Grout Joints and Tile Protection
Grout joints are more than a visual part of an installation; they help to protect your tiles from damage. Unless a tile has been machined to be perfectly square, which slate tiles with their rustic edges have not, they can move against one another when the house settles or shifts. This rubbing will cause cracking, flaking or spalling of the slate tiles, with small pieces breaking away. The grout will help the tiles to absorb movement, preventing this problem.
Grout Joints and Tile Size
Most stone tiles, including slate, are not perfectly square and consistent in size. In fact, many slate tiles are ungauged, meaning that they can vary as much as 1/2 inch in size between two tiles. Grout joints make up for irregularities in tile sizes during the installation process.
If slate tiles were to be installed without grout joints, they would shift in their lines. This would create a crooked-appearing installation, with rows of tile that would not line up. Grout joints keep the slate tiles even by absorbing these size irregularities.
Slate Tiles and Uneven Edges
Ungauged, cleft and tumbled slate tiles frequently have broken or chipped corners and uneven edges. These characteristics help lend the tiles their rustic appearance. Because the corners of the tiles may be missing, or the edges of the tiles may be severely uneven in line, grout joints are necessary to fill these gaps.
Even if slate tiles that are ungauged, cleft or tumbled were to be lined up against one another, there would be large gaps or spaces between them where the missing pieces are. Grout fills these gaps and keeps the installation sound.
Minimizing the Grout Joint
If you don't like the look of grout joints, consider using honed, gauged slate tiles. While they are less rustic in appearance, they have a cleaner, straighter edge. Corners are square and intact, and the size difference between tiles is kept to a minimum of 1/16 to 1/8 inch.
It would be possible with honed, gauged slate tiles to use a very narrow grout joint of 1/16 inch without damaging the tiles or causing uneven rows. Using a non-sanded grout in the same color as the slate can further help to disguise the joints.