Green Film and Algae on Glass Roofs
North-facing and tree-shaded roofs go green. It is organic growth on a damp surface that never fully dries, and it is cosmetic rather than structural — but it blocks a surprising amount of light.
Why it happens
Shade plus retained moisture plus airborne spores. Overhanging trees accelerate all three, and shallow-pitch roofs hold water longer than steeper ones.
Cleaning it safely
Warm water, mild soap and a soft brush on a pole. Do not use abrasive pads, bleach or strong solvents, which damage coatings and perish gaskets.
Reducing recurrence
Trim back overhanging branches, keep the gutter clear so water does not sit, and consider self-cleaning glass if the roof is hard to reach.
When to get help
Anything requiring a ladder over glass is better handled by someone with the right access equipment.
Frequently asked questions
Does algae damage glass?
Not the glass itself; long-term growth in gasket junctions can hold moisture where you would rather it didn't.
Will self-cleaning glass stop it?
It slows it considerably on daylight-exposed roofs, but does not eliminate it in deep shade.
Is a pressure washer OK?
No — it drives water past gaskets and can lift edge coatings.
How often should I clean?
Twice a year in most gardens; more under trees.
Next step
Ask about self-cleaning glass if your roof is high or shaded.
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