If you've cleaned your rendered walls and the black streaking was back within a year, nothing went wrong with the cleaning — the problem is that cleaning alone doesn't address what's growing there.
It's alive
The black streaks under eaves and the green film on southern walls are mould, mildew and gloeocapsa algae. They have root structures and they release spores. Remove the visible growth with pressure and you've cut the grass — the roots are still there and the spores are now spread across the rest of the wall.
Why Sydney render is a target
Render is slightly textured and mildly porous, which gives spores somewhere to sit and moisture somewhere to linger. Add Sydney humidity, mild winters, tree canopy and long shaded elevations and you have year-round growing conditions.
North-facing walls that get sun usually stay reasonably clear. South-facing walls, enclosed courtyards and anything under a dense canopy go first.
Treatment, then removal
A soft wash applies a biodegradable biocide at low pressure and gives it time to kill the organism through its full depth. Only then is the dead material rinsed away. The wall looks the same the day after either method — the difference shows up 12 months later.
Reducing regrowth
- Trim vegetation back from walls to improve airflow and light
- Fix leaking gutters, downpipes and overflowing valleys
- Redirect sprinklers that spray directly onto walls
- Clear debris from the base of walls that holds moisture against them
- Schedule a soft wash every 18–24 months rather than waiting for visible staining
Need this done?
Free quote, fixed price, no obligation
We cover 183 Sydney suburbs. Send photos and we'll usually quote without a site visit.
