Home → Exterior Cleaning Advice → Why Houses Go Green

Why Toowoomba houses go green faster than Brisbane houses

That green film is alive

The first thing worth understanding is that what is on your wall is not dirt. Dirt washes off. What sits on a rendered or painted wall in Toowoomba is a living colony — usually green algae, often black mould, and on older surfaces a lichen, which is a fungus and an alga living together as one organism.

That distinction is the whole reason a hose does nothing. You are not trying to shift a deposit sitting on top; you are trying to kill something that has a grip on the surface and, in the case of lichen, has pushed into it. Blast the visible growth off with high pressure and the root structure stays behind, which is why a pressure-washed wall greens up again within a season while a properly treated one stays clean for years.

What 700 metres actually does

Three things change on the way up the Range. Nights are colder, so the air hits its dew point earlier in the evening and stays there longer into the morning. Fog rolls across the top regularly rather than occasionally. And the cool, moist air sits rather than moving on, because the escarpment holds it.

The practical result is a wall that is damp from about nine at night until mid-morning, most nights, for most of the year. In Brisbane that same wall dries within an hour or two of sunrise. Algae does not need much — it needs surface moisture and low light — and a Toowoomba wall gives it eight or ten hours of both, daily. That is the entire explanation.

It gets more pronounced with height. Highfields, Cabarlah and Hampton sit higher and colder again, with real frosts and longer dew than the city. Houses up there green up noticeably faster than the same house would in Gatton or Withcott down off the range.

Why the south wall always goes first

Walk around almost any Toowoomba house and the pattern is the same: the southern and south-eastern walls are green, the northern wall is comparatively clean. Southern elevations in the southern hemisphere get the least direct sun, so they stay damp longest and dry slowest. Add a shade tree or a neighbouring fence line and you have a wall that barely sees direct sun from April to August.

The same logic explains the rest of the property. The shaded run of path down the side of the house goes slippery before the open driveway does. The roof greens on the southern pitch first. The fence greens on the side facing away from the sun. If you want to know how long a surface has been damp, look at how green it is.

What actually works on it

• A soft wash, not a pressure wash — a treatment applied at roughly garden-hose pressure that kills the organism at the root, then a gentle rinse. On render, paint, weatherboard, Colorbond and roof tiles this is the only method that both works and leaves the surface intact.
• High pressure only where the surface can take it — concrete, pavers, brickwork and hard stands. Never render, never paint, never a roof.
• Treating the whole elevation, not the visible patch — the colony extends beyond what your eye picks up. Cleaning only the obvious green leaves a tide line and a head start for next year.
• Wetting down plants first and rinsing after — garden beds along the wall line get pre-wet before anything is applied and rinsed again at the end.
• Getting the airflow back if you can — a shrub pressed against a south wall keeps it permanently damp. Pulling planting back half a metre from the wall does more for the next three years than any product.

What does not work

Hosing it, scrubbing it with sugar soap, or hiring a pressure washer for the weekend. The first two do not kill anything, and the third damages the surface while leaving the roots behind — high pressure on render drives water into the wall, strips the surface off paint, and on a tile roof it takes the glaze with it. A roof that has been pressure-blasted will grow moss faster afterwards than it did before, because the surface is now rougher and holds more water.

So how often does it need doing?

For most Toowoomba homes, a house wash every two to three years keeps it looking right, and spring is the best window — before the summer humidity, after the wet winter. Heavily shaded houses and anything under dense tree cover run closer to two years. A well-exposed northern-aspect house in full sun can stretch to four.

Roofs run longer, because a proper soft-wash roof clean kills the lichen right through: four to six years on tile, longer on Colorbond. House washing starts from $495 and roof cleaning from $880, and doing both in one visit costs less than two separate trips — which also gets the order right, since the roof has to come first.