Which Side of a Melbourne House Goes Green First, and What It Tells You

Walk around almost any house in Melbourne after winter and the green growth is lopsided. One wall has it, the opposite wall does not. The path down one side of the house is slippery and the path down the other is fine. People often read this as a sign that one part of the house is dirtier than the rest, and then wonder why the same side needs doing again a year later.

It is not about dirt. Green growth follows a short list of conditions — shade, moisture and still air — and those conditions are fixed by the way the house sits on its block. Once you can read the pattern, you know where to look each spring and, more usefully, what is feeding the growth before anyone cleans it.

Why the south side goes first

In Melbourne the sun tracks across the northern sky, so a south-facing wall gets little or no direct sun for most of the year and almost none through winter. A wall that is never warmed by the sun dries slowly, and a surface that stays damp for days at a time is exactly what algae and moss need to establish. The north side of the same house, hit by sun for hours most days, dries out between rain and never gives growth the chance.

This is why the pattern is so consistent from house to house. It is not the render or the paint or the age of the building. It is which way the wall faces, and that does not change. The practical consequence is that the south side of a house sits on a different cleaning rhythm to the north side, and treating them as one job on one schedule means either doing the north side unnecessarily or letting the south side go too long.

Shade is shade, whichever way the wall faces

Orientation is only the first layer. Anything that casts permanent shade creates a south-side condition wherever it happens to be. A large tree next to a west-facing wall, a tall fence close to the house, a neighbouring double-storey build, a carport or pergola roof, a deep verandah — all of these produce a strip of surface that never sees sun and behaves exactly like a south wall.

Trees add a second effect on top of the shade. Leaves drip long after the rain has stopped, so the surface beneath a canopy is wetted for longer than the surrounding wall or path, and the leaf litter that lands there holds moisture against the surface as well. The strip of path directly under an overhanging branch is very often the greenest, slipperiest part of the whole yard, even on the sunny side of the house.

The narrow side passages that so many Melbourne blocks have are the worst case of all: two walls close together, a fence on one side, a wall on the other, little sun from any direction and almost no air movement. Those paths stay green nearly year round and are the ones people slip on.

Where the water has been going all winter

The third condition is the one worth paying real attention to, because it is the one that can be changed. A great deal of green growth is not caused by shade at all but by a surface being deliberately wetted, over and over, by the house itself.

Look at where the growth is thickest and follow it upward. A green streak down a wall often starts at a gutter joint that has been dripping, or at a section of gutter that overflows in heavy rain and runs down the wall face. A patch of green on paving usually sits below a downpipe that discharges next to the path instead of into stormwater, or under a downpipe that has come apart at a joint. A stained, green band along the base of a wall can be splash-back from a path or a garden bed that slopes toward the house.

The reason this matters is that cleaning these areas without fixing the water is a short-term result. The wall goes green again quickly because it is still being wetted every time it rains. Looking at gutters and downpipes at the same time as the walls is the difference between a clean that lasts and one that does not.

Reading the pattern surface by surface

Different surfaces show different things, and each one says something about what is going on.

• Render and painted masonry show a thin green or grey-green film first, usually as a haze rather than a defined patch. It spreads across the whole shaded face and is darkest where water has been running.
• Weatherboard greens along the lower boards and the underside edges of each board, where water sits, and around any board that has lost its paint.
• Brick grows in the mortar joints before the brick face, because the mortar is more porous and holds water longer. Green joints on an otherwise clean wall are an early sign.
• Paving and concrete go green in the joints and the low points first, then across the whole surface in permanent shade. The slip risk starts before the colour is obvious.
• Timber decking and steps hold the most moisture of all and get slippery earliest, particularly on the shaded end and around the fixings.
• Colorbond fences and metal rarely grow much themselves but show drip lines from whatever is above them, which is a useful clue to a leaking gutter or a dripping tree.

Why it shows up in spring

The growth has been there since about June. What changes in September is that people are outside, the light is better, the garden is being tidied and the house is being opened up after months of coming and going in the dark. Growth that was invisible in July is obvious once the days are longer.

Spring is also a fair time to deal with it. The surfaces have been wet for months and the growth is at its thickest, so a clean now removes a full winter of build-up in one go. It is also the point in the year where the surface will actually dry afterwards, which is what stops it re-establishing straight away. Cleaning a south wall in the depths of winter is possible but it goes back green quickly because the drying that would normally follow never happens.

What to do with what you find

The useful first step is a walk around the house with the pattern in mind. Note which faces are green, which sections are worse than their neighbours, and whether the worst patches line up with a gutter joint, a downpipe, a branch or a fence. That five minutes tells you whether the job is a straightforward seasonal clean or a clean plus a drainage fix.

On plain concrete a careful wash is a reasonable home job. On render, painted timber, softer brick or old mortar it is worth going gently — a domestic pressure washer at close range can strip coatings and force water into a wall, which is how a cosmetic problem becomes a repair. A soft wash that treats the growth and then rinses, rather than blasting it off, is kinder to those surfaces and tends to keep them clean longer. That is the approach a roof and exterior house clean takes across the whole building, including the sections nobody can reach from the ground.

And once the house is clean, the pattern is still worth remembering. The south side and the shaded passage will need looking at again before the north face does. Working with that rhythm rather than against it is what keeps the whole house looking even.

Frequently asked questions

Is the green stuff on my walls mould, moss or algae?

Usually more than one thing at once. The thin green film that appears on render, weatherboard and paving is generally algae, which only needs light, moisture and a surface. Moss is the thicker cushioned growth that needs somewhere to root, so it favours mortar joints, cracks and the gaps between pavers. Lichen is the crusty grey-green patches that grip hard and come off last. Black spotting in a sheltered corner is more likely mould. They all point at the same thing, though, which is a surface that stays damp for long enough for something to establish.

Will it come back after cleaning?

It will come back faster on the same side than on the others, because cleaning removes the growth but not the conditions. What extends the gap is dealing with what was keeping the surface wet: a gutter that overflows onto that wall, a downpipe discharging next to the path, a tree branch holding shade and drip over one section, or a fence line that stops the wall ever drying. Where the cause is simply that the wall faces south, the answer is a cleaning rhythm for that side rather than a one-off.

Is a green path actually a problem or just ugly?

The slip risk is the real issue. An algae film on paving, concrete or timber decking is far more slippery wet than the same surface clean, and it stays wet longer because the film holds water. Paths in permanent shade at the side of the house, the shaded end of a driveway and steps under a tree are the usual places people slip, and it happens in the mornings when the surface has not dried. On walls the concern is different: growth holds moisture against render and timber, which is what eventually damages the coating underneath.

Should I pressure wash it myself?

On plain concrete, probably fine with care. On render, painted weatherboard, softer brick or anything with old mortar, a domestic pressure washer at close range can strip the coating, scar the surface or drive water into the wall, which makes the growth worse next year. The safer method on those surfaces is a low-pressure wash with a treatment that kills the growth, followed by a rinse, and letting the surface dry before judging the result. If in doubt about the surface, that is worth asking about before pointing a pressure washer at it.