If you’ve been looking at your house and wondering why is my render going green, the answer is almost always the same thing in a different coat. It’s algae. Microscopic spores that arrived on the wind, found a damp shaded wall, and started a colony. Within a couple of seasons that colony is visible from the pavement. Within three, the house starts to look tired.
This is not dirt that washes off. It is a living layer on the render surface. The job to remove it is a soft wash, not a pressure wash, and we’ll come back to why that matters.
Why is my render going green when the neighbours’ isn’t?
Three things decide which house on a Thirsk street goes first.
The first is aspect. Algae loves shade and damp. A north-facing wall holds moisture for hours longer than the south face after rain, which is exactly the environment algae spores need to germinate. A south-facing front can stay clean for ten years while the north gable goes patchy in three.
The second is trees and overhanging hedges. They drop shade, drop debris, and trap humidity against the wall. The closer the canopy, the faster the algae colony establishes.
The third is render age and finish. Older render with micro-cracks, or a textured silicone finish, gives algae more surface area to cling to. Smooth, newer render takes longer to show.
If you have a north-facing render wall under a tree on a textured finish, that’s the wall that goes first every time.
What “green” actually is, in two sentences
It is a thin film of algae, mostly the genus Trentepohlia in the UK, sometimes joined by lichen on stonework or roof tiles. Both are biological, both feed on moisture and microscopic surface debris, and both will keep coming back until you kill them at the spore level, which a hose will not do.
Why a soft wash beats a jet wash
We hear this one every spring: “I’ll borrow my brother-in-law’s pressure washer and blast it off.” We see the result a year later.
Pressure washing strips the visible film but leaves the spores. It also opens up the render surface, so when the spores re-establish (and they will), they embed faster. The result returns darker within a year.
A proper render cleaning uses a low-pressure biocide that breaks down the cell walls of the algae. It dies, it lifts, and crucially the next round of spores has nothing to grip to. Done right, a soft wash lasts three to five years.
When to act and when to leave it
Some judgement calls. A faint shadow on a north wall after a wet winter is not an emergency. We’ll usually say to leave it another season and watch. A green tinge across whole walls, with streaks under sills, has been there a couple of years already and is not going to stop on its own.
If you want eyes on it, we cover Thirsk and the surrounding villages for free quotes. A photograph of the worst wall is usually enough for a price range without a site visit.