Gap Between Hearth and Firebox

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New Member
Dec 19, 2023
Nashville, TN
Hi. We recently had a really scary event with our fireplace. The joint between the Isokern firebox, and the stone slab hearth, which is approximately 7/16”, is a mortar joint. It evidently had broken down and some embers had fallen down into the hairline cracks and started a small fire under the house. We were able to extinguish the fire but want to ensure naturally it doesn’t happen again.

I cleaned out the old mortar and filled it in with Rutland cement. However after a couple of fires I have noticed some cracks and a separation from the stone slab. I would appreciate any advice. I attached some photos for reference.

Gap Between Hearth and Firebox Gap Between Hearth and Firebox Gap Between Hearth and Firebox Gap Between Hearth and Firebox
I'd like to also add that the larger two cracks in the mortar are where I had to fill a void with paver sand. Not sure if that was smart but I didn't know a better option to filling the void where the subfloor was burned out under the hearth about the size of a tennis ball in diameter. Probably not a coincidence that's where the two cracks are. I also have a concern as to why there is a separation of the mortar joint and the hearth as seen in the photos. Thanks.
It sounds like someone messed up on the installation. This is dangerous. The fireplace is supposed to be sitting on a thick concrete slab with no wood underneath.
Gap Between Hearth and Firebox Gap Between Hearth and Firebox