Before installing tile on your floor you must make sure your floor is properly prepared.  A properly prepared floor does not have to be level. It must, however, be flat.

The only time the levelness (is that a word?) must be taken into consideration is when drainage is an issue, such as on a porch or in a shower. In those cases you must make sure your floor is not level – it has to be angled toward a drainage area.

If your floor will not be subjected to water regularly, such as a kitchen or bathroom floor, it does not necessarily have to be level. That does not mean you can have a 45 degree angle from your door to the cabinet (although I suppose you could if you wanted), it just means if your floor is not absolutely level it will not negatively affect your tile installation.

One of the things you must make sure of, among other things, is that your floor is flat. If it is not it will be difficult to set your tiles without what we call “lippage”. That’s a ridiculous word, isn’t it? Lippage simply describes the difference in the height of two adjacent tiles. If you have a tile that sticks up higher than the tile next to it you have lippage. You don’t want that. Starting with a flat floor helps prevent it.

When prepping your floor for tile trade your level for a straight edge. Don’t be concerned with how level your floor is, be concerned with how flat it is.

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  • Loraine

    Hello- my “new” home is very old has sloped areas. I have a plywood subfloor in place so it is flat. The question … do I add extra mastic/thin set to make it level OR do I keep the slope? Also which is better articles or thin set?
    Thank you much

    • Roger

      Hi Loraine,

      It only requires flat. If you want it level you’ll need to do it with something other than the mortar if it’s more than 1/4″ or so. Most mortars and thinset have a maximum thickness.
      I don’t know what you mean by “articles” or thinset. You want thinset for that.

  • claire kauffmann

    Hey floor elf! Does a stable/good quality tiled floor qualify as flat enough? Slight variation (at grout lines) and the new tiles aren’t the same size, so won’t follow the same pattern. I am hoping the thinset would be OK to level these minor ‘lips’ but now second guessing myself! The new tiles are quite thin (6mm porcelain tile), so I am wondering how flat it will need to be to support the tiles really well (but maybe my fear here is unnecessary?)

    • Roger

      Hi Claire,

      It can be, but it does require prep if it is stable enough. You want to ensure that whatever you put over it will be able to bond to it, that normally requires scarifying the surface of the existing tile. An alternative to that, and much less messy, is to use a proper primer like Mapei eco-prim grip, which can be rolled over it, then thinset will bond to it just fine. The variation of the grout lines will not significantly affect the installation, thinset will fill those as you set the new stuff. You just need to ensure that the surface is properly prepared so thinset will bond to the existing tile.

      • claire

        Great thanks so much for the advice! I was feeling a bit worried that 6mm is a bit thin for a porcelain tile. I see though that they can come even thinner than that these days. If the floor is well prepped and the tiles properly supported, do you think (in general) it’s ok/reasonable to install a 6mm tile in a residential space?

        • claire

          And hello from Australia by the way! :) I don’t think I’ve ever posted on a US blog/forum before!! How is life in your part of the world?!

          • Roger

            Hello Claire!

            Sorry, just saw this.
            It’s the US, so, you know, we’re kind of a shitshow right now. :D

            Not ALL of us are contributors to the shitshow, but there’s enough to give that sweeping description of the country…

  • Allen

    Regarding floor flatness with concrete subfloors, I have some questions:

    1. How do you ideally flatten a large area? Especially when you are working alone or with one other helper?
    2. When flattening the subfloor and possibly using something like leveling compound, how do you handle the sawed in control joints? Just cover them up? Fill them with backer rod, foam, or something else?
    3. What’s the easiest way to check floor flatness over a large area?

    Thanks in advance! Also curious if just learning and using the mud set method is better than worrying about getting the floor perfectly flat.

    • Roger

      Hi Allen,

      1. Self-leveling cement
      2. Duct tape over them (really) or fill them with backer rod (cylindrical foam). They are only there to tell the slab where to crack, either method will keep that intact.
      3. Use a laser level and a 5 gallon bucket. Set up your level, shine it horizontally to a spot on your bucket and make a mark. As you move the bucket around the floor the laser mark relative to the mark on the bucket will show you whether it is high or low or level with your initial spot. Ideally you want to make the mark with the bucket sitting at the highest level of your floor, then the other spots will tell you how much lower they are than the initial high spot.

      • Allen

        Thanks Roger! This is good info.

        Any suggestions on a specific self-leveling cement and process to use? Would that be better than more of a dry pack type screed?

        • Roger

          Hi Allen,

          Define ‘better’. :D For me a dry pack screed (mud floor) is the best option 99% of the time. However, most diy’ers who are on my site are not equipped to do anything like that. Slc is the easiest option. In the end, both should give you a flat substrate to work with. Just prime your subfloor, let it cure, then pour the slc following the directions on the bag (they differ).