Green algae / staining appearing on rendered corner – possible damp, splashback or drainage issue?

[george121212]

Hi all,
I’d be grateful for some advice regarding green algae and staining that has appeared over the winter on the corner of my house.


Background
The house was fully rendered around 5 years ago and the issue is isolated to one corner only.
Behind the render is a 1970s extension.
The problem area has developed a green algae-type growth – no cracking visible in the render.

Checks I’ve already done:
Checked the guttering above – no blockages, leaks or obvious faults.
Checked the area below – I can’t see the damp-proof course clearly where the blue engineering bricks are. Dug a little around the base and the mortar lines seem dry.
There is a damaged drainage channel nearby with a small crack which I have removed. It’s right at the top of the run so didn’t carry water and the channel has been draining.
Patio has been relaid recently and the highest point is one brick away from the render.

Other observations:
There is also green growth visible slightly further along the wall around the corner as per photos making.
This makes me wonder whether it’s:
Splashback from the raised patio or Ground levels bridging the DPC?
There are no obvious leaks from the nextdoor neighbours.
Moisture tracking from the cracked drainage channel?
Or possibly moisture getting behind the render?

My current thoughts
I’m planning to:
Repair the cracked drainage channel.
? Use dry rods in the mortar line below the rendering.

Questions
Does this sound more like a splashback / ground level / drainage issue rather than a render failure?
Could bridging the DPC by the new patio height be the real cause?
What would be the best way to confirm the source of the moisture?
Any advice on what to check next would be much appreciated.

 

[Wayners]

No airflow to dry out that bit after rain.
Could you create a rain cover?

 

[jj4091]

Always happens in shady areas. Pressure wash off.

 

[^woody^]

Is that render a silicone one coat?

Don't pressure wash render, it will erode the surface making it rougher and more susceptible to future dirt and algae stains.

Just brush on some biocide - Wet and Forget or Path Clear or similar. That will give lasting protection.

 

[george121212]

Thanks for the great advice , sounds really reassuring will try some.biocide first and see how it holds up, don't think it's is silicone has got some masonary paint on.