When installing a new tile tub surround it is critical to make sure the substrate or tile backer is installed over the lip of the bathtub and the and the joint between the tub and bottom row of tile must be filled with grout caulk not just grout. Grout caulk can be purchased from the grout manufacturer to match any color of grout they sell. The purpose of using grout caulk is that it is flexible and will not crack like grout will. If the proper caulk was not used on the original installation and cracks appear it is important to have the grout removed and replaced with grout caulk right away. If you do not repair the crack promptly water will penetrate the wall and cause damage to the framing or the wallboard behind the tile making it impossible to ever get a waterproof seal.
