"joining" old backer board to new backer board

[alq137]

greetings, all.

i'm in the process of replacing an old bathtub which has tile surround. i've removed the required 6" of old tiles and backer board to expose the tub. i will need to replace the old backer board that i removed with new backer board, but i'm not sure how to 'join' the old and new backer board. the old backer board was cut flush with the bottom of the old tile. is it sufficient to use the thinset adhesive to fill in the gap between the old and new backer board?

[jdm]

You've got a bit of a problem there.

The proper way to join two sheets of backerboard requires taping the seam with alkali-resistant fiberglass tape and then filling the joint and covering the tape with thinset. The fiberglass adds tensile strength to the joint.

But, as you realize, there is no way to get the tape into position over a backerboard joint that is flush with the edge of the tile. And without the tape, the tile grout joint that falls right over the backerboard joint (it has to since the old tile ends a the joint) is almost guaranteed to crack.

Since you are replacing the tub, don't you really want a new surround, too?

And, BTW, is there a vapor barrier in there somewhere? It would either be a sheet of plastic or tar paper behind the backerboard and over the tub flange, or a film that was "painted" onto the backerboard before the tile was installed?

As usual, any pictures you have or can take and post would be very helpful. As some of the old-timers are fond of saying, "It's hard to see from here."

[alq137]

thanks for your reply, jdm.

are you thinking that i could fill in the gap of the removed backer board with new backer board, shimmed it to be level with the old tile not removed and placed an one-piece acrylic tub surround on top of that, instead of the initial plan of replacing the removed tile and backer board with new tile and backer board? i hope that's what's possible, because tearing down the rest of the old tile and backer board than installing new backer board for the tub surround would be so much more work.

[Saldibs]

Can you remove another row of tile to expose some backer board in order to allow you to tape the seam. This can prove to be very difficult, depending on how well the tile is attached to the board, it is possible to do it at times though it's the luck of the draw.

[jdm]

One-piece surrounds can be installed over almost anything -- some can even go over bare studs. But this is a tile forum, and most everyone here is not going to encourage you to go that route unless the alternative is your joining the great unwashed masses.

While removing and replacing the surround is work, it is a job that is doable by most any dedicated (and not-so-dedicated) do-it-yourselfer. The folks right here have helped scores and scores of folks who never thought that they could do a tile job through projects that far outclass plastic surrounds with results ranging from respectable to spectacular. And all with a quality level that surpasses that of many folks who call themselves pros.

But Sal's suggestion is a good one, if you can successfully remove a row of tiles without destroying the underlying backerboard. And if you can either clean the backs of the removed tiles or obtain matching new ones.

And the folks around here like to have first names to call each other. It's one of the things that makes this the friendliest forum on the web. So go to the dark blue bar at the top of the page, click on "User CP" and enter a name in the "signature" field. The system will automatically add it to every post you make.