Concrete slab - prime, screed or sand?

Discussion in 'Subfloor Preparation' started by Meadowbarn, Jul 19, 2021.

  1. Meadowbarn

    Meadowbarn New Member

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    Hello

    I intend to install engineered oak flooring by glueing down using sika 54 wood floor glue, onto our concrete slab. Can anyone advise me how they think I should prepare the slab before doing this job? The floor seems level and flat to me.

    I have attached some pictures which are a random spot on the floor, I went quickly over it with a rubbish vacuum. I tried rubbing with a crow bar and not a lot of extra dust came up, just a little. So seems pretty hard. Don’t seem to be able to upload video to show you

    I’ve had suggested I should sand with very low grit to remove Laitence, but I don’t know we really have that issue (?) , I should use a self levelling screed just to clean everything up, or possibly just prime and then glue. Or maybe just glue!

    Thanks in advance for any help
     

    Attached Files:

  2. tarkett85

    tarkett85 Well-Known Member

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    Grind off laitance if required, do a Rh% test on the concrete, screed ardex k301,surface dpm (which one depends on Rh% reading), prime dpm Ardex p4, screed again to protect dpm Ardex K11 then start install, if you grind the concrete smooth as a billiard table you can skip the first screed.


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  3. Meadowbarn

    Meadowbarn New Member

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    So you reckon this isn’t smooth enough to go straight onto really? I should have mentioned there is DPM and insulation under the slab
     
  4. tarkett85

    tarkett85 Well-Known Member

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    No that’s too rough to bond to, when installing any solid or engineered wood directly to the subfloor you HAVE to use a surface dpm or moisture suppressant (depending on Rh% values which you use) the concrete has to be below 65% Rh all year round and it won’t be which is why you dpm to remove it as being a potential failure point


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  5. merit

    merit Well-Known Member

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    If it’s flat enough you can dpm it and bond straight to it. Silane dpm or epoxy resin. How old is it? Ufh ? Need a moisture test first amd if you have ufh it needs to be fully commissioned and run for 2 weeks


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  6. Meadowbarn

    Meadowbarn New Member

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    It’s over a year old and we’ve had the underfloor heating running. Although not this week as it’s so hot. It is pretty flat! I don’t quite get the idea behind the surface DPM if there’s one underneath it - that’s just to say I don’t understand rather than I disagree with experts!
     
  7. merit

    merit Well-Known Member

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    For engineered it probably doesn’t need one especially if it’s ufh and it’s been down a year. But by the book the subfloor needs to read below 60 %rh which is unlikely.


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  8. dazlight

    dazlight Super Moderator

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    Because all screeds have water in them and if to wet you can’t lay anything on them. So they need to dry out to under 65%RH for wood or use a liquid Dpm on top of the concrete.
     
  9. tarkett85

    tarkett85 Well-Known Member

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    Think of concrete like a big sponge, it will absorb moisture from the ground (which is what the mechanical dpm is there to help with) and the air then it will release it over time going path of least resistance which is usually straight up. If the moisture content in the concrete base is too high it will be released into your flooring causing it to cup and swell, you use a surface dpm to remove this issue from the installation leaving you to only deal with the air humidity which if too high can cause tenting and swelling you want to keep this between 35-40% all year round going too high can cause the issues mentioned, going below can remove too much moisture from the flooring causing shrinkage and possible cracking.


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  10. Distinctive Adam

    Distinctive Adam Well-Known Member

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    What you reckon merit ? Murexin msx3, k509 whammy
     
  11. merit

    merit Well-Known Member

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    Job done. Works a treat


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