What are we actually removing?
Exterior surfaces accumulate multiple types of contamination over time. Understanding what's on the surface is fundamental to choosing the right approach.
Algae
Single-celled organisms that establish quickly in damp conditions. Often presents as green or black streaking. Responsive to appropriate biocidal treatments.
Lichen
A symbiotic organism combining algae and fungi. Physically bonds with surfaces over time, making it significantly harder to remove than surface algae.
Mould and mildew
Fungal growth that thrives in shaded, humid environments. Can penetrate porous surfaces and cause staining that persists after the organism is killed.
General grime and atmospheric deposits
Airborne particles, pollution, dust and environmental contaminants that accumulate on surfaces and dull their appearance.
The role of chemistry
For most biological contamination, pressure alone doesn't solve the problem; it moves it. Effective exterior washing uses appropriate biocidal and surfactant solutions that kill organisms at a cellular level, breaking down the cellular bonds that hold them to the surface. The water then rinses away the dead material.
This is why soft washing, using lower pressure, specialist chemicals and appropriate dwell time, is more effective and safer for most premium surfaces than simply hitting them with high-pressure water. The chemistry does the work; the water is the delivery and rinse mechanism.
Pressure vs soft washing: when each is appropriate
The choice between pressure washing and soft washing depends on the surface, its condition and what needs to be removed:
| Surface | Recommended Approach |
|---|---|
| Painted or coated cladding | Soft washing; high pressure risks surface damage and paint delamination |
| Roofing (tiles, iron, membrane) | Soft washing; high pressure lifts tiles and damages coatings |
| Concrete driveways and paths | Pressure washing appropriate for hard, unpainted surfaces |
| Timber decking | Soft washing or low-pressure; high pressure can raise timber grain |
| Brick or block | Context-dependent; usually low-pressure with appropriate chemistry |
| Stone | Soft washing; high pressure can damage soft or porous stone |
Post-treatment and residual protection
Professional exterior washing typically includes a biocidal treatment that continues to work after the clean, inhibiting the regrowth of algae, lichen and mould for a period of months to years, depending on the product and the exposure of the surface. This residual effect is a key reason why professional treatments last significantly longer than a simple water rinse.
Prevent's approach
We select chemistry appropriate to each surface, apply it correctly and use the right technique for each situation. For high-value homes, that almost always means soft washing, protecting your cladding, coatings and surfaces while achieving a genuinely clean result.
Learn About House Washing