I've taking an extra layer of bricks from the front of my fireplace. I want to replace the fronting with marble. The original surface is really rough. My question is should I apply a mortar to smooth out existing front and then put the marble on that, or should I just apply concrete board then put the marble on that?

This is a working fireplace that's approximately 80 years old.

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As long as the fireplace draws well you won't have impossible & ugly staining of the marble, so go for it. A rough base is great & will provide the best holding power, an uneven base is bad & needs to be leveled for a uniformly even coat of Polymer Modified White Thin-Set or Mortar...white so the marble doesn't get stained from the backside by your choice of adhesive. This area shouldn't get anything more than warm with a raging fire going.

If you have room for 1/4" or more cement board then great, but be conscious of screw (Tapcon) depth not entering the fire chamber & don't just mortar the cement board on. Screws should only go into the bricks mortar joints & not into the bricks themselves.

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