Rendered walls are designed to protect buildings from weather exposure while creating a clean, modern exterior. However, across the UK, render is also one of the most commonly stained exterior finishes, often developing green, black or uneven discolouration within just a few years.
This guide explains:
- Why render gets dirty
- Why it stains faster than brick or stone
- How professional render cleaning works
- How to protect rendered walls long term
If your render looks tired, stained or uneven, understanding the cause is the first step to restoring it safely.
Why Does Render Turn Green, Black or Patchy Over Time?
Render discolouration is primarily caused by moisture retention combined with biological growth.
Because render is porous, it absorbs rain, humidity and condensation. When this moisture lingers, it creates ideal conditions for staining.
The most common causes include:
- Green staining from algae, especially on north-facing or shaded walls
- Black streaks from pollution and airborne contaminants
- Patchy colour loss caused by uneven drying
- Rainwater tracking beneath sills, pipes and gutters
- Persistent damp conditions caused by poor airflow
Unlike dirt, algae and lichen do not simply wash away - they embed into the surface and continue growing unless treated properly.
What Causes Algae, Moss and Lichen to Grow on Render?
Biological growth develops when moisture, shade and airborne spores combine.
- Algae forms first, spreading green staining across damp surfaces
- Moss thickens over time, holding moisture against the wall
Lichen forms crust-like patches that bond tightly to the render
Importantly, what you see on the surface is only part of the problem. The root system often penetrates the render, meaning surface washing alone won't solve it.
Why Does Render Get Dirty Faster Than Brick or Stone?
Render stains faster because it behaves very differently from traditional masonry.
Key differences include:
- Render has higher porosity
- Brick and stone shed moisture more effectively
- Mortar joints allow evaporation
- Masonry textures hide light staining better
- Render presents a flat surface where stains are obvious
Even modern silicone or acrylic renders, while more water-repellent, can still stain if algae is allowed to establish.
This is why rendered properties often appear dirty years before nearby brick buildings.
How Is Render Cleaned Safely and Professionally Without Damaging the Surface?
Professional render cleaning is designed to clean thoroughly while preserving the integrity of the render, not to strip or force contamination away. Unlike pressure washing, which relies on impact, professional methods use controlled, low-impact techniques tailored to the specific render type and condition.
The focus is on treating the cause of staining, not just removing what's visible on the surface.
The core goals of professional render cleaning are to:
Remove visible staining and discolouration
This includes green algae, black streaks from pollution, and uneven patchiness caused by moisture retention.
Kill biological growth at the root
Specialist treatments eliminate algae, moss and lichen spores embedded within the render, preventing rapid regrowth.
Preserve the render surface and finish
Low-pressure methods avoid surface erosion, texture loss and colour inconsistency — especially important for coloured and monocouche renders.
Maintain breathability and moisture balance
Proper cleaning allows render to dry naturally, reducing the risk of trapped moisture, damp and freeze-thaw damage.
Professional Render Cleaning Methods Explained
Not all render should be cleaned in the same way. The safest and most effective render cleaning method depends on the type of render, the level of staining, and the age and sensitivity of the building. Using the wrong technique - particularly high-pressure washing - can cause permanent surface damage, colour loss and moisture problems.
What Is Softwashing and Why Is It Used on Render?
Softwashing is the preferred cleaning method for most modern render systems.
It uses:
- Low-pressure water (similar to a garden hose)
- Specialist biocidal solutions
- Extended dwell times to break down algae and spores
Benefits of softwashing include:
- No surface erosion
- Even colour restoration
- Long-lasting results
- Reduced regrowth
Because it treats the cause rather than the symptom, softwashing typically keeps render clean for several years.
What Is Steam Cleaning and When Is It Suitable for Render?
Steam cleaning uses high temperature and very low pressure to lift surface contamination.
It is often suitable for:
- Light staining
- Delicate finishes
- Areas where chemical use is restricted
However, steam alone may not eliminate deeply embedded algae spores, so it's sometimes combined with follow-up treatments.
What Is TORC Cleaning and When Is It Used on Render?
TORC cleaning is a specialist, conservation-grade system that uses gentle micro-abrasives with precise control.
It is typically used on:
- Lime render
- Historic buildings
- Listed properties
- Highly sensitive surfaces
TORC cleaning allows careful stain removal while preserving original material.
Is It Safe to Clean Render Yourself?
DIY render cleaning carries significant risks and often causes long-term damage.
Common DIY issues include:
- Surface erosion from pressure washers
- Colour inconsistency on monocouche render
- Moisture driven deeper into the wall
- Faster algae regrowth
- Reduced render lifespan
Professional cleaning prioritises preservation, breathability and prevention.
