How tight can grout lines be?

Are grout lines or gaps required for expansion with ceramic tiles?

An article I read from a tile master said that grout had two reasons:

The first is that the tiles are not perfectly even, so the grout will help make up for the size mismatch. To check this, place nine tiles close together in a 3 x 3 pattern and see if there are any differences in the dimensions horizontally, vertically, and diagonally.

The second reason was the expansion and slight movement could cause the edges of the tiles to peel or crack if they butted directly against each other.

I don't know if I'm completely convinced of the second reason as I've seen some tile installations that don't have grout lines, especially marble. The first reason is particularly useful for tiles with loose dimensions such as stone tiles. Otherwise, your lines could be uneven.

Here is a link to a really good book by an actual tiling master: = 1 -1 & keywords = setting + tile


Personally, I have a feeling that the only way to reduce the expansion and movement of the subfloor is by tiling on concrete. Then you can get away with no grout lines. Otherwise ... that seems unwise. The thickness of your decoupling also makes a difference. For example, if you're using Ditra XL, you could get away with smaller grout lines than if you were using an old-fashioned concrete backing board that's "glued in" with thinset (since the former is made of plastic and has a lot more resilience than the latter that are 3 layers of concrete).