Render comes in several formats from different suppliers. K-Rend, Wethertex, Weber or Monocouche and many more. They can be colour through or painted surfaces. They can stay clean for several years as the organic colonisation is hard to spot in the first few years. But as it tends to grow exponentially, you will usually see it make a fast appearance from about year 7 to 12 in our experience.
If your render has turned red and black, it is likely due to the growth of algae, moss, or other types of organic matter. The red colour is usually caused by a type of algae called Trentepohlia, which can grow on exterior surfaces such as render, walls, walkways and roofs.
The black colour is likely due to the growth of black mold, which thrives in damp and humid conditions. These types of growth can be caused by a variety of factors, including poor ventilation, high humidity, and insufficient sunlight exposure especially on elevations that suffer a lot of shade.
To remove the red and black growth from your render, you can use a combination of cleaning methods such as pressure washing, scrubbing with a brush, and using chemical cleaners. Be sure to follow the instructions on any cleaning products you use, and use appropriate protective gear like gloves and goggles. Great care should be taken if you try this with your domestic pressure washer as the surface can easily be damaged. A Cleaner Exterior Ltd use bespoke built professional pressure washers that can generate super heated steam which reduces the need for pressure. thus reducing the risk of damage to the surface. Pressure washing is only part of the cleaning process. There are a variety of chemical cleaning solutions to choose from based upon the actual biological conditions present on the wall. Pressure washing alone will not clean the wall. A combination of super heated steam and chemicals will clean the render.
To inhibit the growth from returning consider using a biocidal wash to help reduce future growth. By adopting a maintenance regime using biocidal cleaners the need for ever having the pressure wash the surface in future years is virtually eliminated.
Whilst there is nothing wrong with the home owner attempting to clean the render, and many videos are available on the internet, the combination of using chemicals and pressure can easily lead to permanent surface damage or risk to your health.
You may also want to consult with a professional who can assess the cause of the growth and provide further advice on how to remove it and inhibit it from returning.