Updated
Green Algae On A Conservatory Roof
The conservatory roof panels have gone green, the glazing bars are streaked, and the room is noticeably darker.
Find a window cleaner
What's happening?
A conservatory roof is the dirtiest glazing on a house and the one nobody can see. It sits almost flat, so rain does not run off cleanly, and it holds a permanent film of algae, moss, lichen, leaf litter and pollen that thickens through autumn and winter. That growth blocks light, stains polycarbonate permanently if it is left, and clogs the box gutter and the drainage channels in the glazing bars — which is what turns a green roof into a leaking one. Cleaning it is genuinely dangerous. Conservatory roof glazing and old polycarbonate are not designed to carry a person, and falls through roof lights are a well-documented cause of serious and fatal injury. This is the page on this list where the honest answer is that you should not be up there.
What usually causes it?
- Algae, lichen and moss colonising the shallow pitch where rainwater sits rather than runs off
- Overhanging trees dropping leaves, sap, pollen and bird droppings onto the panels
- A blocked box gutter holding standing water against the bottom of the panels
- Blocked drainage channels in the glazing bars so condensation and rain cannot escape
- Moss and debris trapped under the glazing bar caps and in the end closures
- North-facing or shaded roofs staying damp long enough for growth to establish year round
- Old polycarbonate whose UV-protective surface has broken down, so grime keys into it
Is it dangerous?
Yes — to you, not to the house. Conservatory roof panels, roof lights and ageing polycarbonate will not reliably take a person's weight, and falling through one is a fall onto a hard floor through broken material. HSE guidance on work at height is explicit that fragile roof surfaces must not be walked on without proper staging and edge protection. Ladders leant against a conservatory gutter, or a foot on a glazing bar, are how these accidents happen. Separately, a blocked box gutter that overflows into the wall junction causes real damp damage over a winter.
Can I fix it myself?
You can clear what you reach safely from the ground with a soft brush on a pole and plain water — the bottom edge, the front gutter face, and the lower panels of a low lean-to. That is the limit. Do not stand on the roof, do not lean a ladder against the frame or gutter, and do not pressure wash the panels: high pressure drives water past the glazing bar seals and can force the gaskets out of the bar, which creates the leak you were trying to avoid. Never use a solvent or an abrasive on polycarbonate.
When should you call a window cleaner?
Call a window cleaner who does conservatory roofs, with a water-fed pole and roof-safe access, for anything you cannot brush from standing on the ground. Do it before winter rather than after: the box gutter and the bar drainage channels need to be clear going into the wet months, and that is the part homeowners never get to. Call sooner if the room has started to smell damp, if you can see standing water in the gutter, or if a stain has appeared on the wall where the conservatory meets the house.
What will the window cleaner actually do?
- Clean the panels from the ground or from proper staging with a water-fed pole and soft brush — never by standing on the glazing
- Clear the box gutter of moss, leaves and silt, and check the outlet actually runs
- Clear the drainage channels and end closures in the glazing bars so trapped water can escape
- Wash the glazing bar caps, ridge and finials where lichen and moss establish
- Rinse with deionised water so the panels dry without mineral spotting
- Report cracked panels, slipped gaskets, failed bar seals or a sagging bar rather than cleaning over them
Frequently asked questions
We’re happy to answer all your questions.
Sources & References
Need a window cleaner?
Compare local window cleaners on Tradehand — coverage areas, accreditations and phone numbers, no middleman.