Render failing again

[sandro]

My garden wall is about 2 metres high by 10 metres long and made of high density breeze block. When it was constructed one side was painted and the other rendered. It was built in 1979. I moved in about 20 years ago and have had it re-rendered twice. The render is now failing again, blowing from the wall and paint flaking off. The wall itself appears to be quite solid but there is a fair bit of green algae on it. It is not a retaining wall being purely a boundary wall
My thinking is that moisture is getting into the wall from the painted side and causing the problem with the render and also encouraging the algae.
Would it be possible to tank the painted side of the wall to prevent water ingress? I would then have the render removed and redone by a professional.
I have read that tanking should not be applied to painted walls but as this is to prevent the tanking being "pushed" off the wall by moisture seeping through, it would not be an issue here as any moisture would be coming on to it from the outside.
I would be grateful for any advice.

 

[^woody^]

What's on the top of the wall (copings), how much overhang and is a drip (underside groove) provided?

 

[sandro]

Hi
Thanks for your quick reply.
The top has coping stones with a plastic membrane between them and the wall. They fit like a cap in that they slot over the top of the wall. There is a 1-2 cm gap between the coping stones and the painted side of the wall. On the rendered side the gap is filled by the render. The bottom of the render has a plastic finishing strip running along it.

 

[^woody^]

On the rendered side the gap is filled by the render.

Not sure that that means, but any coping to a wall, especially a rendered one should overhang by 50mm from the face/render and there should be a drip groove to stop water running back to the face. THis is a major cause of failure of render and spalling of bricks.

For the wall, render should be on the exposed side of the wall and the back left open for drying. Or both sides rendered.

In terms of failure, several possibilities
poor mix
poor application
poor background (bricks or joints)
wall movement
excessive moisture entering the wall
reaction with the wall mortar

A picture of the whole wall showing how the render has failed may help. Otherwise, its a case of determining which cause or causes were at play and dealing with them before re rendering

An external grade paint should be enough to prevent excess water entering the wall (unless its the more exposed side of the wall). A breathatbel silane water repellent would be better than anything, but I don't' think that would work over the top of paint. Otherwise it could be rendered.

 

[noseall]

Rendered garden walls should be for magazines and the Mediterranean continent only. Conditions in the UK are ideal for a high failure rate.