When can I lay LVT on Screed?

Discussion in 'Subfloor Preparation' started by JoeyW, Aug 19, 2022.

  1. JoeyW

    JoeyW New Member

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    *I'm a total flooring novice, but just wanted to get some advice please*

    I'm in the final stages of having an extension built. 10 days ago the builders laid a sand & cement screed on a damp proof membrane with UFH on. The area is 28m sq and the depth was around 80mm. I'm planning to have an LVT floor laid on this.

    My chosen floorer put down a Hygrometer 4 days ago, but after 72 hours, the readings are still fluctuating between 97% and 87%. (I'm not sure why they go up and down?)

    My floorer is planning to come and lay the LVT in 10 days. This will be only 20 days after the screed was put down. He said that if the Hygrometer reading isn't below 75%, he can put down some damp proofing and still lay the floor.

    After reading this forum, it sounds like this might not be a great idea and I should wait for the Hygrometer reading to drop below 75%. Is this damp proofing really a shortcut?

    We're desperate for the flooring to go down so we can finally finish off the extension, but I also don't want to risk damaging the floor by rushing. Please help!
     
  2. J d clarkson flooring

    J d clarkson flooring Well-Known Member

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    Well there’s a few things here…..
    Firstly if it’s a sand cement concrete mix then I’d leave it a minimum of 4-6 weeks to cure before even doing anything!!!!
    Then I’d be saying get your heating system operational at its lowest temperature and increase by 2 degrees a day to optimum 24 degrees then leave a week then turn it back down by 2 degrees as day to commission the system
    At that point I’d do a moisture test and it’s likely to need either a surface moisture suppressant or sandwich dpm.
    Then I’d fit on top of that
    All depends on your moisture readings but even in warm conditions it’s Usually 1mm per day to dry below 75%RH……
    Belt and braces would be epoxy sandwich dpm system
    That’s if it’s standard sand cement screws and not a rapid dry b
     
  3. Rugmunching

    Rugmunching Well-Known Member

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    Don't 'rush' it then!
    Let it dry naturally or go with a sandwich dpm
     
  4. JoeyW

    JoeyW New Member

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    Thanks so much for the replies. Just to clarify, I should wait 4-6 weeks before even considering agreeing to a sandwich DPM?

    Is there a particular Relative Humidity % that we should be aiming for before putting down a sandwich DPM? Or is 75% still the target?

    I was hoping that a DPM would be a magic shortcut to cutting the time down to 3 weeks post-screeding, but looks like it's not the case.

    Also, is your advice not to turn on the UFH for 4-6 weeks? I've read a few different things on this.

    Thanks again.
     
  5. merit

    merit Well-Known Member

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    Sand cement needs to cure before you run the heating. That’s usually around a month. Your heating should be run for 2 weeks and FULLY COMMISIONED after that and before a dpm and installation can be carried out. Unless it’s a rapid set screed but I don’t fully trust them. I would want a dry certificate from the screeding company before installing over that
     
  6. J d clarkson flooring

    J d clarkson flooring Well-Known Member

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    Advice is to let the concrete screed cure which is 4-6 weeks.
    Then turn your heating in and commission it as described.
    So in total your now 6-8 weeks from installation.
    Test for moisture at this point
    Over 90% sandwich dpm
    75-90% moisture suppressant
    Hope that clears it up for you
     
  7. JoeyW

    JoeyW New Member

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    Thank you again. That's really clear.
     

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