Algae on the facade: clean it or repaint it?

Why facades turn green and black

A greenish film usually comes from algae, grey to black patches from fungi or mould, often mixed with dust and soot. They settle wherever the surface stays damp for long periods: on north and west elevations, behind trees and hedges, below missing roof overhangs and near meadows or water.

Externally insulated facades (ETICS, known in Switzerland as WDVS) are particularly prone. Because the insulation keeps the heat inside, the thin render layer cools down sharply at night. Condensation forms and the surface dries slowly in the morning. This is not a construction defect in the strict sense but a physical consequence of good insulation.

Growth on a facade is first a cosmetic problem. It only becomes a renovation case once the render itself is damaged.

Surface or substrate: the decisive finding

Whether cleaning is enough depends not on the colour of the growth but on the condition of the substrate. A specialist should therefore inspect on site rather than from photos. Ask for the following points to be confirmed in the assessment:

The render is firm, load-bearing and does not chalk when you run a hand over it.
There is no spalling, no hollow areas and no open spots in the insulation system.
Cracks are at most fine hairline cracks and do not run through.
The growth sits on the surface, not within the building fabric.
There are no signs of moisture ingress, for example at window junctions or the base.

Finding	Typical recommendation
Render intact, growth on the surface	Gentle cleaning followed by a protective treatment
Render intact, some hairline cracks	Cleaning plus local crack repair
Render chalks or paint is peeling	Cleaning as preparation, then repainting
Spalling, hollow areas, moisture damage	Repair of the system; identify the cause first

How a gentle cleaning works

Assess the condition and clean a test patch in an inconspicuous spot.
Protect windows, plants, paving and drains.
Apply a cleaning agent suited to the substrate and let it work.
Rinse with regulated, low pressure and warm water.
Once dry, apply a protective treatment such as a water-repellent impregnation.
Inspect the result and record when the surface should be checked again.

Be careful with pressure washers

Excessive pressure roughens the render, can force water into the insulation system and creates exactly the surface on which algae cling even better later. Waste water matters too: check that the products and disposal comply with cantonal and municipal water protection rules.

Case study: cleaning versus repainting

A documented project in the Swiss Mittelland, described by the contractor GEKO GmbH, shows how large the difference can be. Eleven apartment buildings with a combined 12'500 m² of ETICS facade had algae and mould growth, but the mineral top render was intact. The work was done from the ground with telescopic lances reaching 15 m, without scaffolding or lifting platforms, rinsing with pressure-regulated hot water below 20 bar. A water-repellent treatment followed.

Option	Cost	Duration	Scope
Repainting (quoted)	CHF 920'000	28 weeks	Scaffolding, crack repair, two coats of silicone resin paint
Cleaning with protection (carried out)	CHF 248'000	3.5 weeks	Work from the ground, gentle rinse, water-repellent treatment

These figures apply to this one case and cannot be transferred to every building. They do show the pattern: much of the cost of repainting often lies in scaffolding, preparatory work and construction time rather than in the paint itself.

Common mistakes and when to call a specialist

Deciding on appearance rather than substance and asking straight away for a repainting quote.
Having paint applied over growth that has not been cleaned off.
Working with too much pressure and damaging the render structure.
Skipping protective measures and seeing the growth return within a few years.
Ignoring the causes, such as vegetation touching the facade or faulty gutters.

Doing it yourself is at most an option for small, easily reachable areas with a clear finding. As soon as height, area or the condition of the render are uncertain, you need experience with products, pressure and work safety. Ask contractors which products they use, what pressure they work with, whether a test patch is planned, how waste water is handled and what guarantee they give on the result.

For property managers

For larger properties, always obtain two options: cleaning with protection and repainting. Only a direct comparison of cost, construction time and disruption for tenants makes the decision robust.

Frequently asked questions

Can a heavily discoloured facade still just be cleaned?

Yes, provided the render is intact and the growth sits on the surface. The extent of discolouration says little about the condition of the substrate.

When is repainting still necessary?

When the render chalks, the paint peels, cracks run through or the insulation system is damaged. The problem then lies deeper, and cleaning is at most the preparation.

How long does the result last?

That depends on location, orientation, vegetation and the protective treatment. A water-repellent treatment slows recolonisation but does not prevent it permanently. Plan regular visual checks.

Is cleaning disruptive for residents and surroundings?

When the work is done from the ground, disruption is usually limited. Windows should stay closed during the work, plants must be covered and waste water disposed of correctly.

Does an algicidal paint prevent new growth?

It can delay recolonisation, but the active ingredients wash out over time. Structural measures such as cutting back vegetation and proper water drainage remain important.

The key rule is simple: have the substrate assessed first, then decide. Following that order avoids paying for work that is not yet technically necessary.