Bob~We really kinda need to know exactly what you're setting the time on. "Modified" has latex polymers mixed in (Like HD's Versabond) UNmodified thinset....and PLEASE don't use HD's "Customblend" for anything other than a base for cement board on the floor...DitraSet or Laticrete 317 works quite well for installing a Surface-Applied treatment like Schluter's "Kerdi" Membrane...in fact, they recommend it.

Essentially, for MOST installs, a modified is the way to go.

Still, we need more info on exactly WHAT you're doing...

OK, I'm setting a marble shower pan and then tiling walls floor & ceiling. (5x8 bath) plan to hardi board and membrane everything except the area the shower pan sets. Do I need to do anything other than thin set under the pan ?

I would first check with the manufacturer of the pan for advice on setting material. Over wood or slab? They might say anything from thinset to brick morter to sand!

If no advice is available from them, and it's on slab, I would think that thinset would be fine. If the pan IS actually marble, and is light in colour, use white, modified thinset. Versabond from Homer's is satisfactory.

What sort of "membrane" are you using on the walls...Kerdi? Hydroban? If Kerdi, dampen the walls thoroughly and install with only a quality UNmodified thinset like Ditra~Set or Laticrete 317. Install the tiles with the same thinset.

Your HardiBoard should overlap the flange on the pan for proper channeling of the water to the pan and NOT behind it, but that's obvious, eh?

Thanks lazarususa, I haven't bought the Kerdi or the hardi board yet but thats next on the list. Spent the whole day today fighting with a deteriorated toilet flange and sub-floor around it. That was fun HA! HA!. Oh well its right now.The shower base is a simulated marble ( pretty heavy ) it took two of us to carry it upstairs. There were no real instructions with it just one sheet, it says use motor mix or thin set under the base to insure it is level and has full even contact with the floor. So I'm thinking a dry run set ( proving thats good ) thin set on top of plywood sub-floor should be fine. Am I thinking right ? Thanks Bob

"Your HardiBoard should overlap the flange on the pan for proper channeling of the water to the pan and NOT behind it, but that's obvious, eh?"

"Overlap"?? I have seen other's say it should not overlap because it will bump out the backer board at the bottom.

You would have to shim the board above it to make it line up right?

What I read before is that the backerboard should be on top of the flange, then fill the gap will silicone.

I actually saw Mike Holmes say that the gap should be filled with thinset.

Which way is correct or does it make a difference?