Black Spots on Bricks Upsetting the Family?
Black Spots on Bricks Upsetting the Family?
The Science, Figures and Chemistry Behind Those Stubborn Marks
Black spots appearing on brickwork can be very frustrating for homeowners. Many families notice these dark speckles spreading across the walls of their house, leaving the property looking older, dirty, or poorly maintained.
What many people don’t realise is that these spots are not ordinary dirt. They are usually a biological organism that has chemically bonded itself to the brick surface, which is why they can be so difficult to remove.
Understanding the science, figures and chemistry behind black spots on brickwork helps explain why they appear and why specialist cleaning is often needed.
What Are the Black Spots?
In most cases the black dots seen on brickwork are lichen colonies.
Lichen is a symbiotic organism made from two living components:
• Fungus (mycobiont) – provides structure and protection
• Algae or cyanobacteria (photobiont) – performs photosynthesis
Together they create a tough biological crust that attaches tightly to porous materials such as brick, stone, concrete and roof tiles.
Typical species responsible for black spotting on masonry include members of the Verrucaria and Arthonia families, which are known for forming dark pigmented colonies.
The Size and Spread of Black Spots
Although they may look tiny, each spot is actually a small colony.
Typical figures:
• Individual spots often measure 1–5 mm in diameter
• Colonies can contain thousands of microscopic fungal filaments
• Spores can travel several kilometres in the air before settling
• A colony may take 3–5 years to become clearly visible
Once established, the spots can slowly multiply across the wall.
On a typical semi-detached house, a heavily affected wall may contain tens of thousands of individual colonies.
Why Brick Is the Perfect Surface
Brick is particularly vulnerable because of its porous structure.
Typical clay brick properties:
• Porosity: 5% – 30% void space
• Water absorption: 10% – 20% by weight
• Surface roughness allows spores to anchor
The microscopic pores trap moisture, which supports biological growth.
The Chemistry Behind the Staining
Lichen survives by producing organic acids that slowly dissolve minerals in the surface it lives on.
The most common acids involved include:
• Oxalic acid (C₂H₂O₄)
• Lichenic acids
• Usnic acid
These acids react with minerals in brick such as calcium compounds.
A common reaction involves calcium carbonate, which is present in mortar and sometimes brick surfaces:
CaCO₃ + H₂C₂O₄ → CaC₂O₄ + CO₂ + H₂O
This reaction produces calcium oxalate, a very stable compound that can contribute to dark staining.
Over time, pigments produced by the organism also darken the spot.
Why Pressure Washing Often Fails
Many homeowners try pressure washing to remove the spots, but the results are usually disappointing.
This is because:
• The root-like fungal filaments penetrate the pores of the brick
• Pigments can stain the surface minerals
• Dead colonies often leave a visible mark
Typical pressure washers produce 120–180 bar, which can remove dirt but often cannot fully remove the biological staining.
In some cases excessive pressure can even damage brick faces or mortar joints.
Environmental Conditions That Encourage Growth
Black spot lichen thrives in specific conditions.
Typical environmental factors include:
Moisture
Lichen becomes active when moisture levels exceed roughly 20–25% surface humidity.
Shade
North-facing walls often stay damp longer.
Air Quality
Interestingly, many lichens grow better in cleaner air environments, which is why they are common in rural and suburban areas.
Temperature
Most species grow best between 5°C and 25°C, which makes the UK climate ideal.
How Professional Cleaning Works
Professional exterior cleaning specialists often use biocidal soft washing systems rather than relying solely on pressure.
• Breaking down the biological organism
• Oxidising pigments that cause staining
• Killing spores that cause regrowth
Some treatments use oxidising chemistry such as sodium hypochlorite (NaOCl) or specialist surfactant blends that penetrate the porous surface.
The chemical reaction helps destroy the organism and gradually lift the staining.
Useful Figures on Brick Cleaning
To give some perspective on typical brick cleaning work:
• A house wall may contain 5–8 m² of affected brickwork
• Treatment coverage rates are often 4–6 m² per litre depending on porosity
• Visible improvement may occur within 24–72 hours
• Full natural weathering of remaining marks may take several weeks
Why Families Notice It So Much
Although black spots rarely cause structural damage, they can significantly affect the appearance of a home.
Many homeowners become concerned because:
• The spots slowly spread over time
• The house begins to look permanently dirty
• Brick colour becomes dull or patchy
• Kerb appeal is reduced
For families who take pride in their home, these marks can become surprisingly irritating.
Restoring the Appearance of Brickwork
The good news is that specialist exterior cleaning methods can dramatically improve the appearance of brick walls.
With the correct approach, it is often possible to:
• Remove the majority of black spotting
• Restore the natural brick colour
• Improve the overall appearance of the property
Understanding the biology and chemistry behind black spots helps explain why professional treatment is often the most effective solution.
