Extending existing fireplace hearth for wood-burning insert

We cut out the hardwood today. Please see the attached pictures.

Here is the issue I see now: when they poured the slab, the concrete, in the area directly in front of the the fireplace, has oozed over the wooden joists that frame the slab (please see picture 2). So we cannot butt up the Micore directly to the slab all around. There will be a small area (unfortunately right in front of the insert) where there is wood only covered by a an inch of concrete. My husband thinks that it would not be a problem because if the slab gets heated up, it distributes the heat within itself, so no one area of the slab would get very hot. Plus, he says if the area where the concrete oozes were an issue, then we'd have that issue all around the transition from the slab to the Micore-protected area, because the slab is framed in wood about an inch below the surface (the slab as a whole is 10" thick, but there is wood framing around it - I had a picture of it from below earlier in this thread).

If this transition (and especially the area where the concrete has oozed over the wood) is indeed a problem, the only solution would be to lay the Micore all over the hearth, even the concrete slab. With the concrete backerboard that has to go over that, this strategy would raise the whole hearth 1" higher than we had planned. We are trying to keep the hearth low (and were going to tile directly over the concrete slab), but now I'm debating if we will have a fire safety issue because of the transition between the slab and the Micore-protected area beyond.

I hope what I wrote is not too confusing... I would be very grateful for your thoughts!

Thank you, BeGreen. We did what you suggested. It was actually easy (I wonder why in 1924 they didn't do that, and instead cut those planks short??).

So do you think that if we lay the Micore only over the wooden subfloor butting it up to the slab as planned, the transition will not be a problem (as a "heat leak" that might jeopardize the framing)?

Thanks, BeGreen. Our initial plan was to piece in the Micore, and then cover everything with one solid piece of Durock. I was, however, advised by one person on the John Bridge forum, that it would be better to put Durock just over the Micore area, and have an expansion joint between the tiles on the concrete slab and the tiles on the Durock. He said we should accommodate the movement, not fight it by covering the gap with a piece of Durock. That expansion joint puts some constraints on my tile layout, but right now we are planning for it. That also allows us to keep the hearth even lower, because we can tile directly over the concrete. The one issue with this plan, however, is that on the sides, one row of tiles will have to span the old and the new area (please see attached picture, subfloor still to be repaired). We will use a very flexible thinset (1Flex crack isolation mortar by TEC) for the tiles, so hopefully that row won't crack...

To me, the "one solid piece of Durock over everything" method still sounds best, but apparently, a board of Durock overlaying the transition might crack or come loose because of the movement between the concrete slab and wooden subfloor.

Would you ignore the advice from the guy on JohnBridge, then, and use the one piece of Durock?

The Micore plus Durock will bring the surrounding floor up to the height of the cement slab, so we are planning to tile directly over the slab.

Yes, I have a sanded caulk the same color as the grout. By the way, I'm not sure if I have the right type of grout. It is Laticrete Permacolor. Someone told me yesterday that we should definitely use an epoxy grout, since the grout on the hearth would be exposed to a lot of dirt and epoxy grouts are stain resistant. What grout do people usually prefer on hearths? Are epoxy grouts fire safe?

And you also know people who used it on hearths (I'm still concerned how it will hold up to the heat)? Maybe I will return the Permacolor and get an epoxy grout...

I wasn't really worried about a fire with the grout, but more about failure, like discoloration, cracking... But I think I'll go ahead and get some epoxy grout. Thanks again for all your advice!

Thanks, Jocam! We are putting in a Drolet Escape that we bought at Northern Tool. Not a high-end insert, but we are on a budget... and everything I read about it was positive. I also like its plain look.

We are also going to tile the brick surround of the fireplace (which was covered in a glossy brown paint by the previous owner) in the same tile. But that's a whole other adventure that we are only going to tackle after we get done with some other projects in this house. This place is still a bit of a construction site, almost a year after we moved in. For now we just need to get the hearth done, so we can get the insert put in before the end of the winter! (This insert has been sitting in our living room since - gasp - last August! Or was it July? We got side-tracked by taking apart our finished attic.)

I'll post some pictures as we progress...

We have finished the hearth extension and installed the tile. Grouting is tomorrow, but I wanted to post some pictures of our progress. The extended area to the front is built as follows:

- sheetmetal (aluminum flashing) over the wooden plank subfloor, thinset underneath it
- 1/2" Micore 300
- 1/2" Durock laminated onto the Micore with thinset. Everything then screwed down with 2 1/4" Rock-On screws.

We used TEC Superflex thinset to laminate the subfloor sandwich, and to lay the tiles.

We used sand mix mortar to repair the firebox floor, and extend the hearth to the sides.

We also added some cross-bracing to the floor joists below, to minimize any flexing.

We have an expansion joint between the tiles over the concrete slab and the tiles over the Durock/Micore sandwich, which is going to be caulked. The concrete slab was slightly lower than the Durock area, and we tried to even that out by backbuttering the tiles over the slab. Overall, we didn't achieve a completely plane surface (the uneveness of the substrate in conjunction with the slight crookedness of the 'artisanal' tile we had chosen was a bit too much of a challenge for our first tile job!), but we'll view this as adding to the charme of the hearth!

The tiled hearth extension is now about 1/2" higher than the firebox floor. We are wondering if we can set the insert onto 1/2" inch of Durock, to raise it to the height of the hearth extension. If it's non-combustible, it should be fine under the insert, I would think. Has anybody done this?

Thanks for the compliments, Ed and BeGreen!

As far as I know the new Durock is still non-combustible - I believe someone quoted the data sheet in this thread to that effect. And as another member noted on this thread, underneath the insert is probably the coolest spot around it.

Tonight we'll put in the grout. I'll post more pictures then.