You shouldn't. A pressure washer can force water past the seals and gaskets, lift the end caps on the glazing bars, crack or scratch polycarbonate and strip the coating off some glass. A soft brush, low-pressure water and patience get the roof clean without any of that risk.
What actually goes wrong with a pressure washer?
The main risks are leaks and damage to the roof's weak points. A conservatory roof is held together by rubber gaskets, end caps and seals around each panel. A jet of high-pressure water aimed at those joints can push water through them and lift parts that were never designed to take that force.
Once a gasket has been pushed out of its channel, water can track inside along the glazing bar and drip into the room. Lifted end caps let rain into the bar itself. Neither shows straight away; you notice the drip after the next downpour, when it's harder to link to the clean.
Is polycarbonate more at risk than glass?
Yes. Polycarbonate is a plastic, and it's softer than glass. High pressure, especially with a narrow nozzle held close, can scratch the surface, open up the flutes along the ends of the sheet, and drive water and dirt inside the chambers where it can never be cleaned out.
Older polycarbonate is also more brittle than new sheet, because sunlight breaks it down over the years. A roof that's been up for a long time can crack under a jet that a new one would have shrugged off. Scratches matter too: they hold dirt, so a scratched roof goes green again faster.
What about self-cleaning glass?
Some glass conservatory roofs have a self-cleaning coating on the outside. These coatings rely on sunlight and rain to break down and rinse off dirt. Manufacturers generally warn against abrasive cleaners and harsh treatment, and a pressure washer used carelessly is the kind of thing that can wear a coating away. If you're not sure what glass you have, check your paperwork or assume it's coated and be gentle.
Will a pressure washer at least shift the green?
It will move some of it, but it often leaves the worst bits behind. The green film and moss live along the glazing bars and in the joints, which is exactly where you shouldn't aim a jet. What's left grows back from the edges.
A soft brush working along the bars, with plenty of water, gets into the corners without forcing anything. That's slower than a jet but it's thorough, and it doesn't create new problems.
What should you use instead?
A soft brush on a long pole and plenty of clean, low-pressure water. Work from the ground or from a ladder resting on the house wall, never the conservatory. Brush along the glazing bars, not across them, and rinse from the ridge down.
- Clear the conservatory gutter first so the dirty water has somewhere to go.
- Brush off loose leaves and moss while the roof is dry.
- Wet the roof and give the green film a few minutes to soften.
- Brush along each bar and panel, then rinse well.
- Do the side windows last, because the roof water runs down them.
Our step-by-step guide on how to clean a conservatory roof safely goes through it in more detail.
Why do professionals use pure water?
Pure water dries without spots, so no soap is needed and nothing is left on the roof. Tap water in most of Essex is hard. Rinse a glass roof with it on a sunny day and you can end up with a roof covered in white dots, which then hold dirt.
Purified water has had the minerals taken out, so there's nothing left behind when it dries. It also means no detergent, which matters on a roof where the run-off goes onto the patio, the lawn and the plants below.
Can you walk on a conservatory roof to clean it?
No. Conservatory roofs, glass or polycarbonate, aren't built to take a person's weight, and falls through fragile roofs are a well-known cause of serious injuries. Even a short fall through a roof onto furniture or a tiled floor can be very bad. Everything needed to clean the roof can be done without climbing on it.
When is it worth calling someone?
When the roof is too big to reach the middle safely from the ground, when it's badly green, or when you're not sure what it's made of. A professional clean with a long water-fed pole reaches the ridge from the ground and uses pure water, so it's gentle on the seals and leaves no spots. Our conservatory roof cleaning is a one-off visit across Essex, quoted free from a photo.