Lichen on roof tiles: what those crusty grey patches are doing
Lichen is not moss. It is a fungus and an alga living as one crust, and it anchors into the tile with rhizines — root-like threads that work into the glaze and hold water against the surface. Scraping chips that glaze and high pressure strips it. A soft wash with proper dwell time kills the organism, and weather carries the dead crust off over the following months.
Lichen is not moss, and it holds on differently
Moss sits on a tile. Lichen grips it. Lichen is two organisms living as one body — a fungus that builds the structure and an alga that makes food from light. The fungus anchors with rhizines, root-like threads that work into the glaze and the pores beneath it. Between them they need almost nothing: light, occasional moisture, and a surface that stays damp a little longer than it should. That is why lichen colonises bare rock, and why a fired terracotta or concrete roof tile is, from a lichen's point of view, rock.
The consequence is water. A lichen crust holds moisture against the tile face long after the rest of the roof has dried, and it holds it exactly where a roof relies on shedding — the exposed surface and the top of the lap. A tile that stays damp for days instead of hours weathers faster, and a weathered tile is a better foothold for the next colony. Moss you can lift off with a gloved hand in one piece. Lichen you cannot, and that single difference decides how it has to be treated.
Grey crust, orange freckles, south pitch
Lichen on a Sydney roof usually reads as a flat grey-green crust with a slightly raised, ruffled edge, like a splash of paint that has begun to lift. Orange or rusty spotting through it is normal — sometimes a second species, sometimes fruiting bodies of the same one. It is not slime and not a cushion. Run a hand over it and the tile feels scabbed rather than furry. Moss, by contrast, is the plush mound in the valley beside it. Most roofs carry both, and they want different amounts of water.
The map is predictable. Worst on the southern pitch, worst under a canopy, worst on the slope a taller neighbour or a row of gums keeps in shade until midday. Trees add the second ingredient: leaf litter and bird traffic drop nutrients onto the tiles and the growth follows. Lichen also tolerates drying better than moss does, so it spreads onto pitches moss never reaches. A roof can be crusted grey across most of its area while the green stays confined to the shaded valleys.
Scraping chips the glaze and pressure strips it
Because the rhizines are into the surface rather than on it, anything that removes the crust mechanically removes some tile as well. A scraper or wire brush takes flakes of glaze off terracotta and the cement skin off concrete tiles, and it leaves behind pale patches and micro-pitting. That is worse than where you started. A rougher, more porous tile holds water longer and gives the next colony more to grip, so the roof recolonises faster than the untouched sections beside it. The crust goes; the problem accelerates.
A pressure lance is the same mistake with more force behind it. It will strip lichen off a tile, and it will strip the tile off the tile — the surface finish that was doing the shedding. It also drives water up under the laps a roof exists to keep dry, and it puts a person walking on brittle old terracotta, which cracks tiles and shifts ridge capping. The roof looks transformed for a season. Then the growth returns onto a surface that is now friendlier to it than before.
Soft wash and dwell time do the work you cannot see
The fix is chemical, applied at low pressure. The solution is put on wet and even, at the right dilution, and then left. Dwell time is the whole job — the crust has to stay saturated long enough for the treatment to reach the fungal body and the rhizines underneath, not just bleach the visible top layer. Kill the organism and the crust dies and loses its grip on the tile. Rush the contact time and you get a paler roof that is still very much alive.
After that it is weather's turn. Dead lichen does not vanish on the day; rain, wind and thermal movement lift it off over the weeks and months that follow, which is why a properly treated roof keeps improving long after the van has gone. Plants get wet down before and rinsed after, and the gutters take a share of the debris on the way through. The trade is patience for a surface that was never scoured, chipped or walked into pieces.
The tile does not go back to new, and that is the honest part
Where lichen has held one spot on old terracotta for a decade, it leaves ghost marks — outlines a shade different to the tile around them, because the surface under the crust aged differently to the surface exposed to sun and rain. Cleaning removes the organism, not the history. On old concrete tiles the faded look is the colour coating wearing out, and no wash restores that; recoating is a separate job with a separate decision behind it. Anyone promising a new-looking roof from a clean is selling you the wrong thing.
Roof work is quoted per job, because pitch, access, tile type and how much growth is up there change the day completely. The instant quote returns a firm, bookable price in about 60 seconds, and it holds unless the place turns out to be materially different from what you described. Put the gutters on the same visit and the bundle saves 5-15% — worth doing here, since those gutters will be collecting dead lichen for months after the roof is treated.