Can you pressure wash roof tiles? What the blasting actually does
You can, but you shouldn't. High pressure strips the fired surface off terracotta and the coloured slurry coat off concrete tiles, ages the roof by years in an afternoon, spreads moss spores, and forces water under the laps. Soft washing kills growth at the root instead. Schmick quotes roof cleaning per job — the instant quote returns a firm, bookable price in about 60 seconds.
What the jet actually strips
A pressure washer doesn't know the difference between moss and roof. At the pressures most operators run — the same setting they'd use on a concrete driveway — the jet takes the top layer of the tile with it. That layer is the tile's weather skin. It's what sheds rain, resists weathering and slows the growth coming back. Once it's gone, it's gone. The roof looks clean for a season, then starts absorbing water like a biscuit.
The damage is invisible from the ground, which is why the practice persists. The tiles are still red, still in place, still shedding most of the rain. But a blasted tile is a porous tile. It holds moisture longer, grows moss faster and weathers harder every winter after. You've paid to age your roof. A tile roof is built to outlast the mortgage; a pressure-washed one starts its decline the day the trailer leaves.
Terracotta and concrete fail differently
Terracotta is clay fired hard at the surface. That fired face is thin — a few millimetres of glass-like skin over a much softer body. High pressure erodes it, and on older Sydney terracotta it can flake it off in patches. Underneath is porous clay that drinks water, holds it through winter and feeds the next round of lichen. Glazed terracotta fares a little better, but glaze chips under a jet, and a chipped glaze traps moisture beneath it.
Concrete tiles are worse off. Their colour isn't in the tile — it's a thin cement slurry coat applied at the factory, often with a clear sealer over the top. Pressure washing strips that coat unevenly, which is why blasted concrete roofs go patchy and grey within a year or two. No wash restores it; that's a re-coating job, and an expensive one. If your concrete roof still has its colour, the cheapest thing you can do is keep a jet away from it.
The spores survive the blast; the roof doesn't
Moss and lichen don't die under pressure. The jet shears off the visible growth and atomises the rest — spores in the spray, spores in the runoff, spores settling on every tile downwind. On a freshly stripped, newly porous surface, they take hold faster than they did the first time. This is why pressure-washed roofs green up again within a couple of seasons, and why the operators who blast them are rarely around to see it happen.
Then there's the water. A tile roof sheds rain by gravity — down and over each lap. It is not designed for water arriving sideways at high pressure. A jet aimed up-slope drives water under the tiles, into the sarking, onto the battens and, on older roofs without sarking, straight onto the ceiling below. It can also shift and crack tiles outright, and lift the fragile mortar on ridge capping. One clean, several new leaks.
Soft washing kills the growth at the root
Soft washing works the other way around. Low pressure — not much more than heavy rain — carries a biocide treatment across the roof. The treatment kills the moss, lichen and algae at the root, spores included, instead of shearing the tops off. Dead growth loses its grip and washes away with the weather over the following weeks. The tile surface is never touched. Nothing is stripped, and nothing is driven under the laps.
Be clear about what it can't do. Soft washing won't bring back the colour on a concrete tile that has already lost its slurry coat — that's re-coating territory. It won't fix cracked tiles or crumbling ridge mortar; those need a roofer. What it will do is remove the growth properly, keep it off for years rather than months, and leave the roof structurally exactly as it found it. That last part is the whole point.
What a proper roof clean costs
Roof cleaning doesn't price off a flat rate, and anyone who gives you one sight unseen is guessing. Size, pitch, growth cover, access and tile material all move the number, which is why Schmick quotes roof work per job. The instant quote asks about your roof, prices it against the live price book and returns a firm, bookable figure in about 60 seconds. It holds unless the place is materially different from what you described.
If the gutters and windows are due at the same time, bundle them — one visit saves 5-15% across the job. And if a leaflet has promised to pressure wash your roof for cheap, now you know what that price actually buys: a stripped surface, a faster regrowth and a roof older than it was that morning. Get the number for doing it properly instead. It takes about a minute.