Lichen is the crusty orange, grey or yellow-green patchwork that spreads across older roofs, often looking more like paint than a plant. Because it is flat and slow, many owners wonder whether it matters at all. The question has two honest halves. Lichen is not usually the cause of leaks, and on some roofs it is regarded as part of their weathered character. But it can hold moisture against a surface, it can be hard to remove, and on certain coverings the way it is removed does more harm than the lichen ever did. This guide sets out what lichen is, how it differs from moss, what it can and cannot do to clay, concrete, slate and stone, and how to decide between leaving it, treating it and removing it. Protected status adds a step to the decision, and we flag it.
What lichen is, and how it differs from moss
Strictly, lichen is two organisms in one: a fungus and an alga that live together and share the work. The fungus provides the structure and holds on to the surface, and the alga makes food from light. That partnership is why lichen survives on bare, dry, nutrient-poor surfaces where little else will grow, such as a south-facing slope in full sun.
Moss is a different living thing. It is a green, soft, cushion-like plant that thrives in damp and shade and holds a lot of water. Lichen is usually flatter, drier and slower. You can often tell them apart by touch and colour.
- Moss is green or brown, raised and spongy, and tends to gather along edges, in valleys and on shaded slopes.
- Lichen is flat or slightly raised, crusty or leafy, in grey, white, yellow, orange or green, and often grows in rings or patches on exposed faces.
- Algae is the dark, streaky film often called black staining, and we look at it separately in black streaks on roofs.
A roof can carry all three at once. The difference matters because each reacts differently to treatment. Moss can often be brushed or softly washed off. A crustose lichen grips the surface with fine threads and may need a biocide to loosen it. Knowing what you have is the first step in deciding what to do.
Does lichen damage tiles and slate?
The answer turns on the covering, and lichen is usually a slow problem, not an urgent one. Lichen does not eat through a roof or cause a leak in the way a slipped slate or a split lead gutter does. It grows on the surface.
What it can do is hold moisture. A patch of lichen keeps a small area damp for longer after rain, and on a porous surface such as old clay, concrete or soft stone, repeated wetting and freezing under the growth can contribute to surface wear. Lichen also produces mild acids as it grows, which can etch some stone and weathered surfaces over a long period. The effect is gradual, and there is no sourced figure for how quickly it happens, so do not trust anyone who gives you a timescale without looking at the roof.
Rain also matters. Lichen on the lower courses and around the gutter line gets the most water, and the runoff carries spores and fragments downhill, which is one reason a roof tends to be colonised from the eaves upwards. Gutters full of debris keep that zone wet for longer, so clearing them is part of any plan, not a separate chore.
Slate
Natural slate is dense and sheds water, so lichen sits on it with little harm. It can spoil the look, and heavy growth along a ridge or at the lap between slates can hold water where you want it to drain.
Clay and concrete tiles
Porous tiles are likelier to be affected, and thick growth can mask cracked or slipped tiles. A survey is how you find out what is hiding beneath.
Stone tiles
Cotswold-type stone tiles are valued for their weathered surface. Lichen is part of that character, and cleaning them needs care, as our page on cleaning stone roofs explains.
When lichen is worth removing and when to leave it
Removal is not automatic. It is worth thinking through, because a poor clean can damage a roof more than a patch of lichen would.
There are good reasons to treat or remove it.
- Growth is thick along valleys, gutters or the lower courses, where it holds water and can feed moss.
- It is covering tiles you need to inspect, because you suspect cracks or slips.
- It is spreading across porous tiles that already show surface wear.
- The roof is going to be sold, and you want to see what you are selling.
- Debris from the growth is landing in the gutters and downpipes.
There are also good reasons to leave it. On a sound slate or stone roof, light lichen is largely cosmetic, and many owners of period homes prefer the weathered look. If cleaning would mean pressure washing, scraping or strong chemicals on a fragile surface, the risk to the covering may outweigh the gain.
