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Old 08-09-2009, 04:31 PM   #1
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Mixing Mastic and Thinset in a Tile Job


I've got the old tiled area around the tub cutout and insulated, now I'm going to install the cement board to close it all up. Thing is that the old install had the tile only up to about 56" and I'm going to take it all the way to the ceiling so some wall area will be the old greenboard. The new tile is 8x10, so the tile row at the interface between the two will be 6" on the cementboard and 4" on the wallboard. Cutting to 60" and inserting a strip so a full tile is on cementboard is of course possible, but I'm feeling awfully lazy in this DC heat. I have already cut all the cementboard to size up to 56". Do I thinset what I can then switch to mastic for the remaining portion? Any options I'm not considering?

I can also go mastic throughout, though one of the cement boards will have the "rough" side that is typically thinset instead of the smooth. Who would have known that one side of cementboard is for thinset and one for mastc? Certainly not I...


Last edited by smata67; 08-09-2009 at 04:34 PM.
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Old 08-09-2009, 06:04 PM   #2
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Mixing Mastic and Thinset in a Tile Job


Tile is not my area of expertise, but I can tell you that greenboard is in no way a proper backer for tile, even if it's at the upper portion of the shower. If it were mine, I'd use cement backer to the ceiling....

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Old 08-09-2009, 10:38 PM   #3
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Mixing Mastic and Thinset in a Tile Job


Quote:
Originally Posted by bjbatlanta View Post
Tile is not my area of expertise, but I can tell you that greenboard is in no way a proper backer for tile, even if it's at the upper portion of the shower. If it were mine, I'd use cement backer to the ceiling....
Amen. It is also code...No drywall of any kind behind tile in wet areas.

Mastic is a big no-no in wet areas as well. Use thinset for the entire application. The mastic will fail.

Sometimes it helps to add a stud or two to support the cut edges of the drywall and backerboard at the joint. I'd suggest an overlap of an inch or so to hide the seam between the substrates but be sure that the majority of the tile is supported by backerboard. Also, don't forget the CRITICAL step of mesh-taping and thinsetting your backerboard seams.
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