It’s likely that if you or your neighbors have a slight roof discoloration, it’s caused by a common type of algae known as Gloecapsa Magma. Categorized by uniform dark gray or black streaking on shingles, Gloecapsa Magma is an airborne algae that can spread spores from home to home within a neighborhood. ARMA representative Tom Bollnow notes, “The algae is mostly an aesthetic nuisance, although with time, the moisture retained in the algae can prematurely age the shingles.”

Asphalt shingles tend to be more prone to discoloration since the algae feeds off the calcium carbonate within this type of shingle. And since the algae thrives in shade and moisture, any tree-covered or north-facing roof angles are much more disposed to growing Gloecapsa Magma. While this type of algae discoloration isn’t ideal, it also isn’t a huge cause for concern.

However if the stains have a green tinge, the source of the problem is more likely moss growth from overhanging trees. As moss grows, it will seek out the cooler and darker areas of your roof to thrive; this could mean creeping underneath shingles causing premature curling or moisture damage to your roof deck. Moss tends to spread and potentially damage shingles at a much faster rate than algae so it’s important to trim back tree limbs close to your roof line.