Mak Wilson wrote:
Thanks for the help Rico. As a reward for your assistance, I pile on a
few more questions....

Lucky me.

Regarding not needing a "big" gap between the wall and the cement
board, I was hoping for a more concrete answer (no pun intended). I
know with plywood you have have to allow for expansion. Is that the
same thinking here with the cement board?

There will be very little movement - nowhere near 1/4". You should
think of it as more of flexing between different wall and floor planes.
In a mud job they'd reinforce the corners with metal lathing and just
continue up the wall. Since backerboard isn't as bulletproof as a mud
job, you should allow for that little bit of movement.

Regarding caulking the wall-to-backer board gaps, is it best to use
100% Silicone or is a tub/tile silicone better? (To be clear, I use
thinset on the backerboard-to-backerboard gaps).

I think you misunderstood me. 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

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.

Thanks again for any help you can offer!

You're welcome.

R