Repairing mortar between metal insert and stone hearth

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New Member
Jan 25, 2021
Hello all,

I have a stone hearth with a metal insert and glass doors. The mortar bonding the metal frame to the stone is basically coming off along the top, I could pick it out in loose chunks. The frame itself seems to be a bit loose as well, when I open the glass doors I see the top part of the frame lean outward a little bit from the weight of the doors and I can jiggle it a bit. There are two bolts on the inside of the metal insert at the top that seem a bit loose.

The gap between the metal frame and the stone is at least 1/2in up to 3/4in or more in places (to follow the contours of the stone). I'd like to remove the old mortar, tighten those bolts (hopefully that would solve the loose frame), then repair this gap. The issue I am running into is that a product like "Quikrete High Heat Fireplace Mortar" specifically says it does not bond to metal. A product like "Imperial HI-TEMP STOVE & FURNACE CEMENT" says that it will bond metal to masonry but only joints less than 1/8in. Castable Refractory Cement seems OK for larger gaps but doesn't seem to say if it will bond metal to masonry (I found an amazon review where the Rutland brand says it does not bond to metal and recommends Furnace Cement)

I had a chimney sweeping company come in and they didn't really identify any of these problems even though I made it clear that I was a total newbie to this. He didn't even tell me that the two openings at the back of the fireplace led to vents outside for fresh air intake (which I think is a pretty good thing to have). So far I've used the fireplace with basically no problems. When I open the glass doors I do not get any smoke in the house, but when I open the metal grate doors to put another log on, I'll get some smoke coming out the top.

I've attached a picture with a yellow square around the problematic mortar - it seems to be fine on the left and right sides, it's really only the top where it's coming loose. Any advice would be greatly appreciated.


Repairing mortar between metal insert and stone hearth
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That steel piece is the lintel, the mortar isn't holding it up, nor does it need to bond to it. The lintel is supported on either side by stone and is holding up the opening. You can tuck point mortar back in the gap.
I believe that you don't want to directly attach the metal to the stone, because each time it heats up (the metal will heat and cool quicker than the high density / mass stone) it will expand and contract, which will cause the mortar to be pulled and pushed and eventually crack.

I bet its cracking on the top because thats the point where the temperature goes the highest, causing the most expansion and contraction. I have also searched around for "high temperature mortar" but I couldn't find any that specifically stated "use this for fireplace / hearth stone work"

Does anyone know if this product exists? or is regular mortar good enough that it doesn't matter?
Thanks for the info. What I found is a tube of "quikrete fireplace mortar" at home depot that says it's good for high temperatures. I figured i couldn't use it because it doesn't bond to metal but from the sounds of it that's probably my best bet for this.