What is Render and Why Does It Need Cleaning?
Render is the outer coating of plaster applied to the exterior walls of a building. It serves two primary purposes: to provide a durable, weatherproof barrier that protects the underlying brickwork or blockwork from the elements, and to offer a decorative, textured finish. In the UK, you will commonly find several types of render, including traditional sand and cement, modern monocouche renders like K-Rend or Weber, and flexible silicone-based systems.
Despite its protective qualities, render is a porous material, which makes it an ideal breeding ground for various types of organic growth and a magnet for environmental pollutants. The discolouration you see is rarely just "dirt". It is typically caused by a combination of factors:
- Organic Growth: The UK's damp climate provides the perfect conditions for microorganisms to thrive on render surfaces. This includes green algae, which is most common, as well as more stubborn red and black algae. You may also see lichens (hard, crusty growths) and moss, particularly around leaking pipes or overflowing gutters.
- Environmental Pollution: Properties located near busy roads or in industrial areas often accumulate a layer of soot, traffic film, and dust. This creates a dull, greyish appearance and can be difficult to remove without the correct cleaning agents.
- Water Staining: Run-off from roofs, leaky gutters, or faulty window sills can create distinct streaks and damp patches on the render. These areas often encourage more concentrated algae growth, resulting in prominent stains.
Cleaning your render is not merely a cosmetic exercise. A heavy build-up of algae and moss can trap moisture against the wall's surface. During the winter, this trapped water can freeze and expand, leading to cracks, delamination (where the render pulls away from the wall), and potential damp issues inside your property. Regular, professional cleaning is a vital part of your home’s long-term maintenance schedule.
The Dangers of DIY and High-Pressure Washing
Faced with a green-tinged wall, many homeowners are tempted to hire a pressure washer from a local tool shop and tackle the job themselves. While the immediate results can seem impressive, using high-pressure water on render is one of the most damaging things you can do. It is a common mistake that can turn a simple cleaning job into an expensive repair project.
Here’s why you should avoid aggressive jet washing:
- Surface Damage: The force of a high-pressure jet can easily blast away the top layer of the render, especially on softer monocouche or coloured renders. This can create a patchy, uneven finish, expose the aggregate underneath, and permanently scar the surface.
- Water Ingress: Pressure washing forces a huge volume of water at the wall, driving it deep into the render's pores and any existing hairline cracks. This can saturate the substrate, leading to internal damp problems that may take months to manifest and resolve.
- It Doesn't Solve the Problem: While a jet wash may remove the visible surface growth, it doesn't kill the underlying spores. It effectively gives the algae a "haircut", and because you've blasted the surface and introduced more moisture, the growth often returns thicker and faster than before.
- Invalidated Warranties: Manufacturers of modern render systems, such as K-Rend, explicitly state that high-pressure washing should not be used. Doing so will almost certainly invalidate any product warranty you may have.
A professional render cleaning service understands these risks. They will never use brute force to clean your walls, opting instead for proven, gentle methods that deliver superior, longer-lasting results without causing damage.
Professional Render Cleaning Methods Explained
A reputable render cleaning specialist will assess your property and select the most appropriate cleaning method based on the type of render, its condition, and the nature of the staining. The two most effective and widely used professional techniques are soft washing and steam cleaning.
Soft Washing: The Gold Standard for Render Cleaning
Soft washing is the preferred method for cleaning almost all types of residential render. It is a highly effective and gentle process that prioritises killing the organic growth at its root, rather than simply blasting it from the surface.
The process involves the low-pressure application of a specialised, government-approved biocide. This chemical solution works to break down and kill the algae, lichen, and moss spores embedded within the render's surface. After a period of time to allow the biocide to work, the residue and dead organic matter are rinsed away using clean water at a very low pressure, often little more than a garden hose.
The benefits of soft washing are significant:
- Kills Growth at the Root: By eliminating the spores, soft washing provides a much deeper and longer-lasting clean. Results can last for several years, whereas pressure-washed surfaces can show regrowth within months.
- Extremely Gentle: The low-pressure application ensures there is no risk of damaging the render surface, making it safe for K-Rend, monocouche, pebble-dash, and painted surfaces.
- Uses Less Water: Soft washing is a more environmentally friendly option, using a fraction of the water required for high-pressure washing.
- Uniform Finish: The treatment cleans the entire surface evenly, avoiding the risk of the streaky, patchy results often associated with jet washing.
Steam Cleaning (DOFF System)
Another highly effective professional method is steam cleaning, often performed using a system like DOFF. This technique uses superheated water, reaching temperatures of up to 150°C, which is then applied to the surface through a special nozzle at very low pressure.
The intense heat kills all organic matter instantly and is also extremely effective at melting away stubborn contaminants like paint, graffiti, and heavy grease or oil staining without the need for chemicals. Because the volume of water used is minimal, the surface is not saturated and dries within minutes, reducing the risk of water ingress. Steam cleaning is a specialist technique that requires a highly skilled operator and is often used for heritage buildings or for tackling specific, challenging stains.
The Render Cleaning Process: What to Expect
Hiring a professional ensures a smooth, safe, and efficient process from start to finish. While every project is slightly different, a typical render cleaning job follows a clear structure.
- Initial Assessment and Quotation: The first step is for the tradesperson to visit your property. They will inspect the render, identify the type and severity of the soiling, check for any damage, and assess access requirements. Following this, they will provide you with a detailed written quotation outlining the proposed method, the work involved, and a full breakdown of the costs.
- Site Preparation: On the day of the cleaning, the team will begin by preparing the area. This involves protecting windows, doors, sensitive stonework, and covering plants, flowerbeds, and lawns with waterproof sheeting to prevent any damage from the cleaning solutions or run-off.
- Treatment Application: The team will then apply the chosen treatment. For a soft wash, this will be the low-pressure spraying of the biocide onto all rendered surfaces. If steam cleaning is required, they will set up the specialist equipment.
- Dwell Time and Rinsing: For a soft wash, the biocide is left to dwell on the surface for a specific period to ensure it has effectively killed all the organic spores. The walls are then thoroughly rinsed down with clean, low-pressure water to wash away the cleaning agent and all the dislodged grime and algae.
- Post-Treatment Application (Optional): For maximum longevity, some companies offer an optional application of a clear, preventative biocide. This soaks into the render and helps to inhibit the regrowth of algae, keeping your walls cleaner for longer.
- Final Inspection and Site Clean-Up: Once the cleaning is complete, the team will remove all protective sheeting, tidy the area, and conduct a final walk-around with you to ensure you are completely satisfied with the results.