Talk To Our Team
If your render has turned green or patchy, Ultra Cleaning Services provides professional render cleaning that removes biological growth safely and effectively.
How Often Should Rendered Walls Be Cleaned?
Most rendered properties benefit from cleaning every 3 to 5 years, depending on:
- Exposure to rain and shade
- Proximity to trees
- Local pollution levels
- Render type
- Previous maintenance
Regular cleaning prevents heavy staining and reduces the need for repainting or repairs.
Can Render Cleaning Replace the Need for Repainting?
In many cases, yes - professional render cleaning can remove the need for repainting, or at least delay it significantly.
Over time, many homeowners assume render needs repainting because it looks faded, dirty or uneven. In reality, the underlying render is often still structurally sound - it's simply discoloured by algae, pollution and trapped moisture, not failing paint or render material.
Professional render cleaning targets the root cause of the discolouration, rather than masking it.
Render cleaning can:
Restore the original colour of the render
Algae and atmospheric staining often dull or darken render. Once removed, the true colour beneath is revealed - especially on monocouche and silicone renders that are coloured through.
Remove biological staining at the source
Softwashing and specialist treatments kill algae, moss and lichen spores within the render, preventing rapid regrowth that repainting alone cannot stop.
Delay repainting by several years
By fully cleaning and treating the surface, many properties avoid repainting for 3–5 years or longer, depending on exposure and environment.
Reduce long-term maintenance costs
Cleaning is significantly more cost-effective than repainting and avoids repeated paint failures caused by trapped moisture.
For properties where the render is intact but visually tired, professional cleaning is usually the first and most effective step - with repainting only considered if there is genuine surface degradation or colour loss that cleaning cannot resolve.
Why Do Heritage and Listed Buildings Need Specialist Render Cleaning?
Heritage and listed buildings require a very different approach to render cleaning because historic render systems are fundamentally different from modern materials. Traditional buildings were designed to breathe, allowing moisture to move naturally through walls rather than being sealed in.
Historic render - most commonly lime-based render - manages moisture by absorbing and releasing it gradually. This natural process protects the structure, but it also means the surface is far more vulnerable to damage if cleaned incorrectly.
Heritage-safe render cleaning is essential because inappropriate methods can permanently alter the building fabric.
Specialist heritage render cleaning requires:
No high pressure
Pressure washing can erode lime render, wash away surface detail, and open the structure to accelerated decay.
No aggressive chemicals
Harsh cleaning agents can react with lime, causing long-term chemical instability, discolouration and surface breakdown.
Minimal abrasion
Abrasive cleaning can strip away original material, erase historic tooling marks, and reduce the lifespan of the render.
Full moisture vapour permeability
Cleaning methods must preserve the render's ability to breathe, allowing moisture to evaporate naturally rather than becoming trapped within the wall.
Specialist heritage render cleaning focuses on gentle, conservation-led techniques, such as controlled steam cleaning or TORC systems, chosen specifically to suit the age, condition and composition of the render.
How Long Do Render Cleaning Results Last?
Professional render cleaning typically lasts 3–5 years, depending on:
- Render type
- Environmental exposure
- Shading and moisture levels
- Treatment method used
Biocidal treatments significantly extend cleanliness compared to surface-only cleaning.
Does Professional Render Cleaning Increase Property Value?
Yes - professional render cleaning can directly and indirectly increase property value by improving how a building looks, performs and is perceived by buyers, tenants and surveyors.
For many properties, especially rendered homes, the exterior is the first thing people notice. Stained or patchy render can give the impression of poor maintenance, even when the structure itself is sound.
Clean, well-maintained render improves:
Kerb appeal
Removing green, black and uneven staining instantly refreshes the appearance of a property, making it look newer, brighter and better cared for.
Buyer confidence
A clean exterior reassures buyers that the property has been properly maintained, reducing concerns about damp, neglect or hidden defects.
Survey outcomes
Surveyors often flag heavily stained render as a potential moisture or maintenance risk. Professional cleaning helps demonstrate that the issue has been addressed correctly.
Overall property presentation
Whether selling, letting or refinancing, clean render enhances photographs, viewings and marketing materials, helping properties stand out in competitive markets.
Crucially, professional render cleaning delivers these benefits without the cost, disruption or risk of repainting. It is typically far more affordable than external redecorating and provides visible, long-lasting results.
Ready to Restore Your Render?
If your render is looking tired, stained or uneven, now is the time to act - before algae, moisture and discolouration lead to bigger problems. Professional render cleaning is one of the simplest and most cost-effective ways to transform your property's appearance while protecting it long term.
Ultra Cleaning Services provides specialist render cleaning using safe, proven methods tailored to your building, whether it's a modern home, commercial property or heritage site.
Contact Ultra Cleaning Services today to book a professional render cleaning assessment and quote.