Mak Wilson wrote:
The only place that needs to be caulked
is the floor tile to wall joint...unless you're looking for a
waterproof installation and then I'd go a totally different way

I'm trying not to go too far off topic here, but the backer board is
going over my entire bathroom floor in preparation for ceramic tile. I
also plan to put the baker board under the tub primarily because it's
the easiest way to keep the tub level with the rest of the bathroom.
(Tub is new install).

When you say keep the tub level with the rest of the bathroom, what
does that mean? The backerboard can be done either way - either under
the tub or butting up to it. Doesn't really make a difference other
than you don't have to worry about a precision cut for the backerboard
if it extends under the edge of the tub.

I assume you mean waterproof as in what you'd do
with a shower floor or something like that, so no, there isn't a need
to water proof. I'll leave a slight gap between backer board and wall
1/8"-1/16". I hadn't considered the reason for the gap is for the
expansion of the wood wall studs. That was a good insight.

Oh, when the time comes shouldn't I also caulk where the tile meets the
tub?

Absolutely. Caulk all changes in wall planes, wall to floor
transitions, around all edges of the tub inside and out of the wet
area, etc.

Finally, in true amature form, I don't know the difference between
modified and non-modified thinset, but from what I've read I'm to use
non-modified thinset to bond the backerboard to the plywood. Another
source said to just use constructive adhesive. Any thoughts on that?

Modified thinset has latex additives. It allows for the materials to
flex more - a good thing. I use it exclusively.

I'll probably go with modified then. Although, if you google on
"modified" and "backer board" you might be surprised to find that
there's some debate on this. I don't have the knowledge or field
experience to sort it out.

There's debate on everything, but having a little leeway with flex
between the floor and the backerboard can't hurt. The converse is not
true. It also simplifies your job by not having to keep switching
materials. If you're only doing one bathroom it's not a big deal.

The John Bridge Forums are another good place to get tiling
information.

I guess I'm ready to move forward from here. Thanks again. I really
appreciate people like you taking the time to help out newbies like me.

People like me? People like me?! Crap - here I thought I was unique.

We were all newbies at one point. Answering questions here is my way
of trying to pass on some of the help and instruction I've received
over the years.

Do a good job and have some fun with it.

R