The short answer
Lichen is rarely urgent. It is a slow-growing crust, grey, yellow or orange, that grips the tile surface tightly. Unlike moss, it does not build thick cushions that hold water or block valleys. It is mainly a matter of appearance, though heavy cover can roughen tiles and hold damp. Treat it when it spreads or you want a cleaner roof, and never scrape it hard.
How to recognise lichen, how slowly it grows, when it is safe to leave and how a roofer deals with it when it is not.
How lichen differs from moss and algae
Lichen looks like paint that has dried in rough circles. The patches are crusty or leafy, usually grey, pale green, yellow or orange, and they sit close to the tile with no stems or soft cushion. Scratch one with a fingernail and it barely moves, which separates it from algae, a smear that rubs off, and from moss, which lifts away in clumps.
Lichen is a partnership between a fungus and an alga. It grows on stone, bark and tile where the surface is stable and rain is the only supply of moisture. That is why it likes old roofs, particularly slate, clay peg tiles and the weathered face of concrete tiles. It also appears on ridge tiles, hips and the top of chimney stacks, where it is rarely disturbed. Bird perches and the tops of walls collect it in the same way, which is a clue to how it likes to live.
Moss and lichen often share a roof, and a close look from a ladder or with binoculars from the garden is the way to tell which you have. Our page on moss on concrete roof tiles shows what the cushioned kind does, and survey photos make the difference plain.
Why lichen grows slowly and why that matters
Lichen takes years to spread across a roof and grows at a pace you can only see by comparing photos season to season. It does not need much water or food, so it can survive on a surface that moss would struggle on, but it also does not spread quickly.
That slowness is the reason it is seldom an emergency. A roof with a little lichen has time for you to plan, compare prices and pick the weather. There is no need to book the nearest roofer at the first sight of it.
Slow does not mean harmless. Lichen anchors itself into the surface, and over many winters a dense covering can roughen the face of a tile and hold moisture against it. Rougher, damper tiles are then easier for moss to colonise and more exposed to frost. A thick crust on a soft old tile can also be a sign that the tile is already breaking down underneath.
A dated photo each spring tells you whether it is moving at all. Compare the same patch, taken from the same spot, and the answer is usually clear within a couple of seasons.
When to leave it alone
Plenty of lichen can stay. If the tiles are sound, the ridge is sound, the gutters run and the cover is patchy, many owners decide to live with it, and on an old slate or clay roof it can look natural. Where a roof is in a conservation area, or the house is listed, a heavily cleaned roof can look out of place, and a gentler approach may suit it better. Check with the council's planning team if you are unsure whether any change counts.
Do not scrape or scrub it off dry. Lichen grips so tightly that a stiff brush or a scraper takes the tile surface with it, leaving a rough patch that grows back faster. High-pressure washing is no better, because tile makers such as Marley advise against it for the wear it causes to the surface.
Leave it if it is only patchy and the roof is watertight. Review it each year, and act if cover is spreading, if tiles feel crumbly, or if moss is starting around it.
Treating lichen when it needs it
When it is time to act, the method is a biocide treatment on a dry roof, applied by someone who follows the label. Lichen dies back slowly, often over months rather than weeks, and a treated patch can look unchanged for a long time before it fades and washes off. That is normal, so judge the result after a season, not a fortnight.
Where lichen has taken hold on cracked or damaged tiles, those tiles are replaced rather than cleaned. The survey pictures show you how the tiles under the crust are doing, and the quote comes as one sum. For national orientation only, Checkatrade's April 2026 table gives typical UK roof cleaning prices of £8.50 to £14 per m², and it pays to see that VAT and scaffold are accounted for in any offer.
Our roof cleaning and moss removal service page covers the full job, cleaning a clay tile roof deals with the older tile type where lichen is most common, and black streaks on roof tiles covers the other staining you may see. Send us a photo of the patch and the tile beside it.
More on this in Roof Cleaning & Moss Removal in Crawley
Two more lichen questions
Does lichen mean my roof tiles are failing?
Not by itself. Lichen settles on stable, weathered surfaces, so it shows a roof has been left alone for a long time, not that it is failing. Check the tiles beneath it: if they are crumbly, flaking or cracked, the tile is the problem. A sound tile with lichen on it is a cosmetic matter.
Can I remove lichen with a stiff brush?
It is not advisable. Lichen is attached firmly, so a brush strips the tile face with it and leaves a rough surface that regrows quickly. A brush also means working on the roof, which is a fall risk. A slow, label-guided treatment applied from safe access does less harm, and the lichen fades over time.