I'm installing luxury vinyl planks with an underlayment in my basement. They will go over top of an existing tile floor. I've heard mixed opinions on whether filling in the gaps in the tile is necessary, especially if you use an underlayment. However, the gaps in this case are quite large - 0.75" wide and 0.1" deep, so I'd like to fill them in. I'm hearing from a few sources that a cementitious product would be best to use like a self-leveling cement, thin set mortar, or grout. However, I'm a little skeptical of how well those will bond to the old grout below and I'm concerned about it cracking over time since it'll only be 0.1" thick, but perhaps those concerns are unfounded. The other products I've considered are silicone or a construction adhesive, which would likely be a lot more expensive given the amount that I would need but I think they'd bond better and hold up better over time. Can anyone provide any insight?

  • Is the bottom of the gaps clean cement/grout, or full of dirt?
    – bobflux
    Commented Dec 31, 2021 at 12:27
  • "Embossing leveler" is usually the correct product. It's designed for exactly this.
    – isherwood
    Commented Dec 31, 2021 at 15:34

2 Answers 2


"Embossing leveler" is usually the correct product. It's designed for exactly this. Read some labels and choose one that claims to work on ceramic.


This may be a little unconventional, but I believe it will work will. Since this is just to fill in gaps and then will be covered with underlayment and planks, I would use polymeric sand. Pour and brush it in all the gaps, then wet it down with water. Let it set up and finish up the rest of your project.

  • 1
    I would not use anything that's going to disintegrate into dust and grit with movement.
    – isherwood
    Commented Dec 31, 2021 at 15:35

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