During the process of selecting a method, you will encounter height differences between existing or non-involved floors and the new installation. If the new installation does not involve a height difference in excess of 3/4 and the adjacent areas are carpeted, a threshold may not be necessary (as in Figure A).
Tile Doctor Tip: In all cases where the new construction might pose a tripping hazard, a threshold or ramp is necessary. Slip and fall injuries exceed all others in public injuries.
For installations like those in Figure B, C, and D, a threshold will be necessary. The threshold is designed to bridge the height difference at the intersection of dissimilar floor heights. Thresholds are commonly available in stone materials like marble that can complement the tiled installation. They can also be made of wood or metal. Thresholds are selected by thickness depending on the required installation. If I were choosing a threshold for Figure B, I would desire a thickness not in excess of 3/4″. This allows me to set the threshold in a mortar bed at the height of the new tiled surface. Remember that the joint between the new tile and threshold should be treated as an expansion joint and a sealant used instead of hard grout. Thresholds of marble can be cut with a tile or dry cut off saw to the desired width. The joints at the doorjambs should have a sealant applied also.