Green algae on render / K-Rend? Here's the Johnstone Fix
Most Johnstone enquiries for green render algae follow the same pattern: it looks bad, it's slowly getting worse, and a couple of quick-fix jobs from generalists made no difference. On sandstone terrace and ex council stock in Johnstone, the fix is method-specific and the guarantee is only useful if it's underwritten. Here's what we actually do.
How to tell if it's green render algae
Four tells we look for on every Johnstone survey before quoting.
- A green cast on north- or east-facing gables, worst after wet weather
- Pink or orange staining on south walls — that's a different algae but same fix
- Darkening below window sills where water drips
- A textured, slightly rough feel where the render was originally smooth
Why it happens on Johnstone properties
North-facing walls that dry slowly stay damp for weeks after rain, and Scotland's ~170 wet days a year keep them within the algae's growth window most of the year.
Splash-back from paved paths and driveways drives spores upward onto the lower 1–2m of render, which is why the greening is often worst just above ground level.
In Johnstone specifically, the black cart valley funnels weather over johnstone — windblown leaves fill gutters fast each october, and tree-shadowed roofs hold moss longer than open sites — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Johnstone fix — step by step
- 1Test patch
We test a 1m² patch first to confirm no colour lift on your specific render batch — silicone renders vary.
- 2Soft-wash application
Low-pressure application of a render-safe biocide, worked from bottom up to avoid streaking.
- 3Dwell
10–20 minutes contact time depending on staining severity.
- 4Low-pressure rinse
Rinsed at under 500 psi — high pressure damages the render's silicone binder.
- 5Protective biocide film
A residual biocide is left on the surface to slow regrowth for 12–24 months.
Chemistry & materials we use
- Sodium hypochlorite at render-safe dilution (typically 3–5%)
- Non-ionic surfactant so the biocide sheets evenly, no drip marks
- Neutralising rinse compatible with silicone render binders
- No acid-based cleaners — they etch the finish and void manufacturer warranty
Half a day for a typical semi, one full day for a large detached with wrap-around render.
Johnstone quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Johnstone quote is fixed-price in writing — no on-the-day surprises.
Johnstone-specific considerations
Solid mix of stone-fronted Victorian terraces in the centre and 1970s ex-council semis through Spateston, with private new-builds around Brookfield.
The Black Cart Valley funnels weather over Johnstone — windblown leaves fill gutters fast each October, and tree-shadowed roofs hold moss longer than open sites.
Quick run from Paisley along the A737 — we usually combine PA5 visits with neighbouring Elderslie and Linwood jobs the same week. Nearest motorway is the M8, with routes via A737 and A761.
What NOT to do
Common mistakes we see on Johnstone properties before we're called in.
- Never jet-wash silicone render — it blows the finish off in patches, and re-rendering costs thousands.
- Never use household algae killer diluted from a garden centre — the pH is often wrong and marks the render.
- Do not paint over the algae — it grows straight back through the paint film within 6 months.
Green Render Algae — questions from Johnstone customers
Is soft-washing safe on my K-Rend?+
Yes — soft-wash is the manufacturer-recommended method for K-Rend and Weber renders. We test a small patch on your Johnstone property first and only proceed once we've confirmed no colour lift.
How long before the render greens up again?+
On a north-facing Johnstone wall, expect 18–24 months clear with the biocide left in place. South-facing usually stays clear longer.
Do you clean brick and stone as well as render?+
Yes — sandstone, blonde stone and brick all clean well with the same soft-wash chemistry, which matters in Johnstone where mixed façades are common on older streets.
Will run-off harm my planting?+
No — biocide is neutralised on rinse and diluted heavily by the water volume. We still sheet plants where they sit directly under the wash zone.
Get green render algae sorted this month
Fixed-price quote for green render algae on your Johnstone property. Fully insured, Renfrewshire Council-covered, 12-month guarantee on the work.
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