Algae streaks vs moss buildup
Black streaks are usually Gloeocapsa algae — ugly but often superficial on architectural shingles.
Thick moss holds moisture against shingles, can lift edges in freeze–thaw, and is harder on older 3-tab roofs.
Safe cleaning
- Low-pressure wash with products meant for roofs — not a DIY pressure washer on high
- Work from the top down; protect plants and siding below
- Never scrape moss off dry shingles — granules come with it
- Consider zinc or copper strips at the ridge after cleaning to slow regrowth
When cleaning is not enough
If shingles are cupping, bare spots show granule loss, or the roof is past 20–25 years on a south-facing slope, cleaning will not buy much life.
We will tell you honestly if you are better off planning replacement in the next few seasons.