The short answer
The orange, yellow or rust-coloured crust on roof tiles is almost always lichen, most often a species that thrives where bird droppings and airborne dust add nutrients. It is a partnership of a fungus and an alga, and it grips the tile surface far harder than moss does. It rarely causes a leak, but it slowly roughens the tile and holds moisture against it. It can be removed, though it needs a patient treatment rather than a quick scrape.
What you are actually looking at
Lichen is not a plant. It is a fungus and an alga living together as a single organism, and that partnership is why it copes with conditions that defeat moss. It needs almost nothing: light, air and a little moisture. The orange and yellow kinds are often found on the sunnier, more exposed slopes of a roof, which surprises people who expect all roof growth to be a shade problem.
The patches are flat, dry-looking and crusty, usually round and slowly widening. Grey, white and green lichens are common too. You will often see the orange kind along ridges, on chimney tops and on the top edge of a slope, where birds perch and leave droppings that feed it. If you can scratch the patch with a fingernail and it feels like a tightly stuck scab rather than a soft cushion, it is lichen rather than moss.
Algae can look orange too, particularly in a damp spell, but algal films wipe off more readily and are normally green or black. A close look from a ladder or with binoculars tells the two apart.
Colour comes from pigments the lichen makes to shield itself from strong light, which is why the orange kinds favour bright, open slopes. The same pigments explain why a patch can look brighter after rain, when the crust swells and darkens a little.
Does it harm the roof?
Lichen is much slower to damage a roof than moss. Its threads, called hyphae, work into the tiny pores of concrete and clay and hold on there. Over many years that roughens the surface and can pull away tiny particles of the tile face when the lichen dies or is forced off. The effect on a sound tile is gradual, and a few patches are cosmetic.
The bigger issue is that a rough, crusted surface keeps water against the tile for longer, and that gives moss and algae a better place to start. On older roofs whose surface has already worn thin, heavy lichen can make a poor tile worse. It is not usually the cause of a leak, which almost always comes from a cracked tile, a failed flashing or a damaged underlay. See our answer on whether moss can cause a roof leak for how growth and leaks relate.
On clay and slate the effect is slower still, and many lichens sit on old slate for a lifetime without causing a problem. What deserves a closer look is lichen on a tile that is already flaking, because the crust can hide the true condition of the face.
Why it grows, and why it comes back
Lichen favours clean air, so it is a sign of fairly good local air quality as much as anything. It grows slowly, and some patches take years to reach any size. It likes stone, slate and rough concrete, and it likes places where the surface stays slightly nutrient-rich. A roof near trees with roosting birds, or one with a chimney used as a perch, will pick it up first.
It also comes back, because the spores arrive on the wind and the tile surface stays the same. Removing the crust without treating the tile leaves the root threads in the pores, which is why lichen returns in the same place. We describe the three growths and how they differ on the lichen, algae and black streaks page.
Gutters and flat ledges collect the dust and droppings that feed lichen, so a clean roof edge matters as much as a clean slope. Keeping perches such as aerials and chimney pots free of nesting material is a small help too.
How lichen is removed
Scraping and pressure washing are the usual mistakes. Lichen holds on so tightly that a scraper tends to take the tile surface with it, and a hard jet does the same and drives water under the tiles. The better route is to apply a biocide that soaks into the crust and kills the organism, and then allow it time. The lichen dies, loosens and weathers off over the following weeks or months, with a soft brush used on any stubborn patches.
That makes it a slow job to finish, and the roof may look patchy for a while. We explain the products and the wait on the page about roof biocide treatment. If you want it assessed, the survey comes first, with photos of the patches, and the fixed written quote follows. You can read the full service on our roof cleaning in Basingstoke page.
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