Symptoms
- Dark green, black, or streaky patches on shingles
- Fuzzy green moss clumps, especially on shaded north-facing slopes
- Streaking that runs down from the ridge
Likely causes
- CommonAlgae growth (often black streaking)Airborne algae spores thrive on shingles in humid climates, especially on shaded roofs.
- CommonMoss growth in shaded, damp areasOverhanging trees and limited sun exposure keep shingles damp enough for moss to take hold.
- PossiblePoor roof drainage or debris holding moistureAreas that stay wet longer encourage growth.
What to do
- Identify whether growth is thin streaking (likely algae) or thick fuzzy patches (likely moss).
- Trim back overhanging tree branches to increase sunlight and airflow on the roof.
- Gently remove loose moss with a soft brush from a ladder rather than scraping hard, which can damage granules.
- Rinse the roof with a garden hose on a low setting; never use a pressure washer on shingles.
- Apply a roof-safe cleaning solution made for algae and moss, following label instructions, or a diluted bleach-water solution if the product allows it.
- Consider zinc or copper strips near the ridge for future prevention, since rain washing over them inhibits regrowth.
- Avoid walking on the roof more than necessary, since wet moss-covered shingles are slippery.
Tools and parts
- Ladder
- Soft-bristle brush
- Garden hose
Roof cleaner for moss and algaeSearch: “roof moss and algae cleaner”
Zinc control stripsSearch: “zinc roof algae strips”
When to call a pro
Call a roof cleaning service for larger jobs or steep roofs, especially if moss has been growing for years.
Safety
Use caution
Wet, mossy roofs are slippery; use caution on a ladder and consider hiring a pro for steep or tall roofs.