The glass over my veranda has gone green, how do I clean it?

Why it grows where it grows

Look at where the green is and it explains itself. The north facing slope stays damp longest, the lower edge above the gutter holds a film of water, and anywhere shaded by a tree or a fence dries slowest. Add the fine organic dust that settles on any horizontal surface and there is a ready made growing medium. A shallow pitch makes it worse, because water lingers rather than sheeting away, which is one practical argument for a slightly steeper roof.

The method that works without damaging anything

Do it on a cool overcast day so nothing dries on the glass mid clean. Work from the ground with a soft bristled brush on an extending pole, wetting the surface first so grit floats rather than grinds. Warm water with a little washing up liquid or a proprietary glass safe cleaner is enough for a normal film, applied and left for a few minutes to soften the growth before brushing. Rinse fully, because residue left behind is what causes streaking once the sun comes out.

1. 1Clear leaves and moss from the roof and gutter first.
2. 2Wet the whole panel from the ground before touching it with a brush.
3. 3Apply a mild solution and leave it several minutes to soften the film.
4. 4Brush gently along the panel, never scrubbing at a fixed spot.
5. 5Rinse thoroughly from the top down and let it air dry.

What not to use, and why

A pressure washer is the single most common cause of avoidable damage to a veranda roof. It drives water past gaskets that were sealing perfectly, lifts the protective coating on polycarbonate and can strip sealant from a joint in seconds. Abrasive pads and wire wool leave a haze on glass and permanent scratching on plastic. Bleach and strong acidic cleaners attack the rubber gaskets and can mark powder coating, which is a much bigger problem than the green film you started with.

• No pressure washers, at any setting, near joints or coated panels
• No abrasive pads, scourers or scraping blades
• No bleach or acidic cleaner on gaskets, seals or powder coating
• No climbing on the roof to reach the middle

Self cleaning glass, and what it really does

Self cleaning glass carries a coating that helps daylight break down organic dirt and helps rain sheet across the surface rather than beading. It genuinely reduces how often a roof needs attention on an exposed pitch. It is not magic, and it does far less under heavy shade or where debris sits on the pane, because the coating relies on both light and rain to work. If a roof is under trees, plan on cleaning it regardless of specification.

Self cleaning coatings reduce the frequency of cleaning on an open, well lit roof. Under trees, they simply mean less scrubbing when you do it.

Slowing the regrowth

Prevention is mostly about drying time. Cutting back the branch that shades one corner, keeping the gutter clear so water leaves quickly, and washing the roof at the end of autumn rather than waiting until spring all make a visible difference. On structures where the green returns within weeks, it is usually because debris is sitting in a low spot, and correcting the fall solves both the algae and the standing water at once.