1st timer: slate over cbu over drywall and firebox

[shaunflanders]

OK, I'm finally ready to take a crack at a tile project. I've got the old tile off of our fireplace surround and hearth, the new gas insert is in place and it's time to replace the tile. We've got some really smooth slate (or so Lowe's called it) that has a fairly consistent thickness that we want to use. I'm planning to do the hearth first and then the surround. On the surround, I want to tile all the way up to the insert face (actually behind it a 1/4" or so - you can pull the insert out to get behind the face).

Hearth
The subfloor in the hearth area was damaged a little when I demo'd the existing tile out (see pic). There are some chips in the subfloor material, but nothing very deep. There is also a bit of the old mud/mortar left over in spots. I'm thinking I can just go thinset mortar over the subfloor to get a level surface, lay down a 1/4" hardibacker board, then another layer of thinset and then the slate. Anyone see any issues/complications w/ that piece?

Surround
I think this is going to be the tricky piece. From the gas insert out I have: gas insert, space b/t insert and firebox (~1/2" above the insert and 1 1/4" on the sides, drywall (a bit of which is damaged - see pic). The drywall and firebox are very close to being on a level plane, w/ the firebox sticking out ~1/16" - 1/8" from the wall. My plan at this point is to somehow affix a 1/4" hardibacker board to the wall so that it covers the entire area I want to tile over, including the drywall, firebox and open space b/t insert and firebox. Once that's set, I'd apply thinset mortar to the hardibacker and lay the tile on that.

Questions
1) Do I need to fix the hole in the drywall or can I just put the hardibacker up over it?
2) How should I affix the hardibacker to the firebox?
3) Is there any issue w/ the overhang of hardibacker into the space b/t the insert and the firebox?
4) Any other issues I'm missing?

Thanks for any help..... - Shaun in Seattle

[muskymike]

Hi Shaun, you don't need the Hardi on the walls. Besides you need some solid backing to screw it to which the drywall won't provide. You can skim the drywall with some thinset first, let it dry then put the slate on that. You can do just what you said for the floor.

[shaunflanders]

Will the thinset adhere to the firebox and stay adhered when it heats up/cools down over and over again? Also, what should I do about the hole in the drywall?

Thanks for the advice!....... - Shaun

[MNTileGuy]

Is that subfloor concrete? In the pic it looks like it is. If that's the case, the hardi isn't even necessary-just set the slate right on the concrete.

As far as thinset adhering to the box, thinset doesn't bond well to metal. What I do on fireplaces is run a bead of black silicone around the inside where the tile meets the box. This hides the gap and gives you something that will provide an adequate bond on the inside edge. Just make sure you tape off the edge of the slate and box first, leaving only the joint exposed. Then smooth the bead of silicone with your finger and remove the tape. You'll have a real mess on your hands if you don't tape everything off, no pun intended.

Brad