Installing a tile fireplace hearth
We tore out a "main level" raised hearth that had been poured onto a concrete slab foundation so we could lower the hearth to floor level. The surface of the slab is mostly level but has a few small imperfections. My contractor suggested a cement board, Home Depot recommended a flexible liner in a roll to use as a liner between our new ceramic tiles and the original slab. The carpet surrounding the hearth will butt up to the tiles for now until we tile the whole floor someday, if ever.
I want to make certain everything is level (now and for possible future tie-in), no movement, minimal risk of tile and grout crack, and whatever has proper moisture (and heat?) barrier that might be needed.
The finished height is not an issue because the contractor will cut the tiles to fit around the base of our mantle, but there may be a slight "level from one side to the other" issue. I've read NOT to use (Hardi) cement board (they never said why not) on a slab somewhere, and somewhere else read that it's OK.
So what would you guys recommend and why: The flexible stuff, cement board, or nothing at all and just adhere the tile to the slab with thin-set?