The Right Way to Fix Green algae on render / K-Rend in Airdrie
How to tell if it's green render algae
Four tells we look for on every Airdrie 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 Airdrie 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 Airdrie specifically, sitting 470ft up, airdrie is consistently colder and frostier than glasgow — north-facing slate roofs hold moss for longer here than almost anywhere we cover — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Airdrie 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.
What NOT to do
Common mistakes we see on Airdrie 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.
Airdrie quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Airdrie quote is fixed-price in writing — no on-the-day surprises.
Airdrie-specific considerations
Sandstone tenements around the town centre, large 1970s ex-council schemes in Petersburn and Rawyards, and a wave of new-build silicone-render homes around Calderbank and Plains.
Sitting 470ft up, Airdrie is consistently colder and frostier than Glasgow — north-facing slate roofs hold moss for longer here than almost anywhere we cover.
Junction 5 of the M8 puts us on site fast — most ML6 quotes are scheduled within seven days. Nearest motorway is the M8, with routes via M8 and A89.
Green Render Algae — questions from Airdrie customers
How long before the render greens up again?+
On a north-facing Airdrie 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 Airdrie 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 Airdrie property first and only proceed once we've confirmed no colour lift.
Talk to the North Lanarkshire team
Fixed-price quote for green render algae on your Airdrie property. Fully insured, North Lanarkshire Council-covered, 12-month guarantee on the work.
Related pages for Airdrie
Jump to a specific service, a nearby town, a postcode mini-hub, or an audience-specific package — all fully local to North Lanarkshire.