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I've been looking for the method of how to put in 4x5 sheets of cement fiber board (1/4" thick) on top of cement slab, which then the vinyl plank flooring will be installed.

Can I just use a adhesive caulk to fasten them to the cement slab and not worry about the joints?

I need to raise the floor by 1/4" due to the entry-way is that much higher due to it being tile floor.

If I need to worry about the joints, what method should be used?

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    Why are you putting down cement board? Is it to raise the floor? Generally the LVP is good on top of concrete floors.
    – RMDman
    Sep 28, 2023 at 17:08
  • I would not worry about the joints, they will not bother the planks. Why you want the sheets down might help with the answer.
    – crip659
    Sep 28, 2023 at 17:10
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    My brain was thinking of putting in "why", but my fingers didn't get the message. I need to raise the floor by 1/4" due to the entry-way is that much higher due to it being tile floor.
    – Michael
    Sep 28, 2023 at 17:43
  • If you're looking at 1/8" thick LVP, then do 1/2" instead. - You want there to be no transition... but really, all of this for 1/4"? And if you ever want to tile it, then there's this mess all over a perfectly good slab. Do you already have plank and underlayment, and with both in situ, it's still 1/4" shy? Or now it's closer to 1/8" and a threshold will forgive you on that?
    – Mazura
    Sep 29, 2023 at 6:30

1 Answer 1

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You can just use mortar. Spread it with a 1/4 square notch trowel, then lay the cement boards on top. Around here, cement boards come in 3x5' size, not 4x5. Don't worry about the joints.

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  • What if the "cement slab" is sealed?
    – popham
    Oct 29, 2023 at 1:26
  • How much thickness would be added by the mortar/mastic? Might the OP end up with the new flooring being higher than the existing he's trying to match it up to?
    – FreeMan
    18 hours ago
  • @FreeMan The wet mortar displaces a bit when the boards are pressed into it, so at the end will be about 1/4". Regardless, the whole idea is idiotic. The OP should just put a threshold.
    – Cheery
    17 hours ago
  • 1/4" total thickness, including the 1/4" board? Just checking before the OP goes and does this and ends up with the new floor too high. Agreed that it may not be the best option, just confirming the how so the OP doesn't end up very unhappy...
    – FreeMan
    17 hours ago

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