I had piping re-done at this house and unfortunately the plumber had to open up a lot of walls, including the ceiling in and around the water heater, which I need to repair. I can quite easily do the drywall repair, but I am not familiar with installing the flue pipe collar.

What do I use that’s available at the big box stores? I think maybe a collar that I’ll have to split and install? Is there flashing I need to put around the flue collar as well? What are the guidelines? I am in California.

Also, what is the gray substance right at the top of this flue pipe… Kind a looks like concrete but I didn’t really feel it.

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It IS "concrete", or rather Transite asbestos cement pipe. Just cut two pieces of drywall with the hole where they meet and install them around the pipe.

Awesome! Thanks!

Do I need to do anything with the u-joint pipes at the ceiling to the right of the flue? Flashing or aluminum tape?

Whoever did the repiping did NOT consider how you, or anyone, was going to replace the drywall. I do not know why that "U" is there, but if you can work the sheetrock around it, there are hinged flanges you can place around them.

I know. Owner and I are both so frustrated. But...onto patching things up. Thanks for the help!