What Are Those Black Marks On My Roof!?

What Are Those Black Marks On My Roof!?

Does your roof have black streaks running down it?  When I’m conducting home inspections on Cape Cod and Martha’s Vineyard, I frequently see what looks like ink which spilled on the roof and ran down towards the gutters.  Some homes only have a few streaks, and others looks like the roofs were painted black.
So what is this phenomenon? Has something changed in the shingle manufacturing process?  The black marks you see are what we call roof algae or gloeocapsa magma and it travels with the wind and tends to collect on north facing roofs on Cape Cod and Martha’s Vineyard.  Gloeocapsa Magma has become more of a problem in the last few decades because shingle-makers now use limestone filler in the manufacturing process, which is food for this destructive algae.   These black streaks are roof stains and it means that your shingles are being eaten, which can result in a shortened lifespan for your expensive roof.  This algae can affect your roof’s reflective properties that will inevitably end up costing you more in energy bills as the algae grows and covers the roof’s surface.   There are roofing manufactures that now make an algae resistant (AR shingle) so when your roof needs to be replaced this is the shingle to choose.
How do I get rid of it? There are several local roof cleaning companies that can remove the algae with a chemical cleaner.  Remember to avoid the high pressure washers as this may cause more damage to your roof.

Helpful information for Homeowners – Licensed Cape Cod Home Inspector – Scott Hunt – SEASIDE HOME INSPECTION offering home inspections on Cape Cod, Martha’s Vineyard, and Southeastern Massachusetts