(Also see Trowel Sizes and Type and Size of Tile)
Taking the time to back-butter large tile can make all the difference in having a successful, long-lasting tile installation. Back-buttering is a process of spreading a thin coat of mortar on the back side of the tile with the flat side of the trowel in order to achieve required mortar coverage and proper bond. Back-buttering is generally recommended for tiles that are 15″ or larger on any one side.
Back-buttering is also an essential step when installing clear and translucent glass tile, if the tile does not have an opaque coating on the back. The application of a void-free coat of a white mortar such as MAPEI Mosaic & Glass Tile Mortar on the back of the glass helps to ensure that the installation will have a consistent color and appearance.
Do not back-butter ceramic tile with curved surfaces such as chair rails and rounded, decorative listellos. Back-buttering these shapes can cause cracking.