The Right Way to Fix Green algae on render / K-Rend in Houston
Green algae on render / K-Rend is one of the most common calls we get from Houston homeowners, and it peaks in spring/summer. Houston sits 13 miles from Glasgow in Renfrewshire, with the valley holding damp far longer than the map suggests. This page walks through why it happens on PA6 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 Houston 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 Houston 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 Houston specifically, shielded by the gryffe valley — gutter debris from mature trees and slate moss on shaded sides are the headline maintenance issues — which is the single biggest reason this issue is more stubborn here than in a drier inland town.
Our Houston 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 Houston 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.
Houston quote to job completed is usually under two weeks.
£220–£480
£ per typical rendered semi
Every Houston quote is fixed-price in writing — no on-the-day surprises.
Houston-specific considerations
Conservation-area cottage terraces in the old village, premium modern detached through Crosslee and Barochan, and rural-fringe stone cottages towards Bridge of Weir.
Shielded by the Gryffe Valley — gutter debris from mature trees and slate moss on shaded sides are the headline maintenance issues.
Off the A761 — Houston jobs combine well with Bridge of Weir, Kilbarchan and Bishopton stops on a single tight route. Nearest motorway is the M8, with routes via A761 and A737.
Green Render Algae — questions from Houston customers
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 Houston property first and only proceed once we've confirmed no colour lift.
How long before the render greens up again?+
On a north-facing Houston 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 Houston where mixed façades are common on older streets.
Get green render algae sorted this month
Fixed-price quote for green render algae on your Houston property. Fully insured, Renfrewshire Council-covered, 12-month guarantee on the work.
Related pages for Houston
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