The short answer
Green algae on a roof is mainly cosmetic at first, so it is not an emergency. It does show that the surface stays damp, and over time the film can thicken, darken and give moss and lichen a foothold. Left alone for years it can add to the wear on tiles, so it is worth cleaning gently rather than ignoring, though there is rarely any need to rush.
What the green film is and where it starts
Green algae is a thin, soft film that forms on surfaces that stay wet. On a roof it tends to appear first on the shaded side, along the lower courses and in the lee of chimneys and dormers, where water collects and dries slowly. It looks like a green tint on the tile, sometimes powdery, sometimes slimy after rain.
It is the youngest stage of the growth that later becomes darker or mossy. Spores are always in the air, and a clean, dry roof gives them nothing to hold. A damp one lets them settle, feed on dust and rainwater, and spread. For what happens when the colour deepens into dark lines, see our answer on black streaks on a roof.
On its own, a green film does not weaken a tile. The question is what it leads to and whether the roof underneath can cope.
When algae is just cosmetic
A light green tint on a sound, well-fixed roof is mostly a matter of appearance. If the tiles are in line, the gutters are flowing and the loft is dry, the film is unlikely to cause a leak or a failure. Plenty of roofs carry it for years without trouble.
It can still affect how a house looks and how it sells. A green, patchy roof looks tired from the road even when it is perfectly sound. If appearance matters to you, a gentle clean is an easy fix, and it reveals whether anything else is going on underneath.
There is no need to treat it as urgent. A sensible approach is to note where it is, photograph it once a year and decide whether to clean at your convenience.
When it starts to do harm
The worry is persistence, and the clock it runs on is measured in winters, not weeks. A film that holds moisture against porous tiles keeps them wet, and frost can then flake the surface. It also gives moss a bed. Once moss has rooted among the algae, the growth becomes thicker, heavier and harder to remove, and it starts to affect the laps and the gutters.
Clues that the film has moved beyond cosmetic include raised cushions of moss among the green, a rough or crumbling tile surface, granules or fragments in the gutter, and damp patches in the loft under the affected slope. If you see any of these, the roof deserves a closer look.
Lichen is another companion growth, and it grips more firmly than algae. Whether it matters is covered in our answer on lichen on roof tiles.
Sheds and garages follow similar rules but often have less margin, since the felt or boarding beneath is thinner; see moss on a shed roof.
How green algae is cleared without harming the roof
Gentle methods work best. A soft wash applies a mild cleaning solution at low pressure, lets it break the film down and rinses it off. It treats the growth rather than blasting at it, and it is kinder to old tiles than force. On MyBuilder's 2026 figures a soft wash costs about £15 to £20 per m², while pressure washing, at about £10 to £15 per m², carries a warning about fragile tiles.
On slate, clay and stone we avoid high pressure altogether. A jet strips protective surface and can push water under the laps. Where algae sits among thick moss, hand scraping comes first, so the bulk is lifted before any liquid is applied, at about £5 to £10 per m² on the same guide, and the soft wash follows.
The overall figure depends on the size and pitch of the roof and on access, and MyBuilder puts whole-roof cleaning from about £300. Your own quote is written after the survey.
Stopping the film from coming straight back
A clean surface is only as good as the conditions around it. The film returns fastest where the roof stays wet, so cutting back overhanging branches, clearing gutters and valleys and letting light in make the biggest difference. If one slope always goes green first, our answer on moss on the north side of a roof explains why.
Some owners add a treatment after cleaning to slow regrowth. Whatever is used, it should suit the covering, and we would choose it to suit the material, not to keep the bill down.
A yearly look from the ground, with binoculars if need be, is enough to catch the first signs. Catching algae early means a lighter clean, less time on the roof and a smaller bill. It also means you spot a slipped tile or a blocked gutter while it is still a small repair.
Asking for an opinion on your own roof
When you cannot tell whether it is algae, moss or something else, email or message us pictures of the worst slope and the gutter below it. We will say what it looks like and whether it can wait. When you want a price, you can request a roof cleaning quote.
The full service is described at roof cleaning and moss removal in Stroud. You can ring 01453 933613, and our workmanship guarantee runs for 10 years, in writing, on the work we carry out.