Tile Question: mosaics with a thicker tile border

[Lolaeliz]

I’l setting 1/4” thick mosaics inside a border of 3/8” thick 8x48” planks. I'm thinking I need to build up the floor for the mosaics by as much as 1/4". The planks are a little more than 1/8" thicker, and I imagine there will be a lot of thinset. My concerns are that I don’t want the planks to crack. And I don’t want the mosaics popping up because of house settling or floor movement. (It’s a circa 1970 upstairs bath. previous tile was set in 1.25” of mud / cement).

I thought of 3 options, but I don't know if any of them will work:

1. install uncoupling membrane, cut to sit where the planks will be, with a layer of 1/4" plywood fixed to where the mosaic tiles will be, then uncoupling membrane on top of that.
2. cut 1/2" Hardie backerboard where the thicker tile goes and 1/4" backerboard where the mosaic goes, connecting them with mesh tape and joint compound.
3. put 1/2” backerboard on the whole floor, with 1/4” backerboard on top of that where the mosaic goes.

Any ideas of the best way to do this? Thanks.

 

[ben.e]

You're right to think about tile height transitions and long-term movement. Of the three options, Option 3 is the most straightforward and stable: using ½" backer board throughout and layering ¼" only where the mosaics go gives you solid support and avoids introducing seams between different board thicknesses across the floor. Just be sure to thinset and screw both layers properly, and feather any transition areas.

Also, an uncoupling membrane like Ditra could work well instead of backer board if height is a concern—it helps with movement and cracks, especially in older homes. Whatever you choose, make sure your subfloor is solid and flat first.