Grout joints are a controversial subject within the ceramic tile and natural stone communities. While there are overall industry guidelines, they are nothing more than a reference point when it comes to small-scale, residential projects such as kitchen backsplashes. While some installers will install a jointless installation, others choose to run a 1/4-inch joint, or 3/6-inch, depending on the type of tile. The actual size of your grout lines will depend on personal preference and the type of material used.
As a general rule, the guidelines established by the various tile organizations are that the actual grout joint size should be at least three times the actual range of facial dimensions for rectified tile, or those tiles which are manufactured to be relatively the same size. For example, tiles with at least 1/16-inch variation in size should have a minimum joint width of 3/16 inch to hide the inconsistencies. However, these are guidelines, not mandatory requirements.
Jointless installations are a matter of personal preference, and they can range from polished marble and granite tiles installed tight to each other to tumbled marbles and ceramic or porcelain tiles. Jointless installations require special considerations because there still needs to be some type of joint on any inside corners to allow for seasonal movement due to expansion and contraction with the heat and cold of the weather.
Rectified tiles are man-made tiles that are manufactured to a specific dimension. It is for rectified tiles that the guidelines of the various tile organizations exist. While these man-made tiles are designed to be as close to the same size as possible, there will always be variations in the face dimensions. The tighter the joint, the more evident these variations will be, which is why the recommended guidelines state that joint width should be a minimum of three times the size of any discrepancies in size.
Nonrectified tiles are natural stones or clay tiles, such as Saltillo tiles, that are either quarried or in some way manufactured with large discrepancies in their size. For example, tumbled marble or slate tiles can often vary in size by upwards of 3/16 inch for 12-inch tiles. The larger the joint is, the less noticeable these differences are. As a result, the grout line size for natural stones and nonrectified tiles is largely based upon personal preference depending on what kind of look you are going for: rustic or commercial.
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