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I'm building a new fireplace with a 2cm granite surround (around the firebox opening). This leaves (by design) a 1/2" gap between the back of the granite and the face of the firebox & smoke dome (gap is several inches wide x 1/2" thick).

My question is: To seal off this gap (to avoid a "false chimney" fire hazard), what type of grout should I use? Will the same standard thinset that I use to attach granite-to-fireplace work, or do I need some type of special heat-resistant thinset or mortar?

Or, instead, or in conjunction with, is there a caulk that is suitable for high temperatures that would work in this application?

  • What is a "false chimney" hazard? My quick search turned up nothing useful. – isherwood Feb 11 '16 at 19:48
  • the air gap at the back of the granite can cause a draft and pull smoke from the fireplace into the living area. – Ed Beal Feb 11 '16 at 20:05
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I would use fire brick mortar. It will hold up to the heat and seal the gap so you won't have the false chimney issue you are concerned about.

  • Is fire brick mortar suitable for the relatively large gap (1/2" to 3/4") or will it shrink too much as it cures? – JD33 Feb 22 '16 at 19:35

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