QUESTIONHow much thin-set mortar adhesive contact should there be between the tile and its substrate in order to meet industry standards and be properly installed? Is spot bonding an acceptable installation method?
ANSWERANSWER - Per ANSI A108 and per the TCNA Handbook for Ceramic, Glass, and Stone Tiles they say that the thin-set adhesive coverage under the tile should be 80% with full support along tile edges and corners for interior dry residential applications. But for interior wet areas, exterior areas, and for natural stone applications the thin-set coverage should be 95% with no voids larger than 2 square inches (size of a golf ball) and that all corners and edges are fully supported.
Commercial applications should also be required to achieve 95% thin-set contact too considering the higher and diverse traffic it will be subjected to.
Full adhesive contact is defined as continuous contact between the back of the tile and its substrate. Spot bonding tiles is not acceptable unless an epoxy is being used for interior wall applications only. Spot bonding is where only dabs of thin-set are applied in the four corners and in the center of the tile.
If the tile is installed correctly by troweling the thin-set mortar perpendicular to the short side of the tile, and then forcibly embedding the tile and shifting it back and forth perpendicular to the thin-set mortar trowel ridges, you can easily obtain substantially 95% thin-set contact; as long as you use enough thin-set mortar. You need to periodically lift freshly installed tiles to verify you are achieve the correct thin-set coverage. After installing the tile use your margin trowel to fill in any voids along the edge of the tile prior to installing a tile adjacent to it.
The NTCA produced a 7 minute video showing how to properly install tile to achieve substantially 95% thin-set contact. It is called the NTCA Trowel and Error installation method.
Click on the following link for the English version: NTCA Trowel and Error English
Click on the following link for the Spanish version: NTCA Trowel and Error Spanish