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
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
1. Should I try my best to squeeze thinset INTO the seam? I'm not
sure if the argument is to have a good seal or if there would be some
need for expansion between differnent materials.
2. What do I mud with? On the BB side I'll go with the thinset, but
do I also use thinset on the Green board side? Thinset, to my
understanding, dries to a hard fine grain hard cement surface. How am
I supposed to sand that down and get a nice smooth surface that will
accept paint? Is the idea to put drywall joint compond on top of the
dried thinset and then sand that flush? Will the indented portion be
deep enough to allow this? This is my main question and it's been
answered in different ways, so if you're kind enough to offer me some
advice, please back it up with as much justification as you can.
3. If the tile overlaps the greenboard seam slightly and if I'm using
the indented side of the greenboard, then maybe I could intentionally
leave a slightly gap between the tile bottom and the greenboard and
fill that gap with caulking? Or is that just making it harder than it
needs to be?
Thanks for your help!
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
Apply the wonderboard (cement) anywhere that water or moisture may present a
problem including ceilings. Tape the edges with green fiber mesh
(waterproof for wonderboard) and tape all seams with durabond setting
compound. Feather edges.
NEXT STEP:::: USE DURABOND TO ADHERE TILE TO BACKERBOARD!!!! Do not use
thinset or acrylic pastes. The setting compound will adhere the tiles far
better than anything else.
Use siliconized tile grout when finished.
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