A practical test is to ask what the lichen is actually doing. If it is holding water against a vulnerable edge or hiding a fault, deal with it. If it is sitting quietly on a healthy surface, it can wait. We tell you which category your roof falls into after a survey with photographs of your own tiles or slate, and we do not recommend a clean for its own sake.
Methods that suit lichen, and methods that do not
Lichen grips, so it is harder to shift than moss. That tempts people to use force, and force is where damage happens.
Pressure washing is the usual culprit. A strong jet can strip the surface from soft or old tiles, drive water under laps and into the roof space, and scar slate. MyBuilder's roof cleaning cost guide (2026) reports typical UK ranges of £15 to £20 a square metre for soft washing and £10 to £15 for pressure washing, and warns that a hard jet can harm delicate tiles. That gap is small beside the price of replacing them. We look at the risks in more detail on our page about pressure washing roofs. The quote is fixed once the survey is done.
Scraping and wire brushing are a second hazard. They can scar the tile and leave a rough surface that holds more growth next time.
Gentler options exist.
- Biocide treatment. A liquid that kills the growth in place and lets it loosen and weather off over weeks. See roof biocide treatments for how it works.
- Soft washing. Low pressure with a cleaning solution, which lifts dead growth without forcing water under the covering.
- Careful hand work. A soft brush on a treated surface, for stubborn patches or fragile edges.
Patience is part of the method. Lichen often fades over a season or more after treatment, and rushing it is what causes harm.
Expect the roof to look patchy for a while. Dead lichen turns pale or greyish before it loosens, and a stain can linger after the growth has gone, particularly on lighter tiles. That is cosmetic. If you are tempted to scrub the stain away, check first whether the surface is sound, because a tile that is flaking will lose more of its face. Work at height also needs proper access and edge protection, which is a good reason to leave anything above the first floor to a roofer.
Lichen on older and protected Bath roofs
Bath has a great many roofs where this matters. The city sits entirely inside its World Heritage Site, roughly half of it is also within the conservation area, and there are around 5,000 listed buildings. Plenty of those roofs are natural slate, stone or old clay, valued for their age and surface.
Owners of listed houses should put the question to B&NES Council before work starts. Its listed buildings guidance explains that a listed building needs consent of its own, quite apart from any planning permission, and treats unconsented work as an offence. Whether a gentle clean counts as work needing consent is a question for the conservation team, not for us or for a salesperson. We cover which methods suit historic roofs in our page on cleaning listed roofs.
The conservation area appraisals B&NES has written for villages south of Bath record Cotswold stone tiles still on some roofs and call them rare. Where a roof like that carries lichen, a heavy-handed clean is the bigger danger.
A roof that shows signs of bats or nesting birds, both protected species, is left alone until you have taken advice.
Lichen, coatings and what to do next
A clean roof tempts some owners to seal it, and some sales pitches use lichen as the reason. Be careful. A film over lichen or over a surface that has not been properly cleared can trap moisture, and the growth can return beneath it. Our assessment of roof coatings goes through the claims and the risks.
Mortgage and survey questions sometimes follow a coating, and our page on roof coatings and mortgages explains why. Treat first, and think about sealing only if the roof is sound and the product suits it.
Keep a record as you go. Photographs of the roof before treatment, a few weeks after and again at the end of the season show whether the lichen is retreating, holding steady or returning, and they give a surveyor or buyer something to look at if the question ever comes up. Note which slopes change first, since that tells you where moisture lingers.
Here is a sensible order for dealing with lichen.
- Identify what is growing, and how thick it is.
- Check the roof beneath for cracked, slipped or worn tiles.
- Decide whether removal is needed or whether the lichen can stay.
- Choose the gentlest method that does the job.
- Fix any faults the cleaning uncovers.
- Plan to review the roof after a season.
If you would like the roof looked at as part of our roof cleaning service, we survey it, photograph your own tiles or slate and write the plan down with a fixed price. Where repair is enough we repair, and a dry spell is the best time to book, since a damp roof hides hairline cracks and is slippery underfoot.