What's Actually Growing on Your Wall
Not every dark stain is the same organism, and each one responds differently to treatment:
| Growth | What It Looks Like | Where You'll Find It |
|---|---|---|
| Green algae | Green film or streaks, slimy when wet | Wall bases, near irrigation, shaded faces |
| Black mould | Black or dark grey patches and speckles | South-facing walls, under trees, damp corners |
| Lichen | Crusty grey, white, or orange circles | Capping and tops of panels, long-neglected walls |
| Moss | Soft green cushions | Panel joints, cracks, and ground level |
Why It Comes Back After Pressure Washing
Algae, mould, and lichen don't just sit on the surface — they root into the pores of the concrete. A high-pressure wash without pre-treatment removes the visible growth but leaves the root structure alive inside the surface. With the first good summer rain, it grows straight back.
Worse, blasting at high pressure to "get it all" opens up the concrete surface, giving the next round of growth more places to anchor.
Bleach sprayed straight onto a wall can lighten the stain without killing the root system, and it damages plants and lawn along the base of the wall. Visible doesn't mean dead.
The Right Removal Method
1. Identify the Growth
We identify what's on the wall and how deep it's established. Lichen in particular needs a longer dwell time than algae.
2. Apply a Biodegradable Biocide
The treatment is applied to the affected areas and left to penetrate and kill the organism down to the root.
3. Low-to-Moderate Pressure Wash
Once the growth is dead, it releases from the surface easily. The wash becomes a flushing operation rather than a scrubbing one, using far less pressure.
4. Rinse and Inspect
We rinse thoroughly and check joints, the base, and the capping, where growth likes to hide.
This is the core of our precast wall cleaning service — chemistry first, pressure second.
Slowing Regrowth
- Redirect irrigation: sprinklers hitting the wall feed algae every day
- Trim overhanging trees: more sunlight and airflow means the wall dries faster after rain
- Clear the base: soil and mulch piled against the wall hold moisture at the panel
- Clean on schedule: light growth is quicker and cheaper to remove than an established colony
Look at the base of your south-facing wall after rain. A green tinge in the bottom 30 cm is the earliest sign that growth is establishing.
Frequently Asked Questions
Is black mould on outside walls harmful?
Outdoor mould on a boundary wall isn't usually a health concern in the way indoor mould can be, but it does hold moisture against the concrete, spreads to nearby surfaces, and becomes harder to remove over time.
Are your algae treatments safe for my garden?
We use biodegradable treatments and pre-wet planting along the wall before application. Pets should be kept indoors until the wall has been rinsed.
How long before algae comes back?
After a properly pre-treated clean, most walls stay clear for 12 to 24 months. Heavily shaded or constantly wet walls will see regrowth sooner, which is why we look at irrigation and tree cover too.
The Best Time to Treat
In Gauteng's summer rainfall climate, growth explodes from October to March. Treating your walls in early spring — before the rains arrive — removes the base colony and slows the summer bloom. We cover seasonal timing in more detail in preparing your walls for spring rain.
Algae or mould spreading across your walls? Book a free assessment before the rainy season sets in.