Thinset depth?

Discussion in 'Remodel Forum & Blog' started by Ghost 60, Apr 14, 2009.

  1. Ghost 60

    Ghost 60 New Member

    Messages:
    3
    I have done a few tile jobs and feel comfortable with standard tile but now my brother wants me to do his Kitchen with 17" tiles.
    I was told "the larger the tile the deeper the thinset". But how deep do you go for this size and does it take a special type?
    Does it require a thicher backer board?
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    First tip, visit the experts on the tile forum, www.johnbridge.com.

    From my experience, I would suggest that 17" tiles do need a very firm underlayment. And yes, the notchsize on your trowel gets larger as the tiles get larger. I would assume a minimum 1/2" notch for those. And I think it is considered good practice to backbutter large tiles, to assure complete mortar contact.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,804
    Location:
    New England
    The size of the tile has no bearing on the thickness of the subfloor since the weight is essentially the same. The larger the tile, the flatter the floor you want to have as lippage between tile becomes much more problematic. If you aren't experienced with large tile, you may want to avoid it!

    The biggest thing with any tile is to ensure that the subfloor and building structure are sufficient to prevent deflection that can crack the tile since it is quite brittle. Ensuring full coverage is important as well, as any gaps can lead to point load breakage...all tile needs to be fully supported (you should get at least 90% full coverage on each and every tile and all edges with the goal of 100%).

    Then, any bumps, dips, humps, ridges in the floor will be magnified by trying to bridge those imperfections. You really want to pay close attention to the prep and get things as flat as possible.

    Some larger tile are also not flat, and end up arched or twisted during the firing process...some are as flat as standard glass...knowing what you have will make a difference in how you set things. The flatter they are and the closer in size (referred to as rectified if sized the same), the smaller the grout joint you can use, but the smaller the grout joint you use, the harder it is to prevent lippage (uneven height of an edge of one tile to another).

    With that large of a tile, you'd have to know how flat the floor is, how flat the tile are before you'd know how deep the thinset would need to be. You may want to go with a medium bed mortar (often called granite and marble mortar) instead of thinset since it has more sand in it and supports large tile better while they set. Ultimate strength is about the same, it's just that they won't move as much in a medium bed if you happen to lean on one before it is set. Depending on the condition of the subfloor, you may need larger than a 1/2" notch...can't say without knowing the tile or the floor. Some tile have a waffle pattern on the backside, and require a bit more than one that is just flat as well depending on how deep the patter is. If the floor was glass smooth, and the tile rectified and super flat with no warps, cups, etc., you could get by with a very small notched trowel...but that is rare.

    You will want to backbutter the tile that size to ensure complete coverage.

    Lay out at least a box or so of the tile and check for squareness and consistency as that will impact the gap and mortar requirements.
  4. GabeS

    GabeS Remodel Contractor

    Messages:
    294
    Location:
    Brooklyn NY
    When you start tiling, lay down the first tile, set it, and then lift it back up. Look at the back of the tile and the thinset coverage. You don't want to see any of the tile (it should all be covered in thinset). If you have ridges when you pick it up, then increase notch size. If with a large notch size (let's say 1/2") you are still not getting complete coverage, then butter the back of the tile (like you would a bagel literally) and spread the thinset on the floor with 1/2" notch.

    If you use the 1/2" notch on the floor AND the back of the tile, you will be building the thinset too high and it will be harder to work the job.

    That's why I say just butter the back of the tile so that it's completely covered with a thin layer of thinset and the spread the thinset on the floor with 1/2" notch and you should be okay. This should also give you some manueverability (not sure if that's an actual word) to prevent lippage.

    Like Jim said, first and foremost the floor structure must meet deflection requirements based on tile type and also must be level before you even start.

    Always use 1/4" cement board on the floor, because it doesn't provide strenght, it's only there because it's more stable that wood. If you need to add strength, you will have to add an additional layer of plywood.
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