Render Cleaning 101: What’s Growing on Your Walls and How to Stop It

Hi it’s Ryan from Wash Off N.I, I created this blog post today because I noticed a lot of my client’s don’t think about their render at all.

And when I said “at all” I mean it’s probably the most neglected part of the home.

So if I asked you to go outside and glance at the side of your house…

Would you see a faint green streak creeping up the side of the house?

You can tell yourself it’s nothing, maybe it’s just dirt or maybe a bit of weathering.

All part and parcel of owning a home, yeah?

But here’s the truth:

That green smear is alive.

It’s feeding on your render.

It’s holding moisture against your walls. 

And left untreated, it’s going to cost you.

And it’s going to cost you a very pretty penny 💸

Let’s Start with the Basics: What Is Render?

Render is the smooth, protective coating on the outside of your home.

It’s there to shield your walls from the elements while giving your home that clean, modern finish.

It’s popular across Belfast, Antrim, and throughout Northern Ireland, especially in new builds and refurbished homes. 

You’ll find it on everything from small bungalows to modern estates.

But here’s the thing…

Render absorbs water very easily.

This means that it collects grime, and becomes the perfect home for biological growth like:

• Algae – causes green vertical streaks
  
  
  

• Lichen – white, crusty blotches that bond tightly to the surface
  
  
  

• Black mould – those dark stains in shaded or damp areas
  
  
  

You might think it’s just a cosmetic problem.

But it’s not.

The Real Damage Happens Slowly

Most homeowners ignore these streaks and spots because they seem harmless.

But what’s really happening under the surface?

• Moisture is being held against your walls
  
  
  

• Your render is beginning to erode
  
  
  

• The material underneath is starting to degrade
  
  
  

• Your home is losing insulation and allowing damp to spread inside
  
  
  

Over time, this can lead to:

• Cracks in the surface
  
  
  

• Discolouration that won’t wash off
  
  
  

• Cold spots in your home
  
  
  

• Internal water damage and mould
  
  
  

It’s one of these things that when you spot the signs it’s already too late.

So this means prevention is not just smart but necessary.

Why Northern Ireland Makes It Worse

If you live in Northern Ireland, render issues appear faster than in other places.

Here’s why:

• We get frequent rainfall
  
  
  

• Our homes experience long periods of surface moisture
  
  
  

• There's limited direct sunlight to dry walls
  
  
  

• Many properties are shaded by trees or neighbouring homes
  
  
  

That means your render stays damp longer. 

That’s all algae and mould need to start growing, quietly, persistently, and deep into the surface.

Why Pressure Washing Doesn’t Work on Render

This is the biggest NO NO in exterior cleaning. 

This mistake breaks my heart everytime I hear it happening and I hear it at least once a week.

I’ve had clients notice algae or black streaks and decide to blast it with a pressure washer.

It looks like it works, at first.

But then the render starts to flake.

You see new cracks forming.

Stains come back even faster.

Here’s the problem:

Pressure washing damages the render.

It’s too aggressive. It strips away the top protective layer, drives water into small cracks, and weakens the wall.

That’s why experienced professionals use soft washing instead.

What Is Soft Washing?

Soft washing is a low-pressure cleaning method that combines specialist solutions with gentle rinsing. It’s designed specifically for:

• Render
  
  
  
  

• Roof tiles
  
  
  
  

• Delicate stonework
  
  
  
  

• Painted exterior walls
  
  
  
  

Unlike pressure washing, soft washing doesn’t damage the surface.

It targets the biological growth, killing algae, mould, and lichen at the root.

That means:

• Longer-lasting results
  
  
  
  

• No structural damage
  
  
  
  

• A cleaner, brighter home exterior
  
  
  
  

And because it’s a controlled process, it’s safe for residential exterior cleaning, even on older or fragile buildings.

Try This: The Render Test

Here’s a quick test you can do in under 10 seconds.

Go outside.

Place your hand on your render.

Now ask yourself:

• Does it feel smooth?
  
  
  
  

• Or does it feel dusty, rough, or slightly slimy?
  
  
  
  

• Is it cool and damp, even in dry weather?
  
  
  
  

If you answered yes to the last two, there’s likely biological growth forming. And that means your render is at risk.

This is exactly how stained render starts, and it’s often ignored until deeper damage occurs.

What Happens If You Ignore It?

When algae, lichen, and mould are left to grow, you risk:

• Surface degradation – the render breaks down slowly
  
  
  
  

• Water retention – the wall holds moisture, inviting damp
  
  
  
  

• Loss of insulation – cold gets in, heat escapes
  
  
  
  

• Costly repairs – cracks, re-rendering, and repainting
  
  
  
  

That green streak on your wall might cost you £100s in heat loss or £1,000s in restoration down the line.

Cleaning might seem optional, but it's part of smart home maintenance.

How Often Should You Clean Your Render?

Every house is different, but here’s a general guide:

• Every 1 to 2 years is ideal
  
  
  
  

• If your home is in a shaded area or near trees, aim for every year
  
  
  
  

• If you see green or black streaks appearing, it’s time
  
  
  
  

Render cleaning should also be paired with gutter cleaning to prevent streaks caused by splashback or overflow.

What’s the Best Time of Year to Clean?

Spring and early summer are ideal for render cleaning.

The weather is mild, surfaces dry faster, and there’s less risk of regrowth.

But if you’ve just come through a rough winter, or if you’re preparing to sell your home, autumn works just as well.

The key is not waiting until the problem becomes visible from the street.

Homeowners Across Belfast and Antrim Are Already Acting

Right now, people across Northern Ireland are:

• Booking soft washing to remove render stains
  
  
  
  

• Clearing gutters to prevent algae from streaking the walls
  
  
  
  

• Cleaning paths and driveways before they become slippery in winter
  
  
  
  

• Preparing their homes for summer sales or Airbnb listings
  
  
  
  

Why?

Because they understand something simple:

A clean exterior protects what’s underneath.

Before and After: What Render Cleaning Really Does

The transformation is real. 

A render clean can make a 15-year-old house look five years old.

Without paint. 

Without repairs. 

Just the right technique.

It revives the true colour, wipes away old dirt, and makes your home look sharp.

Better still? 

It gives homeowners peace of mind.

No more worrying about damp.

No more patchy walls.

No more pretending it’s “just the weather.”

The Long-Term Benefits

Render cleaning isn’t just a one-time job.

Done consistently, it:

• Extends the lifespan of your render
  
  
  
  

• Prevents costly restoration work
  
  
  
  

• Improves your home’s energy efficiency
  
  
  
  

• Boosts property value (great if you're selling or refinancing)
  
  
  
  

• Keeps your home looking fresh year-round
  
  
  
  

And in Northern Ireland’s wet climate, exterior home cleaning is one of the smartest investments you can make.

Final Thought: The Outside of Your Home Deserves Attention Too

We clean our kitchens.

We repaint living rooms.

We organise garages.

But when it comes to the outside of our homes, most of us wait until the green streaks are impossible to ignore.

Don’t wait!

Take a walk around your home today.

Check the north-facing wall.

Look for rough patches, streaks, or mould near the base.

Because the sooner you spot it, the easier it is to fix. 

And now that you know what to look for, you’re already ahead.