Red algae on render is becoming increasingly common across the UK. While many homeowners first notice green algae, it’s often the red or orange staining that causes the most concern. These marks can appear suddenly and spread in vertical patterns, giving the impression that something is wrong with the render itself.
In reality, this is usually a biological growth linked to moisture and environmental conditions. In many cases, the red staining is caused by algae from the genus Trentepohlia, which produces the distinctive red, orange, or rust-like colouring seen on exterior walls.
If left untreated, it can make even new or well-maintained homes look aged and neglected.
What Are the Red Streaks on Render?
Red algae on render typically shows up as streaks rather than flat patches. One of the most common explanations is that rainwater carries spores across the surface, and gravity pulls the growth downward as it develops. This is why you often see vertical “run” marks.
These red streaks are most often associated with Trentepohlia, a hardy algae that contains orange-red pigments. These pigments can strongly stain the render surface, making the growth highly visible even from a distance.
Typical appearances include:
- Vertical red or orange streaks
- Rust-coloured staining under windows or rooflines
- Pink or reddish patches on shaded walls
- Faint red lines that gradually darken over time
Unlike general dirt, these marks don’t simply wash away with rain.
Common Types of Algae and Wall Contamination
Different biological growths can affect render in different ways.
| Growth Type | Typical Colour | Appearance |
|---|---|---|
| Green algae | Green | Film-like surface coverage |
| Red algae (Trentepohlia) | Red, orange, pink | Vertical streaks and patches |
| Black mould/algae | Dark grey or black | Dense staining |
| Lichen | White, grey, yellow | Crusty growth attached to surface |
Each type thrives in damp environments, but red algae is often the most visually striking.
Why Red Algae and Red Streaks Develop
Red algae growth is strongly linked to environmental conditions rather than defects in the render itself.
Moisture and rainfall
Render that is regularly exposed to rain or poor drainage creates ideal conditions for algae to establish.
Shaded elevations
North-facing walls or areas with limited sunlight stay damp for longer, allowing Trentepohlia to develop and spread.
Airborne spores and contamination
Spores carried through the air settle on render and begin to grow when conditions are right. Areas near trees, fields, or vegetation are more prone.
Porous render surfaces
Modern render systems are breathable, which helps buildings manage moisture, but also means microscopic water retention can encourage biological growth.
Why Red Streaks Look Different to Green Algae
A key difference is how they spread.
Green algae tends to form a more even film across large areas of wall. Red algae, particularly Trentepohlia, often appears in vertical streaks because rainwater carries the organism down the surface.
This creates the characteristic “run” effect that homeowners notice first.
Is Red Algae Harmful?
Red algae is usually not structurally damaging in the short term, but it should not be ignored.
Main issues include:
- Visible staining that worsens over time
- Moisture being held against the surface
- Increased difficulty of removal if left untreated
- Reduced kerb appeal and property appearance
While it won’t typically break down render quickly, long-term neglect can contribute to surface deterioration.
Why Pressure Washing Is Not Recommended
It might seem like an easy fix, but high-pressure cleaning can create more problems than it solves.
Potential risks include:
- Damaging the render surface
- Removing protective coatings
- Forcing water into cracks
- Only cleaning the surface stain, not the cause
Even when the staining is removed temporarily, Trentepohlia often remains within the surface and returns quickly.
The Safe Way to Remove Red Algae: Soft Washing
Soft washing is the most effective method for treating red algae on render.
It uses low-pressure application combined with specialist cleaning solutions designed to target biological growth at its source.
It works by:
- Breaking down algae at a microbial level
- Treating both surface staining and root growth
- Reducing the chance of regrowth
- Protecting the render finish
This is why it’s widely used instead of pressure washing for modern render systems.
Professional treatments are available through Softwashing UK.
How Soft Washing Is Carried Out
A proper treatment follows a structured process:
1. Inspection
The render type, staining level, and environmental conditions are assessed.
2. Preparation
Loose debris and organic matter are removed to allow even treatment.
3. Application
A specialist biocide is applied at low pressure across affected areas.
4. Dwell time
The solution is left to break down the algae, including Trentepohlia growth.
5. Rinse or natural weathering
Depending on the system used, surfaces are either gently rinsed or left for rainfall to complete the process.
Can Red Streaks Return?
Yes, they can return over time. This is because the underlying environmental conditions often remain the same.
Factors that influence regrowth include:
- Persistent shade
- High moisture levels
- Nearby trees or vegetation
- Poor airflow around walls
Regular maintenance helps slow down or reduce regrowth significantly.
Preventing Red Algae and Red Streaks
While it’s not always possible to prevent completely, you can reduce the risk:
- Keep gutters and downpipes clear
- Trim back vegetation near walls
- Improve airflow around shaded elevations
- Address drainage issues quickly
- Schedule periodic soft washing treatments
Why New Build Homes Are Often Affected
Many homeowners are surprised to see red streaks on new properties. This is usually due to a combination of:
- Close housing layouts reducing airflow
- Modern render designed to be breathable
- Damp UK climate
- Landscaping that traps moisture
These conditions are ideal for Trentepohlia and green algae growth.
Final Thoughts
Red algae on render, particularly the streaking caused by Trentepohlia, is a common issue in the UK climate. It doesn’t usually indicate structural problems, but it can quickly affect the appearance of a property.
The key is early treatment and using the correct cleaning method. Soft washing remains the safest and most reliable way to remove red streaks properly and reduce the chances of them returning too quickly.
If the issue is left too long, it becomes more established within the render and harder to shift, so early action always pays off.