'Drywalling' around a reg 2" air-gap Insulated Flue Pipe

[Stephen N]

Hello,

Previous owners installed a zero clearance wood fireplace out of code, allowing a ceiling joist to touch an insulated flue pipe (which requires a 2"air gap from combustibles). This was discovered after opening a wall we are renovating and leaving open. I know how to solve this problem by cutting in support headers and trimmers to allow the proper air gap for the joists, what I'm having a hard time with is the intersection of the flue pipe to the ceiling drywall.

It is not possible to add a ceiling support box at this point since everything is already installed. My plan is to cut out the area of ceiling drywall colored yellow, and replace it with fireproof cement board, and then caulk the intersection of the flue and cement with fireproof caulking, and mud the transition of cement board to drywall with thin-set mortar. Everything does or will have appropriate 2" air gap to combustibles, but the cement board and flue pipe will touch with fireproof caulk.

Is this a good plan? Am I missing anything? It seems like this should be to code since there will be nothing combustible within the required air gap, but I can't find anyone online with a similar problem to confirm or any other instructables.

FYI, the flue pipe will be painted with stove paint and be exposed, see reference render

Also, I forgot to include the extra mounting points I will be adding around the flue to secure the cement board properly.

Thank you so much for your time.
Columbus, Ohio, USA

 

[begreen]

Cement board, if it has no styrene pellets, would probably work, but so would a simple trim ring that could be painted the ceiling color.

 

[Stephen N]

Cement board, if it has no styrene pellets, would probably work, but so would a simple trim ring that could be painted the ceiling color.

Thank you, Begreen, the trim ring (is it called a flue collar as well?) I had considered, but it wouldn't fit all the way around the back (there's only 3.5" of clearance in the back), but I couldn't find a trim ring in the size I needed anyway so I discounted that. But if the cement board works that makes me very happy, I was having a hard time with this since everywhere I looked a ceiling support box was instructed.

THANKS

 

[begreen]

Just trim the trim ring on the backside before painting it. That's a 2 minute job.