Why modern render goes green so fast
Silicone-based renders (K-Rend, Weber, EWI Pro and similar) hold a tiny amount of moisture in the textured surface — perfect conditions for airborne algae spores to land and grow.
The silicone binder itself is a mild nutrient source. Older sand-and-cement renders didn't have this problem to the same degree because there was nothing for the algae to feed on.
North-facing walls, walls under trees, walls near hedges, and walls behind downpipes always go first — anywhere that stays damp longer after rain.
The green you see is one of several algae species (most commonly Trebouxia). Pink staining is a different organism — Trentepohlia — common on west-facing walls in wet climates like Glasgow and the Scottish west coast.
Why your last clean didn't last
Pressure washing strips visible algae from the surface but does two things wrong: it removes the silicone skin, creating more anchor points for new spores, and it does nothing to the spores in the porous render body. Result: visible green again within 6–12 months.
Cleaning without a biocide finish is the most common mistake. The surface looks clean for one summer, then re-colonises through the next autumn and winter.
DIY render cleaners from a sprayer don't have enough dwell time or the right concentration of sodium hypochlorite to penetrate the spore layer.
The proper fix — what works
Soft-wash with a render-safe biocide blend. Applied at low pressure, left to dwell, then rinsed at low pressure again.
Biocide top-up after rinsing. This is the bit DIY skips and the bit that makes the result last 3–5 years instead of 12 months.
Compliant with K-Rend's own cleaning guidance — pressure washing voids their warranty, soft-wash does not.
Annual or biennial maintenance biocide costs about £100–£200 and extends the life of a full clean to 6–8 years.
What to ask before booking
Do you use sodium hypochlorite-based biocide or just detergent? (Only the biocide kills spores.)
Do you apply a biocide top-up after rinsing? (Without it, you'll be re-cleaning in 18 months.)
Are you K-Rend / manufacturer compliant? (Pressure washing K-Rend voids the warranty.)
What's the guarantee on the result? A confident contractor offers at least 12 months 'visible-clean' guarantee.