Green algae on render / K-Rend? Here's the Bearsden Fix
Green Render Algae on a Bearsden property isn't cosmetic — left long enough, it costs real money to reverse. This page is the version we wish every Bearsden customer read before phoning three firms and getting three different stories. Straight facts, East Dunbartonshire-relevant, no jargon.
How to tell if it's green render algae
Four tells we look for on every Bearsden 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 Bearsden properties
New-build estates share the same render batch across dozens of homes, so once one façade is affected the same streaks appear on neighbours within a season.
Silicone-based renders such as K-Rend, Weber and Parex have a slightly porous face that holds airborne moisture — perfect for green algae (Trentepohlia and Klebsormidium) to establish.
In Bearsden specifically, bearsden's elevation and tree cover make it the moss-heaviest postcode on our entire round — north-facing slate routinely needs full soft-wash plus biocide — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Bearsden 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.
Bearsden quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Bearsden quote is fixed-price in writing — no on-the-day surprises.
Bearsden-specific considerations
Large detached sandstone and inter-war villas dominate, with prestige modern detached around Kilmardinny. Slate roofs, mature trees and big gardens — moss is the headline issue here.
Bearsden's elevation and tree cover make it the moss-heaviest postcode on our entire round — north-facing slate routinely needs full soft-wash plus biocide.
A short run from our base. We schedule G61 jobs in clusters and frequently combine roof, render and gutters in one half-day visit. Nearest motorway is the M898, with routes via A809 and A810.
What NOT to do
Common mistakes we see on Bearsden 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 Bearsden customers
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 Bearsden 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.
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 Bearsden property first and only proceed once we've confirmed no colour lift.
How long before the render greens up again?+
On a north-facing Bearsden wall, expect 18–24 months clear with the biocide left in place. South-facing usually stays clear longer.
Talk to the East Dunbartonshire team
Fixed-price quote for green render algae on your Bearsden property. Fully insured, East Dunbartonshire Council-covered, 12-month guarantee on the work.
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