Gap Between Hearth and Firebox

[Jacksonhouse]

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.

 

[Jacksonhouse]

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.

 

[begreen]

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.