4 min read
There are two main ways professionals clean a roof, and they’re not the same job: high-pressure cleaning and soft washing. Get them mixed up - or let the wrong one loose on the wrong roof - and you can end up with a clean roof that’s also a damaged one.
Here’s the difference, and which one belongs on your roof. (For the full picture, see our professional roof cleaning service.)
High-Pressure Cleaning
High-pressure cleaning uses a powerful jet of water - often 3,000+ PSI - to physically blast moss, lichen, dirt and old chalked coating off the roof surface. It’s fast, it’s dramatic, and on the right roof it’s exactly the right tool.
Where it’s right: as the first stage of a full roof restoration, on sound concrete or terracotta tiles that are about to be re-pointed and re-coated. When you’re going to coat the roof afterwards anyway, you want an aggressive clean back to a sound surface so the new coating bonds properly.
Where it goes wrong: on fragile, old or already-compromised roofs, and in untrained hands. Too much pressure, or the wrong angle, can:
- Crack or chip brittle old terracotta tiles
- Strip the protective colour coat clean off concrete tiles (the colour is only a surface layer)
- Force water up under tiles and through lapped joints - into your roof space
- Blast the protective coating off a Colorbond roof
That last point matters: you generally should not high-pressure blast a Colorbond or painted metal roof. The factory finish is what protects it, and a hard blast can damage it.
Soft Washing
Soft washing uses low pressure combined with a cleaning solution that kills moss, lichen, mould and algae at the root. Instead of blasting growth off, it treats it - the solution does the work, and the dead growth rinses or weathers away. Think of it as chemistry instead of brute force.
Where it’s right:
- Colorbond and metal roofs, where you can’t use high pressure
- Delicate or ageing roofs that wouldn’t survive a hard blast
- Roofs where you’re cleaning to keep the existing surface - not to strip it back for coating
- Killing the moss and lichen at the root so it’s slower to grow back, rather than just removing what’s visible
Where it falls short: it won’t strip a heavily chalked old coating back to bare tile the way high pressure does. If your roof is about to be fully recoated, soft washing alone usually isn’t enough prep.
Which One Does Your Roof Need?
It comes down to the roof and the goal:
| Your roof / goal | Best method |
|---|---|
| Sound tiles, about to be restored/recoated | High-pressure clean |
| Colorbond or painted metal roof | Soft wash only |
| Old, brittle terracotta you want to preserve | Soft wash / gentle approach |
| Just want it clean, keeping the current surface | Soft wash |
| Heavy moss before a full restoration | High-pressure clean |
There’s no single “best” method - there’s the right method for your roof.
The Perth Angle
Perth’s wet winters and shaded southern roof faces grow moss and lichen readily, and our long dry summers bake on dirt and chalk. Both problems are very cleanable - but the moss issue is exactly where soft washing earns its keep. Because it kills the growth at the root rather than just knocking the green off the top, a soft wash tends to stay cleaner for longer than a blast that leaves living spores behind. For badly mossed roofs, see our dedicated moss removal guide.
The Mistake to Avoid
The classic error is a cheap operator high-pressure blasting everything - including Colorbond roofs and fragile old terracotta - because it’s fast and looks impressive. It might look clean on the day. The damage (stripped coatings, cracked tiles, water under the tiles) shows up later.
If someone’s quoting to “pressure clean” your Colorbond roof, ask why they’re not soft washing it. The answer tells you a lot.
The Bottom Line
High-pressure cleaning is the right call when you’re stripping a sound tile roof back for a full restoration. Soft washing is the right call for metal roofs, delicate roofs, and anytime you want to clean and keep the existing surface - especially for killing Perth’s persistent moss at the root. A good roofer picks the method to suit your roof, not whichever one they own a machine for.
Not sure which yours needs? Get an online quote - we assess your roof remotely from satellite and aerial imagery, with a paid on-site inspection available if needed, and we’ll tell you straight.
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