How to clean roof tiles safely (and why moss is a problem)

Quick answer: clean roof tiles with controlled pressure and the right technique, treat the moss and lichen with a biocide so it dies at the root, and stay off the roof yourself. Blasting concrete tiles with a hired pressure washer strips the coloured surface off them and drives water under the laps, which is why half the "cleaned" roofs I see are worse off than before. And a mossy south face is not just ugly, it is holding water against your roof all winter.

Every Melbourne winter puts a green stripe down the south face of half the tile roofs in the suburbs. Come spring, the pressure washer hire places do a roaring trade, and a few months later I get called to roofs with stripped tiles and new leaks. So here is how cleaning actually works, what is safe to do yourself, and when cleaning is the wrong tool entirely.

Why moss and lichen actually matter

This is not just about looks. Moss is a sponge: it holds water against the tile surface for weeks after rain, keeps the tile face damp through winter, and creeps under tile laps where it can wick water past the overlap. Clumps of it break off and wash into valleys and gutters, building the little dams that cause overflows in a downpour. Lichen is worse in one way: it bonds into the tile surface and eats into coatings. A roof that stays green year-round is quietly ageing faster than a clean one, and it is telling you which faces of your roof never dry out.

Pressure washing vs soft washing (the part that ruins roofs)

Here is the thing the hire shop will not tell you. Concrete tiles get their colour from a thin cement slurry layer on the surface, and older tiles hold that layer loosely. Hit it with a 3,000 PSI needle jet from a metre away and you strip colour, surface and any old coating clean off, leaving a porous, blotchy tile that drinks water. High pressure aimed up the roof also drives water straight under the tile laps and into the sarking. I have seen DIY pressure cleans cause the very leaks the owner was trying to prevent.

Professional cleaning is about control, not maximum pressure. Yes, we run a 3,500 PSI Jetblaster, but the work is in the technique: fan tips not needle jets, pressure dialled to the tile's condition, always working with the water flow down the roof, and backing right off on older or previously coated surfaces. On fragile tiles we soft wash instead: biocide and detergent do the killing and lifting, and low pressure does the rinsing. The machine is the least important part of the job.

How we clean a tile roof properly

1. Inspect first: cracked tiles, loose ridge caps and fragile pointing get flagged (and ideally fixed) before any water hits the roof

2. Treat the growth: a roof biocide goes on and gets time to kill moss and lichen at the root, not just the visible tops

3. Wash with controlled pressure, top down, with the flow, keeping water out of the laps

4. Clear the mess: valleys, gutters and downpipes get cleaned of everything the wash sent their way

5. Re-treat where needed so regrowth on the shaded faces is slowed for years rather than months

That flagging step matters more than it sounds: water through a cracked tile during a clean goes straight into your ceiling, which is why cleaning and roof repairs belong together.

Can you clean your own roof tiles?

From the roof? Honestly, no, and it is not gatekeeping. Wet, mossy tiles are about the most slippery surface in home maintenance, old tiles crack underfoot if you do not know where to step, and a slip from even single-storey eaves height is life-changing. Roofers carry edge protection, harness points and the experience of thousands of hours of reading where a foot can go.

What you genuinely can do safely: apply a hose-end moss and lichen treatment from a ladder at the gutter line or from the ground on lower sections, keep gutters clear so the roof drains, and trim back overhanging branches that keep the tiles shaded and damp. That maintenance alone, done each autumn, keeps most roofs off the "needs a big clean" list. Our guide on gutter cleaning frequency pairs with this one.

Killing moss properly (not just knocking it off)

Blasting the green off without treating it is mowing a weed: it looks fixed and comes straight back, usually within one winter on the south face. The roots and spores sit in the tile pores and laps. A proper clean includes a biocide treatment that kills what you cannot see, which is why professionally cleaned roofs stay clean for years while blast-only cleans re-green in months. If a quote does not mention treatment, it is a blast-only clean, whatever it costs.

What roof cleaning costs in Melbourne

Market rates in Melbourne generally run per square metre, and steepness, height and condition move the price:

So an average single-storey concrete tile roof lands in the several-hundred-dollar range, more with treatment and gutter clearing included. We quote each roof cleaning free after seeing the roof, because condition changes the safe pressure and therefore the time on the job.

When cleaning is not enough

If the colour is chalking off the tiles, the surface is porous, or the roof re-greens within a year of every clean, the tile surface itself is gone, and no amount of washing brings it back. That roof does not need another clean, it needs re-sealing: a full restoration puts a new membrane surface on the tiles that sheds water and resists regrowth, and it starts with exactly the same proper clean anyway. Paying for a clean every two years on a roof that needs coating is the expensive way to avoid the real fix.