Liquid screed prep before engineered oak

Discussion in 'Subfloor Preparation' started by AutumnL, Nov 5, 2023.

  1. AutumnL

    AutumnL New Member

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    Hello

    We had a liquid screed poured over our underfloor heating. It was thermio+ from tarmac which I believe is an anhydrite screed. It was sold as ‘Laitance free’ but definitely wasn’t. We’ve been in a battle with the UFH company who were adamant it didn’t need sanding. After lots of back and forth they’ve finally been round to sand. Majority of it looks ok but still a few questionable patches. First question - how do you know if all of the Laitance has been removed?

    We have glue down tongue and groove engineered oak to go on top of it, we bought the adhesive from the company alongside the floor. We cannot find anyone near us who has done this kind of floor over this kind of screed! Second question - what primer is best for the screed before the floor goes down? I want to make sure we are informed before going ahead as we’ve had different advice.

    thank you
     
  2. Rugmunching

    Rugmunching Well-Known Member

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    Can you see the aggregate in the concrete slab?
    Is it consistent?

    Have you gave the questionable bits a further scrape?

    What procedure did the company use to grind of the laitance??

    I've always damp proofed and screeded with the fully fixed wood, can be over kill to some but I take no chances on comeback if it ever wanted to start lifting.
     
  3. AutumnL

    AutumnL New Member

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    Hi

    Thank you for replying. You can see the aggregate in places but then there’s these patches dotted all over. I’ve scraped them with a knife and they don’t seem to be budging. Tried to attach some photos but it says they’re too large.

    The company used a sander I believe. It had black plastic-like sheets rather than the cheapy red grit paper. But definitely not a metal grinder

    Please could you advise which products you’ve used? I read that you can’t use a dpm primer on anhydrite with ufh(?) but then I’ve read that much my brain feels like it’s going to die

    thanks
     
  4. AutumnL

    AutumnL New Member

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    Managed to attach photos now. 1st one is with no sanding, 2nd is a patch we used a hand sander on, 3rd is wider floor area after company sanded the lot (these patches don’t scratch off with a knife)

    IMG_0637 2.jpg IMG_0643 4.jpg IMG_1004 3.jpg
     
  5. Rugmunching

    Rugmunching Well-Known Member

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    My customer couldn't wait for the slab to fully dry so I got the uzin rep in to advise and was costly, not going to lie.
    If I remember rightly it was their epoxy primer, dpm on top of that followed by their grit primer then a nice flow of screed.
     
  6. tarkett85

    tarkett85 Well-Known Member

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    That’s what I would have recommended, I would usually get Ardex out to spec the prep and get it in writing before doing anything for any job that could be problematic.


    Sent from my iPhone using Tapatalk
     
  7. merit

    merit Well-Known Member

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    Speak to Wakol. They have the perfect system for this. It just needs a bit more grinding. Run the heating and fully commission it. Take a moisture reading and when it’s low enough then you can dpm and bond to it
     
  8. dazlight

    dazlight Super Moderator

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    Uzin 425
    Uzin PE404 2 coats
    That’s a system we have used.
     
  9. mjfl

    mjfl Well-Known Member

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