A professional render clean typically lasts somewhere between two and four years before algae or green staining becomes noticeable again, though that range depends heavily on aspect, exposure and whether a residual biocide treatment was applied as part of the job. On a shaded, north-facing wall it can be closer to a year; on a sheltered, sun-exposed wall it can stretch well beyond four.
It's a fair question to ask before booking, because render cleaning isn't cheap and knowing roughly what you're getting for the money helps set expectations properly.
What determines the regrowth timeline
Aspect is the single biggest factor: north and east-facing render stays damp longer and gets less direct sun to dry it out, so algae and green staining return faster there than on south or west-facing walls. Overhanging trees, nearby fences, and general shade all extend the damp window and accelerate regrowth in the same way.
Render texture matters too. Smooth, modern renders tend to hold onto staining less than heavily textured or pebbledash finishes, which have far more surface area and tiny crevices for spores to settle into and establish.
Cleaning alone versus cleaning plus residual treatment
A soft wash or steam clean that removes the existing growth without any residual biocide will typically last on the shorter end of that two-to-four-year range, because nothing's stopping fresh spores from settling and starting the cycle again straight away.
Adding a biocide treatment as part of the clean leaves a residual, spore-inhibiting layer that meaningfully extends the gap before visible regrowth, often pushing a shaded wall from a one-year problem to a two-or-three-year one. It's not a permanent barrier, biocide breaks down gradually with weathering, but it buys real time.
What honestly won't extend the life of a clean
Higher pressure or more aggressive scrubbing doesn't make a render clean last longer, it just risks damaging the render surface itself, particularly on older or more fragile finishes. Soft washing, low pressure with the right chemical treatment doing the actual work, is both gentler and, done properly, no less effective at removing growth.
Be wary of anyone promising results that 'never come back'. Algae and moss are a natural, ongoing process on any damp exterior surface, and no treatment eliminates that permanently, only slows it down.
How to get the most from a render clean
Trimming back any shrubs, hedges or trees growing close to a rendered wall improves airflow and lets the surface dry out faster after rain, which meaningfully extends the gap before visible regrowth regardless of which cleaning method was used. Keeping gutters and downpipes clear overhead also matters more than people expect, since a leaking or overflowing gutter can run water directly down a render face and create a permanently damp streak that greens up long before the rest of the wall.
If a particular section of render is consistently the first to show regrowth, it's worth asking your contractor whether a heavier residual treatment on just that area, rather than a full re-clean of the whole property, might be a more cost-effective way to manage it between full cleans.
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