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I can understand the need for adhesive for wall tiles. But if you have a flat floor, and medium to large tiles, why can't you just lay your floor tiles on the floor without any thinset and grout it. Each tile is fairly heavy so surely that will ensure it stays in place. Also the grout will reduce any movement?

I can't seem to find a reasonable answer as to why it is always necessary to put thinset if the floor is completely flat and tiles are medium to large making them quite heavy.

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    Because if the surface is not completely flat and I, with a mass of 120kg, stand on the edge the tile can tilt or even break.
    – Solar Mike
    Commented Mar 10, 2022 at 11:12
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    Probably because of what you wrote, "grout will reduce any movement". Will reduce, not stop movement. Also would need the floor to be perfectly flat. Over time water might get under the tiles if in a wet room or mopping.
    – crip659
    Commented Mar 10, 2022 at 11:31
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    The bigger the tile is the more area it covers and, therefore, the more likely it is to span a small bump that you haven't noticed. If it's over a bump of some sort, the tipping and cracking mentioned in the previous comments is likely. The thinset fills in those small bumps and dips for you.
    – FreeMan
    Commented Mar 10, 2022 at 12:31
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    Have you laid a long straight edge on your floor? It is not flat.
    – Steve Wellens
    Commented Mar 10, 2022 at 15:18
  • Floor tiles aren't quite as fragile as @SolarMike says (based on my tests with leftovers), but they're not far off
    – Chris H
    Commented Mar 11, 2022 at 20:45

3 Answers 3

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Let's assume your floor is really, totally, flat - to some absurd degree of precision.

Guess what! Your tiles are not. They are nominally flat, but have slight deviations due to the realities of firing ceramic objects, etc. And the larger they are, the more that's true.

They'll likely crack due to inadequate support, but you can lay them down and wait for that to happen, then pull them all up and put down thinset, if you bought enough spares to replace the cracked ones. Or you can just put down thinset to properly support them from the start. Tile is very brittle and cracks easily if not fully supported.

If you really wanted to waste a lot more time, effort, and money than using thinset properly will take, I guess you could grind the floor to some absurd degree of precision and also grind each tile to the same absurd degree of precision. Nobody does that. It's a waste of time, effort, and money. Thinset takes up the minor deviations in floor and tile and supports the tile well as a result.

Or, you could use a different flooring material that's designed to be installed "floating" rather than stuck to the floor.

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    Even if you went the way of grinding and polishing everything, it would only work until the first grain of sand (perhaps a tiny bit of grout that flaked off) got underneath one of the tiles. And then it would be back to square one for you…
    – TooTea
    Commented Mar 10, 2022 at 20:11
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    Morty Experiences True Level
    – Mazura
    Commented Mar 11, 2022 at 0:29
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Thinset is required for several reasons:

  1. Thinset will fill any voids to provide an even support of the tile, regardless the tile's irregulariy or the floor's unevenness.
  2. Glue or thinset will stabilize the tile so that it does not move when grouted, and so that the cured grout is not stressed and brittled due to lateral and perpendicular movement of the tile when under use (e.g. foot traffic)
  3. Thinset will provide additionally required strength, and bond the tile to the subfloor for even more strength. The tile will meet its strength rating when it is installed in thinset, not when it is placed dry.

A floor tile is composed of a hard and durable finish on a brittle substrate (ceramic or porcelain). That substrate will not hold up against drops of hard objects unless it is sandwiched by the finish on top and thinset on the bottom.

Tiles that are suitable for installation without thinset (or a cement bed) are

  • flexible to maintain their integrity as they shape around irregularities, e.g. vinyl click & lay tiles, and as they absorb or yield to shock from dropped hard objects, or
  • thick and strong to provide their own independent support and stability, e.g. outdoor paver tiles
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Small side-reason - Warranty

Most products have a warranty or guarantee against manufacturer's defects, but will also have a clause that the item must be installed to specification.

If a tile broke because it was faulty then the manufacturer would have to replace it.

If a tile broke because you didn't install it right, then that's on you.

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