Chose not to demo a rather large chimney that didn’t look modern or fit the ranch style home. Used a power grinder and 4.5″ carbide blade to cut overhangs, and boxed above the mantle. All angles are mitered/beveled.
There was not a plumb, level, or square angle on the original fireplace. So, the backer was hung with shims, furring-out while still reaching for stability. A large-screen TV will eventually be mounted on the disfigured stone tile in the center.
The ceiling sags a pinch in the center. Again, not my work.
The challenge with something like this is…it’s akin to building a perfect box around a boulder. The shim and support depths are all different, and you can’t see behind the backer board to measure them.
This was an original design and custom build.