Those Black Streaks on Your Shingles: What They Are and What to Do

If you have noticed dark streaks running down your roof, you are not alone. They are one of the most common things homeowners ask us about in Georgia, and the cause is usually not what people think.

What causes them

Those streaks are usually algae, a type that thrives in warm, humid climates like ours. It spreads across roofs by wind and is most visible on lighter shingles and on the slopes that stay shaded and damp the longest. Older shingles often had a filler the algae feeds on, which is why it shows up on so many roofs.

Is it harmful?

Algae streaks are mostly a cosmetic problem, but over time they can hold moisture and make the roof absorb more heat. Moss is a different story. Moss grows thicker, lifts the edges of shingles and holds water against the roof, so it should be dealt with sooner.

How to clean it safely

• Never pressure wash shingles. It strips the granules that protect them and can void a warranty.
• Use a cleaner made for roofs, applied gently, following the shingle manufacturer guidance.
• Rinse plants and landscaping before and after, and protect them from runoff.
• Leave steep or high roofs to a professional. A wet roof is very slippery.

Preventing it

Trimming branches so the roof gets more sun helps. When it is time for a new roof, many shingle lines today include algae-resistant granules designed to keep streaks from coming back. We can show you which options are available in the colors you like.

If you are not sure whether you are looking at algae, moss or worn shingles, we will take a look and tell you, free of charge.