On 11 Feb 2006 08:36:58 -0800, "Mak Wilson"
wrote:

I know this has been discussed her before with different responses, but
I'm not sure how treat the seam between backer board (cement board) and
green board in a shower area. I realize this is a specific and rather
picky question.

Rather than make the seam exactly on the tile edge, my first theory is
that the tile should slightly overlap onto the greenboard, maybe an
inch or less.

I should use fiberglass tape and I should use the seam side of the
greenboard. In other words, the side that's indented to allow for the
tape.

snip

Here's what I do. I'm not a pro, but it's worked for me.

I plan the seam so that the tile will overlap the green board by about
1/4 inch. This requires some care. It's best if the bottom edge of
the greenboard has blocking behind it, but not absolutely required.

I do not tape the seam in any way.

I finish the greenboard before I lay the tile.

I lay the tile, and avoid getting thinset on the area where the tile
overlaps the green board. In other words, the tile is attached to the
backer board and not the greenboard.

I caulk the joint between the tile and greenboard with silicon caulk.

Here's my reasoning: any time you have dissimilar backing materials,
such as greenboard and backer board, they are going to move slightly
in relation to one another over time. The silicon handles this with no
problem. This method avoids all the issues you asked about with what
to finish the seam with.

You do have to make sure the tile (and backer) runs high enough that
you won't be getting a lot of water on the greenboard or on this
caulked seam, but you should do that in any case..

FWIW, another option is to use thicker backerboard and then use cap
tiles (the ones with a curved lip on one edge) to cover the thick
edge and then caulk the seam between the cap tile and greenboard as
above.

HTH,

Paul