Why Algae Grows on render

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  • Post published:November 14, 2016
  • Post category:More Information
  • Post last modified:December 3, 2022

Introduction to why Algae grows on render.

Algae is a natural phenomenon that grows on all external surfaces. In recent years Algae on render  has become more prevalent on buildings due to:

• Clean Air Act: Reduced air pollution, this has resulted  in increase of airborne algae spores

• The way the Climate is Changing
• Excessive  wet weather coupled with a very much milder climate
• “Bad”  building design.

In the construction industry, the term Algae on render is frequently used to describe the occurrence of various organic growths which are commonly found on buildings. These organic growths include a range of algae, moss, lichens and fungi. All of which can be found on any external surface.

image of Algae on render www.cleaning-service.uk.com

How do they colonise surfaces?

In our climate algae only requires high moisture levels or high humidity for growth to occur. The colony is usually spread by the wind, with the initial spores being blown onto the building surfaces, as air borne dust. The damper northern sides of buildings are usually those worst affected; however once established, some can flourish on drier more southern surfaces.

The Algae on render usually occurs where there is frequent wetting of the building surface. This may be caused by maintenance faults (leaking gutters) or design flaws (bad architecture)  which result in poor water shedding or repeated exposure to rain. In many cases , localised growths occur at leaking water pipes or gutters.

Generally  , these Algae on render growths  are unlikely to cause  major structural damage, with their principal effect being  to alter the aesthetic qualities of a building due to unsightly staining of the surface.

Treatment and removal of Algae on render.

Regular maintenance of buildings can help minimise infection in the first instance and reduce growth.
Once established, the best way to treat and remove any growth is to use a surface biocide or algaecide (available from any good builder’s merchant or hardware store). There are a wide variety of such chemicals available; therefore it is important to follow the manufacturer’s instructions closely during use.

Most surface biocides/algaecides will require a few days to be fully effective and are best used during a dry spell to avoid rain diluting the chemical before it has time to act. After treatment is complete the dead growth can be left to weather away, however ,we would recommend a light brushing or mild power wash to remove any residue. In some cases, repeated treatments may be necessary.

Although there is no evidence to suggest that surface biocides/algaecides can damage render when used correctly, it is advisable to carry out a trial in an obscure area to gauge effectiveness and to check the chemical does not damage

the surface.

Typical Algae Varieties  taken from a study by


The following explains the cause and treatment three most common types of algae, all three can appear on any building material and are particularly prevalent on damp surfaces of high humidity.

Green Algae

This is the most common type of algae and can generally be removed with gentle jet washing.

Red Algae

This is becoming more common as the climate gets milder and damper. It is already common in Northern Ireland & Scotland and is on the increase in England. Red algae is more difficult to remove than green algae and will require a good algaecide/biocide for complete removal.

Black fungi

Another common type of growth; Chemoorganotrophic fungi is very stubborn to remove and may require more than one treatment of algaecide.

To summarise

Power washing will generally remove the green and reduce the red but a stronger treatment will most likely be required for the black (Chemoorganotrophic fungi). The problem is that algae will typically return fairly quickly unless an algaecide/biocide with a residual effect is used.

All algae can be removed with algaecide; the black   however will probably require more than one treatment.

There are a number of brands of algaecide available; We would recommend using one that has a continuing residual effect to prevent re-growth. As with all products of this type we recommend a small area is trialled following manufacturer’s instructions to confirm there are no adverse effects.

A good quality algaecide will give a significant period of protection dependent on local conditions.

We are recommended by K-rend to clean their render. Using us to clean your render  will not invalidate your k-rend warranty.

If you would like more information please call 08000933267 for free advice.