Moss, lichen or algae? What's growing on your roof, and whether it matters
A quick guide to what you can see from the garden, and what it means.
What you're looking at
| What you see | What it probably is | Does it matter? |
|---|---|---|
| Soft green cushions, often thickest along tile edges and joints | Moss | Light moss, rarely. Heavy moss, yes: it holds water and fills gutters |
| Flat, crusty patches in grey, silver or grey-green | Lichen | Usually not. It grows slowly and holds little water |
| Bright yellow or orange patches, often where birds perch | Lichen, commonly the species Xanthoria parietina | Usually not |
| A thin green film or powdery green deposit | Algae | Mostly a matter of looks |
| Greenish-brown jelly on a flat roof that turns dark and crusty when dry | Nostoc, an algae-like growth | Can make flat roofs slippery; check the outlets |
| Flat green plate-like growth on a damp flat roof or step | Liverwort | A sign the surface stays wet; check the drainage |
Moss
Moss is the one to watch. The RHS describes the common kinds as "compact, green, cushion-like mosses". On a roof, they start in the joints and along the bottom edges of tiles, where water and dirt collect.
According to NFRC, the roofers' trade body, moss takes hold on rough tiles, where dirt, rotting leaves and windborne spores collect. Marley, one of the UK's biggest tile makers, says light to moderate growth isn't normally harmful, but a heavy build-up "will inhibit the flow of water off the tiles". It also ends up in your gutters.
Lichen
Lichens, in the RHS's words, "form thin, flat crusts, or flat, creeping growths with leaf-like lobes". They grow slowly, grip tightly and hold little water.
The bright orange patches you see on so many roofs are often the lichen Xanthoria parietina. The British Lichen Society describes it as "a bright yellow to orange foliose lichen (often grey in the shade)", found on "rocks, walls and roofs". It grows especially well where it's exposed to bird droppings, so it often shows up where birds perch. The society notes it is "increasing due to nitrate and ammonia pollution".
Algae and dark streaks
The RHS says algae "just form a thin layer on top of hard surfaces", usually as a green film or a powdery deposit. Historic England's research describes "greening" as a sign of more active algal growth, which it links to climate change.
Dark streaks that run down a roof follow the path of the rain, and are often a mix of dirt and growth carried down the slope.
Do any of them damage the roof?
The sources don't all agree, so here's the honest picture.
- The RHS says that on hard surfaces these growths generally "do not damage what they are growing on", though they can make surfaces slippery.
- Historic England's research on stone says "some lichen and bacteria actively cause deterioration", and that moss holds moisture against masonry.
- Marley says "moss, algae and lichen growth can trap moisture and cause roof tiles to degrade over time", and that heavy moss can damage a roof if it's left unchecked.
- Marley also warns that "the removal of moss and lichen can cause further damage to the tiles and the roof in general".
In practice, that means leaving light lichen and algae alone, and dealing with heavy moss carefully, by hand.
What to do about each
- Light moss, lichen or algae, gutters clear: nothing. Look again next winter.
- Moss washing into the gutters, or thick on a shaded slope: have it removed by hand in winter, and the gutters cleared.
- Orange or grey lichen only: leave it. Scraping it off risks more damage than it causes.
- Growth on a flat roof, with water pooling: check that the outlets are clear, because standing water is the real problem.
Not sure which you've got? A clear photo from the garden is usually enough to tell.
How we deal with it
We remove moss by hand, from roof ladders, and treat where it suits the roof to slow regrowth. If what you've got doesn't need touching, we'll say so.
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Sources
- Algae, lichens, liverworts and mosses on hard surfaces (RHS)
- Xanthoria parietina (British Lichen Society)
- Worried about moss on your roof? (NFRC, June 2025)
- How do I remove moss or lichen from my roof? (Marley)
- Roof tile maintenance: a homeowner's guide (Marley)
- Control of biological growth on masonry (Historic England)
Written by Hawkes Roofing, roofers based in St Albans who remove roof moss by hand across Hertfordshire, Buckinghamshire and north-west London. Last reviewed 26 October 2026.