QUESTION

We had porcelain tile installed and they put down Schluter-DITRA uncoupling membrane first and then put thin-set over that but the installer put some down and then came back the next day and finished the laying of the tile. they have not grouted yet and we have stopped the process right now as our concern is there are a lot of hollow sounding areas in the tile that we are afraid the tile did not adhere to the floor. should there ever be a delay after you put thin set down? I would think it would dry out? He has already "prepared" another room in our house with thin set over the Ditra tile and we are fearful of letting him go ahead and finish, Your prompt response would be helpful

ANSWER

ANSWER - Hollow sounding tile is not considered a defect.  It can be an indication that there are voids under tiles or that the tile or the underlying components are not bonded for one reason or another.

Hollow sounds can be indicative of the tile installation materials being used, and/or the configuration of the underlying tile substrate.  For instance membranes, non-bonded mortar beds, framed assemblies, and voids in concrete often give hollow sounds in tile installations.

Tiles installed over an uncoupling membrane will tend to sound somewhat hollow.  Schluter-Ditra mat is an uncoupling membrane.  In this case all of the tiles should sound similar.  If certain tiles or certain portions of a tile sound different, then that might be an indication that there is a void under that tile or that one of the underlying components is not fully bonded.

Per ANSI A108.5 and the TCNA Handbook, residential floor applications in interior dry areas are allowed to have up to 20% voids as long as there are no voids greater than 2 square inches (about the size of a golf ball) and that all of the edges and corners of the tile are fully supported.   Interior wet areas, all exterior areas, and many commercial applications are allowed to only have up to 5% voids.

To determine whether or not a hollow sound is an indication of a defect, then you have to remove some tiles that sound hollow and remove some tiles that don't sound hollow and look for evidence of a defect.