Moss and Lichen on a Melbourne Roof: Why It Grows and How It Is Removed

Come out of a Melbourne winter and look at the roof from the street, and the difference between one slope and another is often obvious. One side looks like it always has. The other has gone green, or is carrying patches of grey and pale orange crust that were not there in March. Very few homeowners notice this happening — they notice it has happened, usually at the end of August.

The growth is not a sign the roof is failing. But it is a sign the roof is staying wet for longer than it should, and it is worth dealing with before spring warmth accelerates it.

Why growth shows up at the end of winter

Moss needs three things: moisture, shade and something to hold onto. A Melbourne winter supplies all three generously. Rain arrives regularly enough that surfaces never fully dry out, the sun sits low enough that southern slopes get very little direct light for months, and the textured surface of a concrete roof tile is an excellent anchor — particularly once the factory coating has weathered and the surface has become slightly porous.

That is why growth is so rarely even across a roof. It concentrates on the shaded southern side, under and around overhanging trees, along the lower courses where water sits longest, and in the valleys where two roof planes meet and debris collects. If your roof is green on one side and clean on the other, nothing is wrong with the tiles on the green side. That side simply never gets the chance to dry.

Moss, lichen and algae are not the same problem

The three get lumped together, but they behave differently and matter differently.

• Algae is the flat black or dark green staining, often in streaks running down from the ridge. It sits on the surface and is mostly a cosmetic issue, though it does darken the roof and hold a little heat and moisture.
• Moss is the soft, spongy green growth with visible depth to it. This is the one that causes real problems, because it behaves like a sponge — it soaks up water and holds it against the tile long after the rain has stopped.
• Lichen is the flat, crusty grey, white or orange growth that looks almost painted on. It is the most stubborn of the three. Lichen bonds chemically to the tile surface and grows into the pores, which is why scraping it off often takes some of the tile surface with it.

Most Melbourne roofs that have gone untreated for a few years carry all three at once, in different places.

What growth actually does to a tiled roof

The damage is slow and indirect, which is why it is easy to ignore. Moss keeps the tile permanently damp, and a tile that never dries deteriorates faster than one that does — the surface coating breaks down, the tile becomes more porous, and more porous tiles hold more water, which grows more moss. It is a loop that tightens on itself.

Moss also grows into gaps. It works into the overlap between tiles and into mortar joints along the ridge capping, and as it swells and dries repeatedly it works those joints looser. On an older roof with already-tired pointing, that acceleration is meaningful.

Then there is the gutter problem, which is usually the one that gets a homeowner’s attention first. Moss does not stay on the roof. Every heavy rain and every windy day breaks material loose and washes it down into the gutters, where it packs into a dense wet sludge that ordinary leaf debris does not form. It blocks outlets, holds water in the gutter line, and shows up as overflow during the first real spring storm. A roof carrying heavy growth will re-block its own gutters within months of them being cleared — which is why gutter cleaning and roof treatment are often better done together than separately.

How professional removal works

The instinct is to get up there and blast it off. That instinct is wrong, and the reasons are worth understanding before hiring anyone — including how they answer this question.

Growth is removed in stages, and the treatment does most of the work rather than the water. The roof is inspected first, because a roof with cracked or slipped tiles needs those addressed before anything wet goes near it. Debris and gutters are cleared. A treatment is applied that kills the growth including the root structure, and then it is given time to work — this is the step that gets skipped by anyone in a hurry, and skipping it is what makes growth return within a season. Only then is the roof rinsed at low pressure, working down the slope so water runs the way the tiles are designed to shed it.

Dead lichen and moss continue to break down and wash off over the following weeks. A roof does not always look finished on the day, and any honest operator will tell you that up front rather than over-promising an instant result.

Why a pressure washer is the wrong tool for most roofs

High pressure removes growth quickly, and it costs more than it saves.

Concrete tiles carry a factory-applied surface coating. High pressure strips it, leaving a rougher, more porous surface — which is a better home for the next generation of moss than the roof you started with. Roofs cleaned this way tend to regrow faster and heavier than they did before.

Water direction is the bigger issue. Roof tiles shed water in one direction, down the slope, relying on the overlap between courses. A pressure washer aimed up the slope drives water backwards under those laps and into the roof space, where it lands on ceiling insulation and framing. Ceiling damage from roof washing is nearly always this, and it may not show up for weeks. High pressure also lifts and cracks tiles, and blows out ridge pointing that was holding on adequately beforehand.

There are surfaces where pressure washing is the correct approach — driveways, paths, some masonry. A tiled roof is generally not one of them, and the distinction between pressure washing and soft washing is not marketing language. It is a different method for a different surface.

Keeping the roof clear once it is clean

Treatment buys time; it does not change the conditions that produced the growth. What extends the result is reducing shade and moisture. Cut back branches overhanging the roof, which removes shade and stops the constant supply of leaf litter that holds moisture and feeds growth. Keep the gutters and valleys clear so water leaves the roof quickly instead of banking up along the lower courses.

Then simply look at the roof from the street a couple of times a year, ideally at the end of winter. Growth caught early is a straightforward treatment. Growth left for several years is a longer job on a roof that has already lost surface coating — and by then some of the damage is not reversible by cleaning at all.

Frequently asked questions

Is moss on a roof actually a problem, or just cosmetic?

It starts cosmetic and does not stay that way. Moss holds water against the tile surface, and anything that keeps a roof permanently damp shortens the life of the tile and its coating. It also breaks loose in storms and washes into the gutters, where it forms the dense sludge behind most overflow problems.

Why does moss only grow on one side of the roof?

Because that side stays damp. South-facing slopes in Melbourne get the least direct sun through winter, so they dry slowest after rain and hold moisture for days. Overhanging trees produce the same effect on any aspect — shade plus retained moisture is the whole recipe.

Can moss be removed with a pressure washer?

It can be blasted off, but that is not the same as removing it. High pressure strips the factory coating from concrete tiles, drives water up under the laps and into the roof space, and leaves the root structure behind so the growth returns. Most tiled roofs need low pressure and a treatment that kills the growth instead.

How long does a roof stay clear after treatment?

That depends almost entirely on shade and tree cover. A roof in open sun stays clear far longer than one under a canopy, and keeping gutters clear and branches trimmed back does more for longevity than the treatment itself. It is maintenance, not a permanent fix.