The exterior walls of your property are constantly exposed to Northamptonshire’s changing weather conditions. Over time, moisture, shade, airborne spores and pollution create the perfect environment for various forms of biological growth. While many homeowners simply refer to these stains as “mould” or “moss”, several different organisms can develop on walls, each requiring a specific cleaning approach.
Stewarts Exterior Cleaning specialises in identifying and safely removing biological growth from residential and commercial properties throughout Northamptonshire. Understanding what is growing on your walls is the first step towards effective treatment and long-term protection.
Why Biological Growth Appears on Exterior Walls
Most biological growth thrives in damp conditions where sunlight is limited and moisture remains on surfaces for extended periods.
Common contributing factors include:
- North-facing walls
- Nearby trees and vegetation
- Poor drainage
- High humidity
- Damaged gutters
- Porous building materials
- Inadequate airflow
Many Northampton properties experience biological staining due to the UK’s damp climate and seasonal weather patterns. Areas that remain shaded throughout the day are particularly vulnerable.
Green Algae
Green algae is one of the most common forms of biological growth found on exterior walls.
How to Identify Green Algae
Green algae typically appears as:
- Green streaks running down walls
- Light green patches on render
- Slippery or slimy surfaces
- Uneven green discolouration
It often develops on:
- Rendered walls
- Painted surfaces
- Brickwork
- Garden walls
- Boundary walls
Green algae thrives in damp, shaded environments and can spread surprisingly quickly if left untreated.
Although it may initially seem like a cosmetic issue, algae traps moisture against the surface, which can contribute to long-term deterioration. Professional soft washing removes algae safely while helping to prevent rapid regrowth. (Stewart Specialist Cleaning)
Red Algae
Red algae is becoming increasingly common on UK properties and is often mistaken for rust or paint staining.
How to Identify Red Algae
Signs include:
- Pink, orange or red streaks
- Rust-coloured staining
- Vertical streaking patterns
- Discolouration concentrated on render
Red algae can be particularly noticeable on:
- White render
- Cream-coloured facades
- Modern rendered homes
- Commercial buildings
Many homeowners attempt to pressure wash these stains away, only to discover they quickly return. This is because the underlying biological contamination remains active within the surface.
Specialist soft washing treatments are often required to eliminate red algae at its source and restore the property’s appearance. (Stewart Specialist Cleaning)
Black Mould and Fungal Growth
Black staining on walls is frequently caused by mould, fungi or atmospheric contamination combined with biological growth.
How to Identify Black Growth
Look for:
- Dark black spots
- Black streaks beneath windows
- Dense dark patches
- Powdery or textured surfaces
Black fungal growth tends to develop where moisture remains trapped for long periods.
Common locations include:
| Area | Risk Level |
|---|---|
| North-facing walls | High |
| Areas beneath gutters | High |
| Shaded side elevations | High |
| Well-ventilated sunny walls | Low |
Beyond appearance, black growth can retain moisture against building materials and contribute to surface deterioration over time.
Lichen
Lichen is one of the most stubborn forms of biological growth found on walls.
Unlike algae, lichen is a complex organism formed by a partnership between fungi and algae.
How to Identify Lichen
Lichen often appears as:
- Circular patches
- Crust-like growth
- Yellow, grey, white or green spots
- Hard, textured formations
It commonly attaches itself to:
- Brickwork
- Stone walls
- Render
- Roof tiles
Lichen bonds strongly to surfaces and cannot usually be removed through standard cleaning alone.
Many Northampton homeowners mistake lichen for permanent staining when, in fact, specialist treatment can often restore the affected area successfully.
Moss Growth
Although moss is more commonly associated with roofs, it can also develop on walls.
How to Identify Moss
Characteristics include:
- Thick green clumps
- Soft, sponge-like texture
- Growth within cracks and joints
- Dense clusters in damp areas
Moss frequently appears on:
- Garden walls
- Retaining walls
- Stone structures
- Older brickwork
As moss grows, it retains significant amounts of moisture. During winter, this trapped water can freeze and expand, potentially contributing to mortar deterioration and surface damage.
The same moisture-retention issues that affect roofs can also impact wall structures if moss is allowed to establish itself. (Stewart Specialist Cleaning)
Biological Growth Versus General Dirt
Not all wall discolouration is caused by living organisms.
Many properties accumulate:
- Traffic film
- Carbon deposits
- Pollution staining
- Atmospheric dirt
These contaminants usually appear as:
- Grey or black staining
- Uniform discolouration
- Surface grime
- Urban pollution marks
Unlike algae or mould, these deposits do not grow or spread. However, they often combine with biological growth, making professional assessment important.
Experienced exterior cleaning specialists can determine whether the staining is organic, environmental or a combination of both before recommending the most suitable treatment. (Stewart Specialist Cleaning)
Why Correct Identification Matters
Different biological growths require different treatment methods.
Using the wrong cleaning approach can:
- Damage render
- Scar painted surfaces
- Leave spores behind
- Cause rapid regrowth
- Increase maintenance costs
For example, pressure washing may remove visible staining temporarily but fail to eliminate the underlying biological contamination.
Stewarts Exterior Cleaning uses specialist soft washing systems that target biological growth at the root rather than simply cleaning the surface. Their approach is particularly effective on delicate materials such as render, where high-pressure cleaning can cause permanent damage. (Stewart Specialist Cleaning)
When to Arrange Professional Cleaning
You should consider professional inspection if you notice:
- Green staining spreading across walls
- Red streaks appearing on render
- Black spots increasing in size
- Persistent damp-looking areas
- Crusty lichen growth
- Moss developing in joints or cracks
Early treatment is often more cost-effective than allowing biological growth to become firmly established.
Regular exterior maintenance also helps preserve kerb appeal, reduce moisture-related issues and protect the lifespan of your property’s exterior surfaces.
Conclusion
Green algae, red algae, black fungal growth, lichen and moss each leave distinctive signs on Northampton walls. Although they may appear similar at first glance, correctly identifying the type of biological growth is essential for choosing the safest and most effective treatment.
With more than a decade of experience cleaning and restoring exterior surfaces throughout Northamptonshire, Stewarts Exterior Cleaning provides expert diagnosis and professional treatment solutions tailored to each property. By addressing biological growth early and using the correct cleaning methods, homeowners can maintain cleaner, healthier and more attractive walls while protecting their property’s long-term condition.