Lichen on Your Roof and Why It's So Stubborn

You've probably seen those crusty, pale patches clinging to a roof and wondered what on earth they are. They look a bit like splashes of dried paint, grey-green or mustard yellow, and once you notice them you start spotting them on roofs all over your street.

That's lichen, and if it's taken hold on your Durham roof you'll know how firmly it grips. Moss you can often brush off, but lichen seems welded to the slate. It's one of the most common growths on roofs in the North East, and it's also one of the most stubborn to shift, which is exactly why it puzzles so many homeowners.

The quick answer: Lichen is a tough, slow-growing organism that bonds tightly to the roof surface, so it can't just be brushed off and needs a proper soft wash and treatment to remove without damaging the tiles.

1. What Lichen Actually Is

Lichen isn't a single plant. It's a partnership between a fungus and an alga living together, which is part of why it's so hardy. The fungus provides the structure and grip while the alga makes food from sunlight, and between them they can survive on bare rock, stone, and roof tiles where little else would.

Because of that toughness, lichen shrugs off conditions that would kill most growth. It handles dry spells, frost, and pollution, and it grows very slowly, which means by the time you notice a decent patch it's been settling in for a good while. That slow, tenacious growth is the root of why it's so hard to remove.

2. Why Durham Roofs Get So Much of It

Lichen loves clean, damp air, and Durham offers plenty of both. Away from heavy traffic and industry, the air here suits it well, and the constant moisture rolling up the Wear valley keeps roof surfaces damp enough for it to thrive. The older, weathered slate on the city's period homes gives it exactly the rough footing it likes.

You'll see it worst on the shaded, north-facing slopes that never really dry out, and on roofs near the wooded riverbanks and greener edges of the city. Out in the former pit villages, the mix of open skies and damp ground makes plenty of roofs a happy home for it as well.

3. Why It Clings So Hard

The reason lichen is so stubborn comes down to how it attaches. It doesn't just sit on the surface like a dusting of algae. It puts down root-like threads that work into the tiny pits and pores of the tile, gripping the surface so tightly that pulling it off can take flakes of the tile with it.

This is why scraping and heavy pressure washing are such a bad idea on a lichen-covered roof. On older Durham slate especially, blasting the lichen off can strip and pit the surface, leaving the tile rougher and more open to the next round of growth. The grip that makes lichen so tenacious is also what makes brute force so damaging.

4. Is Lichen Bad for the Roof

Lichen is slow, so it's less of an immediate threat than a thick moss blanket, but it isn't harmless. It holds moisture against the tile surface, and that constant dampness works away at the tile over time, especially through the freeze and thaw of a North East winter when trapped water expands as it freezes.

On the whole, the bigger issue with lichen is that it points to a roof that stays damp, which usually means other growth is welcome too. A roof crusted with lichen is often carrying moss and algae as well, and together they keep the whole surface wetter than it should be. Dealing with the lot at once is the sensible move.

5. How to Remove It Properly

The right way to clear lichen is patient rather than forceful. A soft wash with the correct treatment kills the lichen and loosens its grip so it can be cleared without gouging the tile. It takes longer than a quick blast, but it protects the roof rather than wearing it down.

A biocide treatment is often the key part, because lichen is so slow to grow and so slow to die. The treatment gets into the growth and kills it off over the following weeks, after which the remains weather away and can be gently cleared. Rushing it with a jet wash might look quicker on the day but costs you in tile damage later.

6. Keeping It From Coming Back

Because lichen grows slowly, a good clean and treatment buys you a long stretch before it returns. Keeping the roof as dry and open to light as possible helps, so cutting back overhanging branches that shade and drip onto the roof makes a real difference on a damp Durham property.

Clear gutters matter too, since a roof that drains freely dries out faster between downpours. Pair a proper clean with a follow-up treatment and a bit of ongoing attention, and you keep the crusty patches from creeping back across your slate for years rather than months.

• What it is: A hardy fungus-and-alga partnership that survives where most growth can't.
• Why it grips: Root-like threads work into the tile's pores, so it can't be brushed off cleanly.
• Durham factor: Clean, damp air and weathered slate make local roofs ideal for lichen.
• Never blast it: Pressure washing strips lichen and tile together, leaving the surface worse.
• Proper removal: A soft wash and biocide treatment kills it at the root and protects the roof.

Frequently Asked Questions

What are the crusty patches on my roof?
That's lichen, a slow-growing organism made of a fungus and an alga together. It bonds tightly to slate and tile, which is why it looks crusty and won't simply brush away.

Why can't I just scrape lichen off?
Its root-like threads grip into the tile surface, so scraping or blasting it pulls flakes of tile away too. That leaves the roof rougher and more prone to fresh growth.

Is lichen damaging my roof?
It's slower to harm a roof than thick moss, but it holds moisture against the tile, which matters through winter freezes. It also signals a roof that stays damp and carries other growth.

How do you remove lichen safely?
With a low-pressure soft wash and a biocide treatment that kills it at the root over a few weeks. The dead growth then weathers away without damaging the tiles.

Will the lichen come back?
It grows slowly, so a proper clean and treatment lasts a good while. Cutting back shading branches and keeping gutters clear helps keep it from returning quickly.

Got stubborn lichen on your roof?
Ben's Roof Cleaning removes moss and lichen across Durham and County Durham, with over 15 years of experience. Get in touch or call 07889 127153 for a free, no-obligation quote.

Patience Beats Pressure Every Time

Lichen earns its reputation as the most stubborn thing on a roof, but stubborn doesn't mean impossible. The trick is treating it the right way, killing it at the root and letting it weather away, rather than attacking it with a pressure washer and damaging your slate in the process. Handle it properly and your Durham roof stays clear for the long haul.