Green film on the north wall. Black streaks running down from the chajja. A patch near the water tank that keeps coming back no matter how many times it gets painted over.
None of that is a paint problem. It is a moisture problem that happens to have grown something.
Why Walls Grow Things
Algae, fungus and lichen need three things: moisture, low light or shade, and something organic to feed on. Indian exterior walls supply all three generously.
The organic food source is usually just settled dust, pollen and airborne pollution. It does not take much.
Remove any one of those three conditions and growth stops. In practice, moisture is the only one you can realistically control.
Algae vs Fungus vs Lichen
Algae is green, sometimes reddish or brown, and forms a thin film rather than a raised growth. It needs the most moisture of the three and appears first on walls that stay damp. Easiest to remove.
Fungus and mould present as black, dark grey or brown spotting, often speckled rather than uniform. It penetrates into the paint film and sometimes into the plaster, which is why surface washing alone rarely clears it permanently.
Lichen is a raised, crusty growth, usually grey-green or orange, that grips the surface hard. It takes years to establish and is the most stubborn to remove. Common on old boundary walls and parapets.
The Three Conditions Growth Needs
Worth being specific, because it tells you where to look.
Moisture can come from rain that does not dry, seepage from inside the wall, an overflowing tank, a leaking AC drain, or simply a wall that never sees sun. Shade comes from orientation, adjacent buildings, or trees. Nutrients come from dust, and the more polluted the air, the faster growth establishes.
Why It Always Starts on the North Side
North-facing walls in India get the least direct sun. They stay damp longer after every rainfall, sometimes for days.
Add a tree canopy or a neighbouring wall three feet away and the surface may never fully dry through the monsoon. That is ideal growth conditions for four straight months.
Same logic applies to the shaded side of a compound wall, the area behind a water tank, and the strip below a chajja that stays permanently wet.
How to Clean It Properly
Pressure washing alone is the most common mistake. It removes the visible growth and leaves the spores, so it returns within months, often stronger.
The correct sequence:
- Dry brush first. Remove loose growth with a stiff brush before introducing water. This stops you spreading spores in a wet slurry across the whole wall.
- Apply a biocidal wash. A dilute sodium hypochlorite solution, roughly one part household bleach to four parts water, or a proprietary fungicidal wash. Leave it on the surface for 15 to 20 minutes.
- Scrub and rinse. Work it in with a brush, then rinse thoroughly with clean water.
- Repeat on stubborn areas. Lichen usually needs two or three applications.
- Let it dry completely. Minimum three to four clear days before any coating.
Wear gloves and eye protection. Never mix bleach with acid-based cleaners, which produces chlorine gas.
Treatment Before Repainting
After cleaning and drying, apply a fungicidal treatment solution and let it soak in. This kills residual spores in the plaster that surface washing did not reach.
Then fix the moisture source. Trim back the tree branches. Repair the leaking AC drain. Seal the crack that is feeding water into the wall. If you skip this, the growth returns regardless of what paint goes on.
Only then prime and paint.
Choosing a Paint With Biocide Protection
Most premium exterior emulsions in India include fungicidal and algaecidal additives in the formulation. These are consumed slowly over the paint’s life, which is one reason protection weakens in later years.
Two things help beyond the additive package. A smoother, less porous film gives spores less to grip and holds less water. And lighter, more reflective shades dry faster after rain, which shortens the wet window growth needs.
Check that the product specifically states anti-fungal or anti-algal properties. Not every exterior paint includes them, and the economy grades usually do not.
Preventing Regrowth
- Trim vegetation back to at least 600 mm from the wall face
- Fix any leaking tank, drain or AC condensate line
- Ensure chajja and sill tops drain properly rather than ponding
- Wash affected elevations once a year, ideally in October
- Accept that north walls will always need more attention than south walls
What This Actually Costs
Biocidal wash material for a typical house exterior runs Rs 800 to Rs 2,500. Labour for cleaning and treatment adds Rs 4 to Rs 9 per square foot depending on severity and access.
Against that, the cost of skipping it is a repaint that fails within two seasons, because growth continues under the new film and pushes it off. Treatment is one of the cheapest steps in the whole sequence and one of the most frequently omitted.
Frequently Asked Questions
How do I remove algae from exterior walls?
Dry brush the loose growth first, apply a biocidal wash such as dilute sodium hypochlorite, leave it 15 to 20 minutes, scrub and rinse thoroughly, then allow three to four dry days before repainting. Pressure washing alone leaves spores behind.
What causes green patches on walls?
Algae grows where moisture, shade and settled organic dust occur together. North-facing walls, tree-shaded surfaces and areas near leaking tanks or drains are affected first because they stay damp longest after rain.
Does anti-fungal paint work?
Yes, but only as part of the solution. Fungicidal additives suppress growth for several years and deplete over time. If the underlying moisture source is not fixed, growth will return regardless of the paint used.
Why does only one wall get fungus?
That wall stays wet longer than the others, usually because it faces north, sits in permanent shade, or receives water from a leak, overflow or blocked drainage detail nearby.
Can I paint over mould?
No. Paint applied over mould bonds to the growth rather than the wall and will fail, while the mould continues developing underneath. It must be killed with a biocidal wash and the surface dried fully first.
