Homeowners · 4 min read · Updated 6 November 2026
Why is my new build render going green? Causes and how to fix it
Why it happens so fast
Many new homes in the UK are finished with thin-coat silicone, acrylic or monocouche renders, often in pale colours. These systems are usually applied over insulation or modern block walls, and they are designed to shed water. However, on well-insulated walls the outer surface stays cold and can collect condensation and dew, especially on north-facing elevations. Algae spores land, find moisture and grow.
- North-facing and shaded walls stay damp longest
- Walls near trees, hedges or open fields collect more spores
- Pale colours show green and black growth clearly
- Ledges, window sills and areas below gutters get extra water
- Nearby farmland or woodland can increase spore levels
Modern estates often have new hedges, young trees and open land nearby, and as planting matures, shade and spore levels can increase. That is one reason growth sometimes appears on walls that stayed clean for the first year or two.
Is it a defect?
Usually not. Most render manufacturers and builders treat algae growth as normal weathering and maintenance rather than a fault in the render. It is worth checking your new home warranty and the render system's maintenance guidance, because some recommend periodic cleaning and specific products. Cracks, debonding, blowing render or water getting in behind it are different and should be reported.
If your home is still within its builder's first years of cover, report any cracking or water penetration promptly and keep photos and dates.
Why pressure washing is risky on thin-coat render
Modern thin-coat renders can be just a few millimetres thick. A pressure washer can erode the surface, wash out the texture, leave tracking marks and force water behind the render at joints and edges. Some systems have a water-repellent surface that pressure can damage, which then lets dirt and growth return faster.
Soft washing at low pressure with a suitable biocide is generally the preferred method for these renders. The growth dies, rinses away gently and the finish is left intact.
Check what is behind the render too. Many new homes use external wall insulation or render-only systems on block, and the edges around windows, doors and service pipes rely on sealants and trims. High-pressure water aimed at those details is the most likely way to cause leaks, so they need particular care during any clean.
Common new build render types
| Render type | Typical look | Cleaning notes |
|---|---|---|
| Silicone thin coat | Fine textured, often white or cream | Low pressure only, check manufacturer guidance |
| Acrylic thin coat | Similar to silicone, less water repellent | Low pressure, may stain sooner |
| Monocouche | Through-coloured, scraped finish | Avoid harsh pressure that erodes the face |
| Painted sand and cement | Smooth or textured, painted | Check paint condition first |
Whatever the render type, ask the contractor to keep the lance well back from sealant joints, trims and window edges, and to rinse gently rather than blast.
Reducing regrowth
Growth will come back on shaded walls eventually, but you can slow it down. Clearing gutters, fixing dripping overflows and cutting back trees and shrubs near walls all help. Some owners have a maintenance biocide applied every year or two after the main clean, which keeps algae from establishing. Avoid sealers or coatings unless the render manufacturer approves them.
Taking care of a new build exterior
New homes have other exterior features that often go green at the same time as the render, including uPVC fascias and soffits, composite cladding, porch canopies, paths and patio paving. Having them cleaned together means one visit, a consistent finish and usually a lower overall price than separate jobs.
- Check the render manufacturer's maintenance guidance in your home pack
- Keep gutters clear, especially in the first autumns while trees on the estate grow
- Fix dripping overflows from boilers and condensate pipes
- Avoid planting climbers directly against thin-coat render
- Book cleaning before green growth becomes heavy, which makes it quicker and cheaper
Keeping photos of the walls each year is also useful if a question about the render ever comes up with the builder or warranty provider.
What it costs
As a guide, soft washing render on a typical semi-detached or terraced house usually costs £300 to £600, and a detached house £600 to £1,100, depending on height and access. Many new estates have neighbouring homes with the same problem, and several neighbours booking together can sometimes keep the cost down.
Frequently asked questions
Can I claim on my new build warranty for green render?
Usually not for algae alone, as it is generally treated as maintenance rather than a defect. Warranties typically cover structural and some other defects in the early years, and cracked or debonded render or water getting in may be covered. Check your warranty terms and contact the builder or warranty provider if you see anything beyond surface staining.
Will soft washing affect the colour of silicone render?
It should not when the right product and strength are used and the render is rinsed properly. Pale and white renders often look brighter after cleaning, simply because the growth has gone. Strongly coloured renders need extra care, which is why a contractor may test a small area first. Avoid very strong mixes that could lighten some pigments.
How soon will new build render go green again?
On shaded and north-facing walls, growth can start to reappear within two to four years, sometimes sooner near trees or fields. Sunny elevations usually stay clean for much longer. A maintenance treatment every year or two can slow regrowth significantly, and fixing gutters or overflows that keep walls wet makes a noticeable difference.
new build silicone render algae monocouche
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