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So we have an old fireplace and are giving it a full rennovation. We're replacing the ugly hearth with something more modern. We're putting in a wood stove insert and adding a more modern (nicer looking) ledgestone facade around everything.

Step 1 is take out the old hearth. I had asked this question last week about the best way to go about it, and got some great answers. So I bought a masonry bit, drilled a bunch of holes into the concrete and started chiseling a small section of the concrete in chunks. I wanted to see how far down the hearth goes and what subflooring is underneath it. Not knowing any better, my guess was it would go down maybe 2" and run into plywood/wooden subflooring underneath. Well...

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Instead we found:

  • the concrete hearth is a solid 4" thick
  • near the bottom is a lot of brick material (wondering if this might have been used for filler material or to somehow reinforce it?)
  • its sitting on top of yet more stone/concrete (foundation?)

I uploaded a YouTube video that hopefully depicts this a little better.

My question

What the heck?! Or, in more formal terms, what is the stone underneath it? Is this a typical concrete hearth installation? I'm assuming I'll need to just keep going and expose the entire stone/concrete foundation underneath it and buy an exceptionally thick slab of hearth stone to replace it, but hopefully its "okay" to add a few layers of plywood to act as a spacer? I just can't imagine how heavy and expensive 4" of hearth stone will be and I'd like to avoid that if at all possible. Thanks for any explanations!

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  • Every heating device has minimum distances to flammable stuff(wood). Cement/stone should reduce those distances, so building up with plywood should be okay. Your local building/fire department will have all the regulations/measurements you need to have. It would be good if you could have a look under the floor to see how it is supported. You are right about the weight. 4 inches of stone/cement dropped on your toes, and you are not a happy person.
    – crip659
    Nov 11, 2022 at 21:39
  • You fill masonry with masonry, not plywood. You could pour concrete, and add thinset and tiles, rather than "an extra-thick hearthstone" which is just silliness. You could simply build up, rather than removing the existing stones, if they are not loose/degraded and you are not height-constrained. And frankly, if you are going to put in a woodstove, just close the fireplace, put in a thimble, and skip the limitations of an insert. Utterly normal for a hearth to be supported by masonry all the way down.
    – Ecnerwal
    Nov 12, 2022 at 17:48

2 Answers 2

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The concrete platform is probably the foundation that's supporting the hearth this is a very common brick fireplace design.

From The Project Gutenberg EBook of Farmers' Bulletin 1889 - Fireplaces and
Chimneys, by Arthur H. Senner and Thomas A. H. Miller

the brick rubble pieces in the mortar are just to save mortar, or possibly there as wedges to hold the stones level while the mortar/grout sets up.

I would not add wood under stone, use stone, brick, or concrete, there instead,

I would just buy a few bags of instant concrete, tip it in dry, get the level right, and then dampen it.

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Given the hearth support is more than 4" deep.

Remove the stones by chipping down and under to release them as if they were tiles.

Get a diamond blade on a circular saw, set depth of cut to the depth of your grout plus new stone thickness.

Run the circ saw and diamond blade and make a series of lines.

Chip out between the lines with a rotary hammer with 2-3" chisel bit to get the approximate depth.

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