Can I lay tile on top of existing tile in a shower?

Have a shower with wall tiles from floor to ceiling. The wall tiles are fine, the floor tiles I like to replace. I don't want to damage the shower pan underneath, so is it possible to lay new tiles on top of the existing tiles (which seems to be solid). If so, do I need to apply some sort of impervious layer first before laying second layer? Do I need to use some special thinset? The existing tiles are small, 1"x1" ceramic tiles, I would like to replace with even smaller stone mosiac tiles (the ones you buy that comes in a 12x12 sheet).
The other question will be the shower drain. The new tiles will raise the floor level in the shower which is not a problem - the curb is 8" tall now. But the drain will be lower than the floor. Should I raise the drain (if so how?) or do something else?
Thanks,
O
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orangetrader wrote:

orangetrader,
There are membrane products that can be used to provide the impervious layer you mentioned. I believe Ultraset, which is made by one of the thinset manufacturers is one of them and can be spread and allowed to set up hard before tiling over it. I inquired about such a product for another purpose but you probably should check with a tile store.
I've seen drain extensions before. Surely these wewre made for just the reason you need it, because thhey seemed to be about the thickness of a tile. They were 2 or 3 piece incl. a metal ring, a square frame to go around it, and a flat strainer. IIRC they were chrome or antique brass in finish.
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If you go over it you need very good prep, a very good cleaning, bleach to kill mold and acid etch.
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