Q:
I would like to use travertine stone tiles over a plywood substrate for a gas fireplace surround. Can I use construction adhesive or modified white thinset to attach them directly, or should I attach cementboard or Schluter-Ditra to the plywood first?
Wayne Hutson, West Montrose, None
A:
Tile contractor Tom Meehan replies: There are a few ways to approach this application: bad, good, and better. Bad: construction adhesive and modified white thinset over plywood. Construction adhesive is fine when repairing a few tiles that have loosened, but not appropriate for a new installation. Modified white thinset can be great on floors, but with the temperature changes around a fireplace, the plywood is going to move. Eventually, the tiles will loosen or crack.
Good: cementboard attached to plywood. This method— with nonmodified thinset used to adhere the tiles to the cementboard— works well. The only concern is the thickness. If planned right from the beginning, the framing can be set back to accommodate the substrate. In a retrofit, the substrate may be proud of the wall, and you’ll need to design a detail to hide the transitions. A gap of less than 1/4 in. can be grouted.
Better: Ditra. This polyethylene membrane from Schluter Systems can accept some movement of the substrate and still maintain a mechanical bond with the tile. Ditra is only 1/8 in. thick, which may help to keep the tile on a close plane with the face of the fireplace.
If you are going to overlap the firebox, however, you need to take additional steps. The problem is bridging from the plywood or cementboard to the firebox. With the heat from the fireplace, that slight space between the substrate and the firebox has the potential for uneven movement, which is where the tile is likeliest to loosen or crack.
There are two ways to address this situation. First, if the fireplace face does not approach a temperature of 300°F (check with the manufacturer), bridge it with Ditra, but when bonding the Ditra to the metal, use Kerdi-Fix, a caulk also made by Schluter Systems. On the other side, bond the Ditra to the substrate with modified thinset. Then bond the tile to the Ditra with nonmodified thinset.
Second, if the fireplace face goes over 300°F, use galvanizedwire mesh, and bridge it over all areas to be tiled. Make sure it is taut, then apply a skim coat of thinset cement the day before tiling to stiffen the wire and to create a flat surface.
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