Key takeaways
- Algae streaks are mostly cosmetic early on.
- Moss holds water and can lift shingles.
- Shade and humidity make it worse.
- Do not pressure wash a shingle roof.
Humid summers and shady yards make algae and moss common here.
Algae
Dark streaks are usually algae. Early on they are mostly cosmetic, but they can trap moisture and heat.
Moss
Moss is different. It holds water against the roof and can work under shingle edges.
What helps
- Trim overhanging branches
- Keep gutters clear
- Improve airflow and sunlight
- Use gentle, appropriate cleaning methods
Caution: high-pressure washing can strip granules and shorten the life of a shingle roof.