Green algae on render / K-Rend? Here's the Glasgow Fix
Green Render Algae on a Glasgow property isn't cosmetic — left long enough, it costs real money to reverse. This page is the version we wish every Glasgow customer read before phoning three firms and getting three different stories. Straight facts, City of Glasgow-relevant, no jargon.
How to tell if it's green render algae
Four tells we look for on every Glasgow 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 Glasgow 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 Glasgow specifically, glasgow averages around 170 wet days a year and sits in a damp west-coast climate — perfect conditions for north-facing roofs to grow moss and for red sandstone to streak with black algae — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Glasgow 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.
Glasgow-specific considerations
A real mix of Victorian sandstone tenements in the West End and Southside, post-war semis in Knightswood and Pollok, and modern detached new-builds across Bearsden and Newton Mearns.
Glasgow averages around 170 wet days a year and sits in a damp west-coast climate — perfect conditions for north-facing roofs to grow moss and for red sandstone to streak with black algae.
We cover the whole city seven days a week and can usually be on site within 48 hours of a quote being accepted, M8 traffic permitting. Nearest motorway is the M8, with routes via M8 and M77.
What NOT to do
Common mistakes we see on Glasgow 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.
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.
Glasgow quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Glasgow quote is fixed-price in writing — no on-the-day surprises.
Green Render Algae — questions from Glasgow customers
How long before the render greens up again?+
On a north-facing Glasgow 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 Glasgow 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 Glasgow property first and only proceed once we've confirmed no colour lift.
Get green render algae sorted this month
Fixed-price quote for green render algae on your Glasgow property. Fully insured, Glasgow City Council-covered, 12-month guarantee on the work.
Related pages for Glasgow
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