Flooring – How to repair uneven tile grout

Although I would never do this myself, eHow has a step-by-step for this.

Keep in mind that doing this will add thickness to the floor, so transitions into other rooms might be tricky. According to This paper "an abrupt increase in height" of 1/4" (6mm) "is considered to be a threshold level for consideration of trip hazard for a pedestrian".

If this is done in a bathroom, the toilet flange (and/or other fixtures) will likely have to be raised to compensate for the new floor height.

Tile Over Tile Floors

1 Inspect the tile floor and make sure there are no loose tiles; the existing tile must be in good shape for re-tiling.

2 Clean the tile thoroughly to remove any wax or build-ups on the tile floor. Scuff or sand the tile to make the surface a little rough; this helps the new tile bond to the old tile. Rinse the floor after it has been sanded to remove any dusty particles.

3 Apply a floor leveler to the whole surface of the floor. If you have a raised area, begin there and work your way outwards in a circular motion. The floor leveler will fill in previous grout lines and make the floor level for the tile over tile application. Floor leveler is usually latex-based cement.

4 Use a screed to level the floor; a screed is usually a long and very straight board or level that will move the excess floor leveler to lower levels and will help you level the floor. Allow the floor leveler to cure according to directions, usually 24 hours.

5 Buy thin-set mortar that has a latex additive in it or replace the amount of water with a latex additive. Latex or epoxy type mortars are best for installing tile over tile. Mix and apply according to manufacturer's directions.

6 Set the new tile in small enough sections as the thin-set mortar directions call for. After the whole floor has been tiled, install the grout according to instructions. Seal the grout if directions call for it.

Tile Over Tile Walls

1 Clean the existing tile thoroughly to remove any wax or soap build-ups. Sand the cleaned tiles to rough up the surface for better adhesion. Rinse the sanded tiles to remove excess dust from sanding.

2 Use a latex or epoxy based thin-set mortar to set tiles into. Apply to the surface of the existing tiles according to manufacturer directions, setting the mortar base at a medium depth.

3 Install the tile a small section at a time, according to the thin-set directions. Apply grout and sealer according to directions.

Pry that tile up, it sounds loose on that corner - i.e. not only unsupported, but some of the thinset in contact with the tile did not adhere.

That looks like unglazed clay tile (aka mexican clay tile), which you must moisten on the bottom to get it to adhere well. If you don't, the tile sucks the water out of the thinset before the thinset can bond, and you get what you have with that "klonking" tile.

If it was just voids at the corners, you could just "pipe" some more thinset under the tile with a pastry bag, but after watching that video, I would pry up that tile. Hopefully you have some spares.