What this guide covers: How to clean render and external walls at home — covering which render types can be pressure washed and which require softwash only, how to apply biocidal cleaner correctly, and why treating the surface after cleaning is as important as the clean itself.
Green algae, black mould streaks and moss on rendered external walls are one of those things that creep up gradually until suddenly the front of the house looks neglected. It's a common problem in the UK climate. Once it takes hold, it comes back faster after each cleaning unless the root cause is treated rather than just the visible surface. With render, the method you use matters more than the pressure itself.
What type of render do you have?
Sand and cement render — thick, hard, relatively tolerant of moderate pressure washing; check for cracks before washing. Thin-coat or monocouche render — finish coat only a few millimetres thick; high-pressure lancing permanently damages the surface texture; use softwash only. K-rend, Weber and similar polymer-modified renders — similarly sensitive to high pressure; once the surface is damaged by pressure, it stains and accumulates biological growth faster. Painted masonry and brick — generally more tolerant but test a small inconspicuous area first if there is any adhesion concern.
What you need
- Pressure washer — V-TUF V3 for sand and cement and painted masonry, or use at lowest setting for thin-coat render
- Garden pump sprayer — for applying biocidal cleaner to thin-coat and polymer renders
- External wall cleaner or biocidal solution with dwell time
- Biocidal maintenance spray or render sealer — for post-clean protection
Softwash vs pressure washing
For sand and cement render and painted masonry, the V3 with a fan nozzle at 30–40cm, biocidal cleaner pre-applied. For thin-coat, monocouche and polymer renders, apply wall cleaner using a pump sprayer, allow 20–30 minutes dwell, then rinse at low pressure from distance. The chemical does the work; the rinse is just removal. Applying a biocidal maintenance spray or render sealer after cleaning significantly extends the clean — most biological growth starts from airborne spores landing on a damp surface and a post-treatment inhibits re-establishment for 12–24 months. If a wall grows algae rapidly, it stays damp from blocked gutters, overflowing downpipes, or aspect — cleaning without addressing the moisture source means the growth comes back faster each time.
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