roof algae will reduce the life of your roof, diminish your home's curb appeal and increase your home cooling expenses.