fireplace construction question

Does anyone know the thickness of the pad poured for under a fireplace and its hearth? I wish to remove my fireplace and rebuild a wall that won't let as much air in as my fireplace wall does.
Also, does anyone have an idea on how deep the foundation that supports the brick of the fireplace firebox and chimney is? It seems to be a separate pour from the regular foundation wall. I'm guessing at that as I do have a regular foundation wall in the basement and that little jut out that the exterior of the fireplace is built on outside.
Thanks
PS: The chimney is capped and I have no intention of going back to burning wood. Also, I am replacing the gas fireplace with a direct vent, zero-clearance model.
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Most of the ones I have seen are a cantilevered pour meaning they are thicker at the back side than at the front. 6-8" tapering to 3-4" would be my best guess. You should be able to determine this from the basement or crawl.
If built with a permit the footer for the fireplace should be below the frost line for your area.
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Colbyt
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