scratch coat render on block work outside wall cracking

H

halfahippy

New Member
HI I have done a scratch coat on a wall the mix was 4.1.1 plastering sand, blue circle premium cement and lime with a glug of water proofer. it is showing some cracks not major ones but still cracks which I believe? is because I stupidly used the premium cement instead of the ordinary. For the top coat my mix is? going to be 5.1.1 with a glug of water proofer but this time with OPC not the premium. Any advice please will I get away with the cracks or will they come through in the top coat? Other than the few cracks the scratch coat is sound I dont really want to have to hack it of and start again if I can help it. Also before the scratch the wall was treated to a coat of rendagrip which again took really well no problems, any advice/tips would be great thanks.
 
halfahippy said:
HI I have done a scratch coat on a wall the mix was 4.1.1 plastering sand, blue circle premium cement and lime with a glug of water proofer. it is showing some cracks not major ones but still cracks which I believe? is because I stupidly used the premium cement instead of the ordinary. For the top coat my mix is? going to be 5.1.1 with a glug of water proofer but this time with OPC not the premium. Any advice please will I get away with the cracks or will they come through in the top coat? Other than the few cracks the scratch coat is sound I dont really want to have to hack it of and start again if I can help it. Also before the scratch the wall was treated to a coat of rendagrip which again took really well no problems, any advice/tips would be great thanks.
Click to expand...


Get some pics up of the cracking to get a better idea.
 
halfahippy said:
HI I have done a scratch coat on a wall the mix was 4.1.1 plastering sand, blue circle premium cement and lime with a glug of water proofer. it is showing some cracks not major ones but still cracks which I believe? is because I stupidly used the premium cement instead of the ordinary. For the top coat my mix is? going to be 5.1.1 with a glug of water proofer but this time with OPC not the premium. Any advice please will I get away with the cracks or will they come through in the top coat? Other than the few cracks the scratch coat is sound I dont really want to have to hack it of and start again if I can help it. Also before the scratch the wall was treated to a coat of rendagrip which again took really well no problems, any advice/tips would be great thanks.
Click to expand...
You haven't stated what substrate you're applying to, that mix could be completely wrong.
 
essexandy said:
You haven't stated what substrate you're applying to, that mix could be completely wrong.
Click to expand...
Sorry thought I had here is a pic of the blocks a type of pumice block
 

Attachments

  • scratch coat render on block work outside wall cracking
    IMG_20250318_115737_HDR.webp
    78.5 KB · Views: 13
  • scratch coat render on block work outside wall cracking
    IMG_20250318_115716_HDR.webp
    468.9 KB · Views: 13
  • scratch coat render on block work outside wall cracking
    IMG_20250318_115718_HDR.webp
    318.3 KB · Views: 13
halfahippy said:
Hi Bob not the best pics but hopefully you can see it ok
Click to expand...

I wouldn't be overly concerned

Not great but not serious.

Look like shrinkage, as long as it's sound you should be ok. The cracks arent continuous are they?

They look real short.




What blocks were they?

Did you do it in strong sunlight or did the sun hit it?
 
Thanks for your reply, they are a pumice type concrete block, done on a cloudy day no sun. The cracks are short so hopefully it is just a bit of shrinkage definitely sound I just hope it doesn't happen on the top coat.
 

Attachments

  • scratch coat render on block work outside wall cracking
    IMG_20250318_115716_HDR.webp
    468.9 KB · Views: 7
halfahippy said:
Thanks for your reply, they are a pumice type concrete block, done on a cloudy day no sun. The cracks are short so hopefully it is just a bit of shrinkage definitely sound I just hope it doesn't happen on the top coat.
Click to expand...

s**t blocks

I wouldn't worry

As long as it's sound, you could always f**k a tight one over that using your same mix again to be sure.

That shouldn't crack theoretically (due to shrinkage anyway).
 
bobski said:
s**t blocks

I wouldn't worry

As long as it's sound, you could always f**k a tight one over that using your same mix again to be sure.

That shouldn't crack theoretically (due to shrinkage anyway).
Click to expand...
If he puts a tight coat of the same mix over the top what mix is he supposed to use for the top coat?
 
bobski said:
s**t blocks

I wouldn't worry

As long as it's sound, you could always f**k a tight one over that using your same mix again to be sure.

That shouldn't crack theoretically (due to shrinkage anyway).
Click to expand...
Yes indeed very s**t blocks not good at all. I think your right about not worrying and giving it a tight one which i will do then il do the top coat probably at the weekend weather permitting and hope it goes ok fingers crossed. Again many thanks mate nice one for the advice very much appreciated (y)
 
halfahippy said:
Yes indeed very s**t blocks not good at all. I think your right about not worrying and giving it a tight one which i will do then il do the top coat probably at the weekend weather permitting and hope it goes ok fingers crossed. Again many thanks mate nice one for the advice very much appreciated (y)
Click to expand...
Bobski is a wealth of advice but I say just go over it with float coat it’ll be fine
Think about it - your worried about a hair size crack yet happy with scratcher size tooth thick furrows?.. if those furrows don’t worry you why would a hair size crack (if scratch coat is sound and chemically bound to subsrtrate)
 
Last edited:
bobski said:
The weaker top coat mix he had planned to.
Click to expand...
I don't think you understand how successive weaker coats work mate. It's not just about what the mix ratio is, it's about the actual strength of each coat. That's why your scratch coat should also be thicker than your top coat.
 
essexandy said:
I don't think you understand how successive weaker coats work mate. It's not just about what the mix ratio is, it's about the actual strength of each coat. That's why your scratch coat should also be thicker than your top coat.
Click to expand...


It's to do with weight of binder mate.

He'll be going

Same
Same

Weaker.

Nothing wrong with that.

Like dub, dub , top.
 
bobski said:
It's to do with weight of binder mate.

He'll be going

Same
Same

Weaker.

Nothing wrong with that.

Like dub, dub , top.
Click to expand...
Sorry mate but you're wrong. A tight coat of 4:1 will be weaker than a full coat of 5:1 going over the top of it.
Like I said before, it's not just about ratios, it's the actual strength of each coat.
 
Monkey Boy said:
Bobski is a wealth of advice but I say just go over it with float coat it’ll be fine
Think about it - your worried about a hair size crack yet happy with scratcher size tooth thick furrows?.. if those furrows don’t worry you why would a hair size crack (if scratch coat is sound and chemically bound to subsrtrate)
Click to expand...
Agreed and that is what bob was saying don't worry it should be fine but if I wanted to be extra sure I could give it a tight one, as for the furrows I get what your saying but the furrows/scratch is there for a reason and I would say are not quite the same or concerning as what cracks are.
I could be wrong but my gut feeling is telling me the reason for the cracking is that I used premium cement instead of the ordinary cement its the only thing i did different, I certainly wont be using it again lol at least not for rendering anyway
 
Last edited:
You must log in or register to reply here.
Back
Top