How do you clean green algae off a roof?
Short answer. First identify the green: a thin flat film is green algae, fuzzy raised spots are early moss, and crusty patches are lichen. A flat film usually washes off with a gentle bleach based or oxygen bleach cleaner rinsed at low pressure, while moss and lichen need a moss specific treatment and time to loosen.
What green growth usually is
- Green algae film. Smooth, flat, slightly slimy when wet. Common on shaded lower slopes and near gutters.
- Early moss. Small raised cushions, often along the bottom edges of shingles and in the slots between tabs.
- Lichen. Pale green or gray crusts that grip tightly. They are slow growing and hard to remove completely.
Cleaning a green film
- Wet plants below the eave and cover delicate beds.
- Apply a diluted bleach solution or oxygen bleach cleaner with a pump sprayer, bottom up.
- Let it dwell per the label without drying.
- Rinse top down with a garden hose at normal pressure.
Dealing with moss and lichen
- Apply a moss killer containing zinc or fatty acid soap per the label.
- Wait days to weeks for it to brown and release.
- Brush gently downhill with a soft brush, or let rain clear it.
- Leave stubborn lichen rather than prying; scraping does more harm than the lichen.
Why it keeps coming back
Green growth means the area stays wet. Trim branches for sun and airflow, clear debris from valleys and the area behind chimneys, keep gutters flowing, and consider zinc or copper strips near the ridge.
See moss and lichen on roofs and trees and your roof for the long term fix.