Green Growth on a Facade: Effects on Render and When to Take Action

The north-facing wall of the building has turned green. In summer, it is particularly visible. The administrator keeps receiving calls from residents. The façade was ten years old and looked good — until it did not. Now the question arises: is this only a matter of appearance, or is there something more to it?

Biological growth on a façade is not just an aesthetic problem. If it is not removed at the right time, it begins to damage the render from the inside. Understanding how this process works helps you decide when to intervene, before the problem becomes expensive.

Where the Growth Comes From: Algae, Moss, and Fungi on Render

Green, grey, or black discoloration on a façade is the result of surface colonization by microorganisms. The most common are:

Algae (Chlorophyta, Cyanobacteria) are the first organisms to settle on a damp or shaded surface. They create a characteristic green or greenish-grey coating, especially visible on north-facing walls and near gutters. They need only moisture and diffused light to survive, so they are not particularly demanding.

Fungi and moulds colonize the façade after algae or independently, especially where the render has absorbed moisture. They form dark, black, or grey-brown stains. Fungi release enzymes that attack the binder in mineral render.

Mosses and lichens appear at a more advanced stage of biological colonization, especially around the plinth and in cracks. Moss rooting mechanically disrupts the render structure.

The conditions that promote biological growth are primarily surface moisture, caused by shading, leaking gutters, or poor wall ventilation; a rough render texture, which provides more anchoring points for microorganisms; and the absence or exhaustion of the biocidal coating in the façade paint.

What Happens to Render Without Intervention: The Degradation Mechanism

Biological growth rarely remains only on the surface. Its effect on render develops in stages.

Stage 1: surface colonization. Microorganisms colonize the outer layer of render or paint. At this stage, the damage is reversible, and chemical cleaning removes the growth without residue.

Stage 2: penetration into the structure. Fungi and algae produce organic acids that react with the binder in cement-lime or mineral render. Micropores begin to form. The render loses cohesion and starts absorbing more water with every rainfall.

Stage 3: repeated freezing and thawing. Water absorbed by the weakened render freezes in winter, expands, and bursts the binder from the inside. In spring, chips, flaking, and cracks begin to appear.

Stage 4: loss of insulating function. Damaged render no longer effectively protects the wall from water. Moisture penetrates into the insulation layer or directly into the load-bearing wall. Repair costs increase many times over compared with early intervention.

In a moderate climate, the transition from Stage 1 to Stage 3 usually takes several years. Under favorable conditions, such as a shaded wall, a leaking gutter, or the absence of biocide in the paint, this period may be shorter.

Rope Access Cleaning Methods

Removing biological growth from a façade requires a combination of methods selected according to the severity of the problem and the type of render.

Pressure washing, or hydrojet cleaning, is effective for fresh algal growth that has not penetrated deeply into the structure. Water pressure mechanically removes the layer of microorganisms from the surface. However, pressure that is too high can damage weakened render, so selecting the right parameters requires experience.

Chemical cleaning with a biodegradable algaecide is the basic method for advanced growth. The agent is applied to the surface, left to act for a specified time, from several minutes to several hours depending on the product, and then rinsed off. It destroys microorganisms in micropores and cracks that pressure washing alone cannot reach.

Application of a penetrating biocide is used in cases of deep biological colonization. Such agents penetrate more deeply and act on fungal mycelium embedded in the porous render structure. They are often used as a preliminary stage before pressure washing.

In every case, façade work above several meters requires access at height. Rope access makes it possible to reach every part of the building, from the cornice to the plinth, without occupying the area around the property.

How to Prevent the Growth from Returning

Removing the growth without protecting the surface means that the problem will return, often faster than the first time, because microorganisms now have ready-made shelter within the render.

Methods that reduce recurrence include the following.

A protective coating with biocide is a paint or impregnation product containing active biocidal substances, applied after cleaning. It slows down recolonization. Its service life depends on the manufacturer and wall exposure, but it usually lasts several years.

Repairing leaks is essential. A leaking gutter, damaged sheet-metal flashings, and missing window seals are the main sources of excessive wall moisture. Removing the cause is more important than applying biocide.

Improving wall ventilation is also important. In ETICS insulation systems, this is achieved through the correct selection of materials. In older buildings, it depends on efficient ventilation of the rooms.

Regular inspections should be carried out every two to three years, especially on north-facing and shaded walls. Early detection of the first signs of growth allows for intervention using an inexpensive method.

A practical inspection date for a wall with a biological problem is one year after cleaning and applying the protective coating. At that point, it is worth checking whether the growth has started to return. If it has, a review of the biocide manufacturer’s recommendations or a decision to change the protection system may be necessary.

When DIY Cleaning Is Not Enough

Façade cleaning agents available in stores and pressure washers from rental companies work well on small ground-floor surfaces. In multi-storey buildings, however, their use has several limitations.

Reach: walls above 3–4 meters are practically inaccessible without equipment for work at height.

Safety: working with a pressure washer from a ladder or non-professional scaffolding carries a risk of accident.

Effectiveness: without properly selected chemicals, surface washing does not kill microorganisms embedded in the render structure, so the problem returns faster.

If biological growth has appeared on the wall of a multi-storey building, it is worth commissioning an inspection from a specialist high-access company. Assessing the condition of the façade, selecting the method, and preparing a quote usually takes only several minutes on site and provides a basis for a rational decision.

Order a façade inspection. We will assess the severity of the problem, propose a method, and price the work without obligation.