new tile over old in second floor bathroom

I want to put new tile on the floor (and in the newly framed in shower with backer board etc) in our second floor bath. The tile that's there was put down about 20 years ago. Now the threshold tiles and some others have come up. In re-doing the bathroom my husband fixed a shower drain leak in the floor near the sink. After fixing it he patched up the floor with some of the same tiles and some mortar. I would like to tile right over all this floor. Question 1: Is it OK to tile over exsisting tile? Or should I tear up all the old tile? Q 2: If I tear up only some of the tiles and the threshold near the door, the surface will be uneven where it meets the tile in the middle of the room. Doing only the entry this way means I won't have to take off the door to shorten it. And I won't have to take up all the old tile. Is this OK? Q 3: It seems that the larger the tile, the more likely it is to crack in a second floor bath, even if a mortar base was put down first. Is this true? or were we unlucky in getting a professional tiler who really didn't do it right the first time?
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Do not tile over old tile. Rip up the old stuff, remove the old mortar, lay down a subfloor and lay the new tile. If you cheat on any of these steps you will just have to do it again in a few months or years.

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"Do not tile over old tile. Rip up the old stuff, remove the old mortar, lay down a subfloor and lay the new tile. If you cheat on any of these steps you will just have to do it again in a few months or years. "
I agree with that.
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snipped-for-privacy@cox.net wrote:

Do not tile over old tile. Rip up the old stuff, remove the old mortar, lay down a subfloor and lay the new tile. If you cheat on any of these steps you will just have to do it again in a few months or years.
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