Green film on render is airborne algae that has found moisture, shade and a slightly porous surface to sit on. In a maritime climate like Plymouth's, all three are usually present on the north and west elevations of a house.
What is actually growing
The green layer is single-celled algae, not mould and not dirt. It feeds on light and moisture rather than the render itself, which is why it returns quickly if you only rinse the surface without killing the organism.
This guide sits under our Wall Cleaning & Softwash work, covering 20-mile radius of Plymouth.
Why pressure washing backs the problem
A jet wash removes the visible layer but also opens the render's surface, leaving more texture and more water retention for the next colony. That is why walls that were jetted often go green faster the second time.
What we do instead
We apply a biocide at low pressure top-down across the elevation, let it dwell, then rinse where the finish calls for it. The treatment kills the colony rather than shaving it off, so regrowth is slow and even.
Questions we get on this
- Will the green come straight back?
- Not straight back. Once the colony is killed rather than rinsed, regrowth starts from fresh airborne spores, which is a much slower process.
- Can I use household bleach?
- We would not. Domestic bleach runs off onto planting and driveways uncontrolled and can bleach coloured render unevenly.
- Does the weather matter for booking?
- A dry, still day gives the best dwell time. We work Monday to Saturday and reschedule if wind would carry product where it should not go.
Related reading
- Black streaks running down walls: what causes them
- Salt and sea air: what it does to coastal exteriors
- Why only one side of the house goes green
- White powder on brickwork: efflorescence explained