Green algae on render / K-Rend? Here's the Milngavie Fix
Green algae on render / K-Rend is one of the most common calls we get from Milngavie homeowners, and it peaks in spring/summer. Milngavie sits 7 miles from Glasgow in East Dunbartonshire, with the higher ground drying surfaces fast but leaving them exposed. This page walks through why it happens on G62 properties, how we fix it, and roughly what it costs.
How to tell if it's green render algae
Four tells we look for on every Milngavie 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 Milngavie 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 Milngavie specifically, pressed against the mugdock and allander hills, milngavie catches the first weather off the west — slate moss and lichen growth here is among the fastest we see — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Milngavie 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.
Milngavie quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Milngavie quote is fixed-price in writing — no on-the-day surprises.
Milngavie-specific considerations
Spacious detached homes, original 1930s villas and a steady stream of premium new-builds around Dougalston. Plenty of K-Rend, slate and natural stone elevations.
Pressed against the Mugdock and Allander hills, Milngavie catches the first weather off the west — slate moss and lichen growth here is among the fastest we see.
Up the A81 from Bearsden — typical G62 jobs are full half-days with roof and gutters combined. Nearest motorway is the M898, with routes via A81 and A809.
What NOT to do
Common mistakes we see on Milngavie 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 Milngavie 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 Milngavie property first and only proceed once we've confirmed no colour lift.
How long before the render greens up again?+
On a north-facing Milngavie 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 Milngavie 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.
Talk to the East Dunbartonshire team
Fixed-price quote for green render algae on your Milngavie property. Fully insured, East Dunbartonshire Council-covered, 12-month guarantee on the work.
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