What happens if you leave it
Green render
Algae and biofilm on render reads as 'tired house' from the road. Painting over it doesn't fix it — the organic layer just reappears through the paint.
Slippery patios & driveways
Moss and algae on paving aren't just cosmetic — they're a slip hazard, especially in autumn and winter. Insurers take it seriously, and so should you.
Jet-wash damage
DIY jet-washing render and old block paving at the wrong pressure pits the surface, blows out joint sand, and accelerates re-growth. The fix costs more than doing it properly the first time.
Quick re-greening
A surface that's just been blasted clean with water re-greens within a season. The biological cause was never addressed.