So, I've got some 12"x12"x1/4" tile laid over a concrete floor. I'm cleaning up between the tiles getting ready to grout and I notice I can see some concrete between and underneath (the small amount i can see) the tiles. I recorded this video showing me striking the tiles. The corners that don't look well supported by thiset have an obvious knock sound.
Is this tile doomed to crack?
Will the grout get under the tile and be supportive enough?
Should I try jumping on the tile and only replace it if it cracks?
There are at least 5 tiles like this I've found so far (not all next to each other). A third party laid the tile so I have no other knowledge than what I can see from above.
Update: These tiles are glazed ceramic (purchased on sale for $.49/sqft). I'm sure that the backs of the tiles were not moistened prior to laying. I found one tile that had about a 6"x6" notch cut out of it to fit a corner that on one of its 'legs' also has a knock but much more pronounced. I stomped on this one as best I could and it is not broken.
Best Answer
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.