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Damp patch on new render

Discussion in 'Plastering and Rendering' started by Jack07, 27 Feb 2021.

  1. Jack07

    Jack07

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    Hi,

    I'm looking for some advice on the cause of a potential problem.

    I recently had my extension smooth rendered and there is a small area where it is not drying out. This also showed up on the scratch coat beforehand. It's about 5ft from the floor. Please see pic attached

    There are no leaks from above and the wall is in the sun most days with no rain fall for a while.
    The wall is block cavity with kingspan.

    There's no sign of damp inside but there are kitchen cabinets in the way so can't be totally sure.

    The plasterer rendered 2 coats with no rendagrip or such.

    The only thing I can think is that due to material shortages during lockdown my builders yard delivered the smooth 100mm blocks rather than the rough ones. Almost like a smooth cement block similar to commons. My builder did say the mortar isn't bonding to them as well as normal blocks.

    Any advice would be greatly appreciated as I didn't want to start painting with sandtex until I'm sure what it was.

    Regards

    Jack
     

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  3. Alastairreid

    Alastairreid

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    Did he put a waterproofer through the scratchcoat?
     
  4. Jack07

    Jack07

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    Not sure if he did, I know he was using it but couldn't remember if he used it in the scratch.

    I know he did spray down the blocks beforehand with a hose.

    Does using it in the first coat help or potentially cause issues?
     
    Last edited: 27 Feb 2021
  5. Alastairreid

    Alastairreid

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    Without waterproofer the render absorbs the moisture into the blocks and shows up as damp patches as it dries back out.
     
  6. Jack07

    Jack07

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    I understand. Thanks for the advice.

    Is this something that could dry out eventually? I suppose it's harder if the waterproofer is locking the moisture the other way!!
     
  7. Alastairreid

    Alastairreid

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    Yes it will dry out but will probably happen again.

    The damp lines you see is the mortar bed joints. There is a new build close to me where you can see the outline of every block after its been raining.
     
    Last edited: 27 Feb 2021
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