Dark streaks can be blue-green algae rather than dirt or a leak. GAF separates this staining from moss and other growth
GAF technical bulletin: Algae staining on shingled roof surfaces.
What this means for your roof
Dark streaks can be blue-green algae rather than dirt or a leak. GAF separates this staining from moss and other growth. Cleaning methods matter because aggressive washing can damage the protective granule surface.
A useful question for your contractor
Ask for the cause of the discoloration to be identified before approving cleaning.
Ask for the answer to be tied to the actual roof condition and the exact product being installed. Keep the agreed repair or installation detail with your estimate and project photos.
Bulletin dated: September 19, 2024. The header date is shown here; the footer carries a different September 2024 date.
Read GAF: Algae staining on shingled roof surfaces
This homeowner summary explains the cited bulletin. The complete manufacturer document and current product instructions govern the installation details. A requirement for one manufacturer or product should not be transferred to another without checking.