Double Bay sits in a natural basin, with ridges rising on three sides and the harbour on the fourth. Cold, damp air settles in low ground, shaded streets and courtyards get little direct sun, and humid summers keep surfaces wet for long periods.
The result is a suburb where mould, algae and moss are a constant maintenance item, not a once-a-decade problem.
What the data says
The Bureau of Meteorology’s long-term Sydney record shows average 9am humidity of 74 percent from February to June, and March has the most rain days of any month. Afternoons are drier, but shaded surfaces in a basin dry slowly even then.
Where growth appears
South-facing walls that never get sun go green first, then black. Courtyards enclosed by walls or neighbouring buildings hold damp air. Paths and steps in shade grow algae and moss and become slippery. Under eaves and balconies where air does not move. Roofs on the downhill side of the ridges, and anything under trees.
Green is not dirt
The green film on walls is algae, a living organism growing on the surface. Washing it off with water alone removes the visible layer and leaves the roots, and it comes back within months. More: the green film on walls is not dirt.
The effective method is soft washing: a biocide applied at low pressure with dwell time, which kills the organism. It then releases and weathers off. Regrowth takes far longer. More: what soft washing is.
What not to do
High pressure on render, paint, timber or old brick damages the surface and opens it to faster regrowth. Bleach mixed at home and sprayed with a garden sprayer burns plants, runs into drains, and often leaves streaks. More: removing mould and algae from render without damage.
What cleaning cannot fix
If a wall is damp from inside, from rising damp, a leaking gutter, a failed balcony or a garden bed against the wall, it will regrow quickly no matter how it is cleaned. Fixing the water source does more than cleaning ever will. Look for: growth that is worst at the base of the wall, staining under gutters and downpipes, and walls that stay dark after dry weather. More: rising damp.
Indoors
The same humidity drives mould on bathroom ceilings and in poorly ventilated rooms. That is a ventilation problem, not an exterior cleaning one. More: mould on bathroom ceilings.
Prevention
Prune vegetation away from walls to let air and light in. Keep gutters and downpipes clear. Keep garden beds and mulch below damp-proof courses and weep holes. Clean shaded elevations more often than sunny ones.
See the Double Bay maintenance calendar and our mould removal service.
