Putting stone face around current chimney; floor support?

We have a vaulted ceiling living room, with a fireplace at one end. Currently the fireplace just has a stone border going around it, with the chimney that goes to the ceiling just standard plaster wall (like the rest of the room). We're hoping to put flagstone or some other stone face all around it up to the top (like at those ski lodges and cabins). We're not sure about whether we'll use real stone or cultured, but one thing I'm concerned about is the weight. In the basement, you can look up and see that there's some type of additional wood between the joists, like a second little subfloor under the first, that one of my handyman buddies thinks is just for the brick floor of the fireplace, because of the weight.
How can we know if the floor will support the stone of what we plan to do? Or, what could we do to just make sure it'll support anything, and not worry about exact specs?
Any ideas, advice, etc. are appreciated, and thanks for reading.
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A structural engineer can look at the framing and tell you what you need to support the extra weight. T
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Consult the building code for the region where you live. Most changes involving chimneys or fireplaces require a building permit i.e. must be inspected for conformity to the current fire safety code.
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Don Phillipson
Carlsbad Springs
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Actual brick fireplaces should NEVER be supported on wooden floor joists, they should be supported by a masonry foundation on heavy footings. The hearth may be supported by wood, and normally this is only until the concrete sets, but this may be the hearth framing that you are seeing. If you have problems discerning what is supported by what, it is time to call in an engineer or somebody who knows.

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