Help! Tiling over tile in bathroom

Seth M

I'm a first time tiler with some questions about laying tile over a tile bathroom floor. It's a small bathroom approx 30sq ft of 4x4 white tile, ceramic I think. I was going to rip the old tile up and start from scratch but it's laid in a thick bed of mud and since the old tile is rock solid I'm just going to use it as the subfloor.
Questions:
1. Do I need to roughen the surface of the old tile? I've read a belt or oscillating sander will work, not sure what type of sandpaper. Is this true and necessary?
2. Should I place a waterproof membrane between the old and new tiles? If so is a liquid or rollout under-lament preferred? Specific brands would helpful.
3. What type of thinset? Does this depend on if a waterproof membrane is necessary? I'll be shopping at HD or Lowes so any brands you recommend that I can find easily would be a big help.

Thanks for the help, I can provide more info if necessary.
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Seth

Bud Cline

Yup! Ruin it as best you can. A silicon-carbide abrasive sand paper would be best and easiest if you can find it. Otherwise aluminum-oxide sandpaper will work. Use about a 60 or 80 grit.

Why?

This will require a quality modified thinset. Something better than the entry-level products they sell. If your scuffing of the old tile is successful (no shiny glaze remaining) Home Depots Versabond should work okay for you.

Seth M

Thanks for the reply. Another question: what size notched trowel will I need to lay the new tile? It's a 12x12 travertine mosaic (9 pieces of approx 4" per piece on each tile plus 1/8" joints)

Bud Cline

Typically travertine is smooth on the backside but thicker than ceramic tile. Therefore a 1/4" X 1/4" X 1/4" should work. Test it and see.