Green algae on render and brick: causes, removal and prevention

A facade that turns green is hard to miss, especially on pale render. It is often less serious than it looks, but it does show that the wall stays damp for long periods. Below you can read where green growth comes from, whether it damages the wall, and how to remove and prevent it.

What is green growth on a facade?

It is a layer of living organisms, mostly algae, sometimes together with cyanobacteria, lichens or moss. Together they often form a biofilm on the stone or render.

Historic Environment Scotland tells them apart by sight (Historic Environment Scotland). Algae form green films, patches or streaks without defined borders, and need moisture and fairly high light levels. Cyanobacteria look similar but also grow in shade. Lichens are crusts or leaf-like forms with sharp borders that grow slowly. Moss grows in small clumps, mainly in cracks, joints and on wet slopes such as roofs.

Why does one facade grow green algae faster than another?

Because algae and moss need water. A facade that stays wet for long periods, because of its position, its material or a defect in the building, turns green faster.

Rough, porous materials. According to Historic Environment Scotland, organisms attach more easily to rough, porous surfaces, which hold moisture. Hard stone with low porosity, such as granite, is generally more resistant.

Building defects. Blocked gutters, damaged pointing and other drainage problems keep masonry wet. Historic Environment Scotland points out that blocked drainage can raise moisture levels enough for algae or moss to establish where they would not otherwise grow.

Shade from vegetation. Trees and shrubs close to the facade keep sun and wind away, so it dries more slowly.

External wall insulation with render (ETICS). A well-insulated wall loses less heat from inside. Fraunhofer IBP explains that this raises the likelihood of condensation on the outer surface, because the surface cools at night through long-wave radiation, and that a film of moisture on the surface is all micro-organisms need to start growing (Fraunhofer IBP). Buildwise, the Belgian building research centre, reports that biological soiling on ETICS render (algae, cyanobacteria or fungi) sometimes appears shortly after installation, and that the usually pale colour of the render makes it more visible (Buildwise).

Does green growth damage the facade?

Mostly not. According to Historic Environment Scotland, most growth on masonry is harmless and mainly a matter of appearance, but some species hold moisture or attack the stone.

Algae generally stay on the surface, although some species produce slime that expands and contracts as it wets and dries, which can cause the stone to flake. Moss holds a lot of water and can keep the wall wet enough to speed up stone decay. Woody plants can open up joints and dislodge stones. Heavy growth often points to another problem, such as a gutter that does not work properly or damaged pointing.

How do you go about removing algae from a facade?

As gently as possible: dry brushing and a low-pressure rinse, or a product suited to the surface followed by a thorough rinse. Always test the method on a small area first.

Historic Environment Scotland advises lightly scraping or brushing the growth in dry conditions with a natural bristle brush, then gently washing down. Wire brushes damage the stone. The US National Park Service says that washing with low-pressure water and a natural or synthetic bristle brush can be very effective if done regularly (National Park Service).

For ETICS render, Buildwise says the treatment depends on the colour and appearance of the soiling: for green growth a (bio)chemical treatment product is often enough, while dark soiling usually needs a prior clean with water under pressure (Buildwise). Enzymatic products break down organic dirt; how that works is explained in enzymatic facade cleaning. How a product is left to work and then gently rinsed off is covered in what is softwash.

A product that kills algae or moss is a biocide. According to the European Commission, a biocidal product cannot be placed on the market or used unless it contains approved active substances and has itself been authorised (European Commission). The Cultural Heritage Agency of the Netherlands adds that the dead remains must then be removed with a water-based cleaning method (Rijksdienst voor het Cultureel Erfgoed). Heritage bodies advise against bleach, partly because it can leave salts in the stone; the reasons are set out in cleaning a facade with bleach.

How do you prevent green growth on a facade?

By keeping the wall drier. Dealing with the source of the moisture limits new growth, and according to Historic Environment Scotland the existing growth may then even die off.

Have gutters, downpipes and pointing checked, and repair whatever leaks or is loose. Cut back vegetation that shades the facade, so it dries faster. Plan regular cleaning as well: the National Park Service describes scheduled cleaning with low-pressure water and a natural or synthetic bristle brush as the traditional way of keeping biological growth at bay.

On water-repellent treatments, the sources are cautious. The National Park Service does not recommend a water-repellent coating on historic masonry as a way to prevent biological growth, and writes that some coatings may even encourage it. The Cultural Heritage Agency of the Netherlands considers the risks of water-repellent treatment on historic buildings very high and advises against it for monuments in most cases. For other facades it depends on the surface and the product; seek advice before deciding.

Frequently asked questions