cement backer board or not?
 

I want place new tile over existing tile for my hearth. I have a 1920's home, the existing tile cannot be removed because that is all there is there. I am extending the tile out onto the old hardwood about 6". do I need backer board to do this? I don't want to put it down because we upgraded to a sealed gas fireplace and I would have to adjust the height of it if I place the backer board- I would only have to slightly adjust the hieght of the front facing without the backer board. If I don't put the backer down will my new tile crack?

How can that be? I suspect there is another reason.:)

You can not install tile over hardwood and you can't really install backerboard over hardwood and expect the tile to last.

Extending the tile over the hardwood while depending on whatever is supporting the tile elsewhere may also not be a good thing.:)

magic tile that sets itself in thin air? I would remove the tile.

The tile is sitting on a concrete base. If you want to extend the tile, you need to extend the concrete base or something of equal structural value.
The tiles will come off with a little effort. If you want the job to last, I suggest you expend the effort.
Ron

There ya go.:)
Crossing over two different substrates using a single tile is probably a mistake.:)