The green is algae, sometimes with moss along the glazing bars. It grows where the roof stays damp and has a little dirt to feed on: shade from trees or the house, a north-facing slope, leaf litter and a gutter that doesn't drain. Cleaning removes it; more light and a clear gutter slow its return.
What is the green stuff?
It's mostly green algae, a thin living film that grows on damp surfaces. On older roofs, moss and lichen join in, usually along the glazing bars, in the joints and in the corner where the roof meets the gutter. Algae needs three things: light, moisture and a thin layer of dirt to hold on to.
A conservatory roof gives it all three. It's shallow, so water drains slowly. It's often near trees or a hedge. And dust, pollen and leaf bits settle on it and stay.
Why does one roof go green faster than another?
Shade and damp are the big factors. A roof that's in the sun most of the day dries quickly after rain and stays cleaner. One that sits in the shadow of the house, a fence or trees stays wet for hours, sometimes days, and that's where algae thrives.
- North-facing roofs get little direct sun and go green first.
- Roofs under trees get leaf litter, sap and bird mess, plus shade.
- Shallow roofs drain slowly, so water and dirt sit longer.
- Blocked gutters keep the bottom edge of the roof wet.
- Scratched polycarbonate gives the algae more to grip.
Is the green harmful to the roof?
Not straight away, but it isn't harmless either. Algae itself doesn't eat through glass or plastic. The trouble comes when it's left for years: moss in the joints holds water against the seals, the gutter blocks with debris, and the room underneath gets darker and gloomier.
Moss and debris in the box gutter or the conservatory gutter can also cause water to overflow, which can lead to damp patches on the wall below. Keeping the roof clean is cheaper than dealing with those.
How do you get the green off?
With water, a soft brush and patience, working from the ground. Wet the roof and let the film soften for a few minutes. Brush along the glazing bars, where the moss is, then across the panels, and rinse from the top down. Clear the gutter at the end so all the loosened muck can drain away.
Avoid pressure washers, scouring pads and strong chemicals. Our guide on why not to pressure wash a conservatory roof explains the damage they can do.
Should you use a moss or algae killer?
You don't need to, and many products can do more harm than good on a conservatory. Some can mark seals and frames, and the run-off lands on the patio and the plants below. If you do use one, choose one that says it's safe for uPVC and glass, test it on a small area first and rinse well.
We don't use them on conservatory roofs. A thorough brush-and-rinse with pure water removes the growth, and nothing is left on the roof to attract dirt.
How do you slow it down coming back?
Let in more light and keep the water moving. You can't change which way the roof faces, but you can usually make a difference with the things around it.
- Cut back overhanging branches and tall hedges.
- Clear the conservatory gutter every autumn.
- Brush off leaves before they rot down on the roof.
- Have the roof cleaned before the green gets thick, not after.
How often should it be cleaned?
For a lot of homes, once a year is about right. A sunny roof in an open garden can go longer. A north-facing roof under trees may want doing more often. The best time is usually after the leaves have come down in late autumn, or after blossom has finished in late spring, so the roof stays clean for longer.
If the job's too big to reach safely, our conservatory roof cleaning is done from the ground with pure water across Essex.