rendering breeze block wall

[cooksta]

Just wondered if anyone could give me advice on rendering a breeze block wall.
I have done scratch coat with waterproofer/plasticiser.
Then the top coat is applied and floated.
All is good until the wall dries........to my horror there are cracks and the render is blown!!!!!!!!!
Do i need to add lime to the mix, if so what lime do i need and in what ratio.

 

[Micilin]

Just wondered if anyone could give me advice on rendering a breeze block wall.
I have done scratch coat with waterproofer/plasticiser.
Then the top coat is applied and floated.
All is good until the wall dries........to my horror there are cracks and the render is blown!!!!!!!!!
Do i need to add lime to the mix, if so what lime do i need and in what ratio.

The bottom coat has come away from teh block

 

[jrplastering]

did you wet down the surface before applying the scratch coat and what waterproofer did you use

 

[Richard C]

I prefer a natural lime mix rather than plasticiser as I think it gives a softer more tollerant mix but with either, hose down & then hose down some more; it's just too dry.

 

[GlenT]

Just wondered if anyone could give me advice on rendering a breeze block wall.
I have done scratch coat with waterproofer/plasticiser.
Then the top coat is applied and floated.
All is good until the wall dries........to my horror there are cracks and the render is blown!!!!!!!!!
Do i need to add lime to the mix, if so what lime do i need and in what ratio.

I got some usefull advice from celcon regarding this.Dont know how to uploaod it on here but i have put it on handyman website under rendering celcon blocks. They state to wet the wall but dont overdo it.6:1 scratch coat.Allow to fully cure(3 days) top coat weeker and thinner than 1st coat.They dont recommend using waterproofer because overdosing can cause problems,but i will be putting some in my scratch coat.Render should not be stronger than the material,hence the weakmix

 

[Micilin]

A 7:1 float coat?

The trouble is - in truth those blocks are not designed for any kind of plastering.

 

[GlenT]

They state that a strong mix makes it too rigid.The more cement you use the more water you use which means more shrinkage when curing which means more cracks

 

[cooksta]

did you wet down the surface before applying the scratch coat and what waterproofer did you use

sorry for late reply i did hose down wall before scratch coat.
I used wickes waterproofer and plasticiser.
Do i actually need a waterproofer??
what is the advantage, I have used waterproofer in both scratch and finish coat but i think this may be why the top coat is coming away from the scratch?????? (because of water resistancy??????)

 

[cooksta]

yes i gave the blocks a good hose down before the scratch coat, when that dried i splashed water and PVA mix on before doing finish coat.
Should i have hosed down scratch coat instead of PVA and water?

 

[marshman]

Did you leave your scratch coat with a open course finish or did you leave it looking nice and smooth ?.

 

[cooksta]

open course finish. Did it with a rendering comb thing!