Pressure Cleaning in Sydney: Roofs, Moss and Lichen

Three different things grow on a Sydney roof, and they do not come off the same way.

Algae is the black or dark green staining that runs down a roof in streaks. It is the earliest stage. It feeds on moisture and on the mineral dust that settles on the surface, and it sits on top of the tile rather than in it. Algae is the easiest to remove and the fastest to come back.

Moss is the green cushion that builds up along the laps, in the valleys and around the ridge. Moss holds water against the roof long after the rain has stopped, and that is the real damage. Constant moisture softens bedding and pointing, keeps the underside of the tiles wet, and in a shaded gutter it turns into a sponge that stops water reaching the downpipe.

Lichen is the pale grey, green or orange crust that looks painted on. It is not sitting on the surface, it is anchored into it. On a terracotta tile that has lost its glaze, lichen grows down into the pores. You cannot blast lichen off without taking the top of the tile with it, which is why a roof that was cleaned hard once often looks worse two years later.

Pressure removes what you can see. It does not remove what grows back.

Moss and lichen reproduce from spores that sit in the pointing, under the laps and in the surface of the tile itself. Washing off the visible growth leaves the spores where they are, so the roof looks clean for a season and then returns, often faster than before, because the surface is now rougher and holds more water.

The other half of the problem is the damage. On terracotta, high pressure strips the fired glaze and leaves a porous tile that takes up more water and grows more lichen. On old mortar it blows the bedding and pointing out of the ridges and hips. On Colorbond it drives water up under the laps and into the screw holes. Under tiles, forced water gets past the sarking and into the ceiling.

So we work the other way around. Treat first, so the growth dies at the root. Then use the lowest pressure that actually lifts it off. On most tiled roofs that is a soft wash, not a blast.

Terracotta tiles. Older terracotta has usually lost some of its glaze. It is porous, lichen gets into it, and it chips at the edges. These roofs get the gentlest treatment and the most patience. If the pointing is already soft, cleaning will show that up, and it is better to find out before the next storm than after it.

Concrete tiles. Concrete holds moss well, because the surface is slightly rough from new. It takes more pressure than terracotta, but the colour coat on an older concrete tile is often chalky and will come away with the moss. That is not a cleaning failure. It is a roof that is due for a coating.

Colorbond and metal. Metal does not grow lichen the way tile does, but it gets algae on the shaded side and salt film near the coast. The risk here is water direction, not surface damage: anything sprayed up a lap or into a screw line ends up inside. Metal roofs get washed down the sheet, never up it.

It depends on what is over the roof and which way it faces, not on the calendar.

• A north-facing roof with nothing above it can go a long time, and may never need more than a gutter clean.
• A south-facing roof, or one shaded by trees or a neighbouring building, stays damp after rain. That is where moss starts.
• Under eucalypts, brush box or figs you are dealing with constant leaf drop and the tannin staining that comes with it.
• Within a few streets of the water, salt film and algae build up together on the shaded side.

Sydney gets its rain in heavy bursts rather than evenly through the year, which is why growth appears quickly after a wet run and then sits there through the dry months. A roof in shade and under trees usually wants looking at every couple of years. One in the open can go much longer.