Rendered garden walls crumbling

[Olive71]

Morning all,

I have some 'professionally' rendered walls in my garden. They are crumbling, paint is flaking off (proper masonry paint) and I fear I need to hack off the render and redo.

1) Or is there an easy way to patch the render that is flaking - keeping in mind that it is a smooth flat finish...

Untitled

• Olive71
• 2 May 2010
• 2

2) I am wanting to know what causes the render to be so crumbly - is this a bad mixture, not enough cement?

3) Also with regards to the painting, the paint has flaked off - I heard I should apply a watered down coat onto the render (once dried) to allow it to 'key' before applying the full coats... is this accurate?

Cheers!

 

[Thermo]

how long ago were they done?

 

[Olive71]

12 months....

 

[Olive71]

Thoughts anyone... hack and redo?

 

[theoldun]

Olive, Give me some answers, and will tell you what I think problem is.
Did you use 100mm dense concrete blocks?
Did you put DPC in walls?
You say they were done 12 months ago.
Please be more specific. Was structure built 12 months ago, if so how long ago was render done, and can you remember what month?
How long has masonry paint been done?
Make of masonry paint?
Is render on both structures crumbly and were they done at same time?
Do you know whether plasterer put waterproof scratch coat on. first day and top coat next day?
Did you notice him put any fairy liquid in his mix, or any admix?
Is fence right hand side facing south or west?
old un.

 

[Olive71]

Answers you want, answers you shall have...

100mm concrete blocks, medium density.
No DPC.
Everything was built about 14 months ago from scratch - Feb.
Paint went on immediately following render (several days)
Sandtex brand paint
Both sides have the issues (but only in patches - some parts are fine)
Scratch coat, unknown and likewise the admix, although the render did look nice and fluffy, so possibly so...
The RH fence is facing West

Thanks!

 

[theoldun]

Olive. Will tell you what I feel your problems are. Am not a scientist or chemist, and my observations are only based from being far, far to long in this industry.
You do not say how long after blocks were laid that they were rendered.
I am going to assume about a fortnight. Now work was carried out in Feb, not a very good drying month, consequently, fair bit of moisture content still in blocks when rendered.
No big deal if only rendering one side, but you have rendered both sides trapping moisture content inside. Now if waterproof scratch coat was also used both sides, this makes it a more effective sealing skin, for trapping moisture in block work.
Now we come to the crux of the problem. No DPC, consequently them old blocks are sucking water up that wall every time it rains.
You gave it a couple of coats of Sandtex two days after render complete. February, no sunshine, no drying. Render to much moister content to really want to accept paint. Masonry paint manufacturers all state that their paint is breathable, in the next breath they say it is water proof. Now if it is water proof one side it must be waterproof the other side thereby trapping moisture inside.
In short, I reckon there is too many little wet men in there and with the help of a couple of mates, Soluble Salts and Calcium Sulphate they have started to dig there way out, as they wish to evaporate back to where they belong.
Would expect sides facing south and west to be worst affected. If you fill beds with top soil, I reckon that the outside walls will be affected even worse than now.
If you want some ideas how to cure problem, then come back. Hope I have been of some use.
Old un. Guidance only.

 

[Olive71]

Good analysis. Probably correct on all counts.

Sounds like I need to take it back coat, allow sufficient drying before painting and go from there? Too late for DPC..