What is the right treatment for moss and mould on a roof?
Short answer. Treat roof moss and mould in order: clear debris, apply a roof safe biocide on a dry day, give it time to work, remove dead moss gently, then soft wash any remaining stains and add prevention. If you also see mould inside the attic, that is a moisture and ventilation problem the outside treatment will not fix.
The treatment plan
- Clear debris. Leaves and pine needles feed mould and hold moisture for moss.
- Apply a moss treatment labeled for your roof type, on a dry day with no rain forecast for the label's required time.
- Wait. Moss browns and loosens over days to weeks.
- Remove gently. Hand or soft brush, downslope only. Let rain handle the rest.
- Soft wash stains. A diluted bleach solution applied at low pressure and rinsed takes care of remaining mould and algae streaks.
- Prevent. Trim branches, add zinc or copper strips near the ridge, and recheck yearly.
Protecting the roof and yard
- No pressure washing or scraping of asphalt shingles.
- Water and cover plants, and flush gutters afterward.
- Keep treatments out of ponds, creeks and storm drains.
When the mould is inside
Dark spotting on the underside of the decking, a musty attic smell, or frost and condensation in winter point to trapped moisture, not outside growth. Common causes are blocked soffit vents, too little exhaust, bathroom or dryer vents dumping into the attic, and roof leaks. The fix is ventilation and leak repair, sometimes with professional mould remediation. See roof ventilation basics and attic heat and shingles.
Treatment in Georgia's climate
Warm, humid conditions mean growth comes back faster than in drier regions, so plan on prevention, not a one time fix. See moss and lichen on roofs.