Facts on Algae Discoloration ...

Today's shingles are being made differently than in the past. Shingles used to be made up of a thick cellulose mat, saturated with asphalt and topped with granular. This make up deterred algae growth.
All shingles manufactured during the past 30 years use a fiberglass mat with a limestone filler saturated with asphalt and topped with granular. This limestone filler is what the algae feeds on.
Referred to as blue green algae by the shingle manufacturers, once established, will continue to spread. It also allows other damaging growths such as mold, mildew and lichens to propagate and damage your roof.
Shingle manufacturers acknowledge this problem and even manufacture an algae resistant shingle. But, in the right conditions even this type of shingle can have algae growth.

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