Advice on Sealing the Joint Between Drywall and Cement Board?

In the process of re-tiling my walk-in shower, I removed the old drywall about 6 feet up the wall (that's as far up as the old tile extended) and replaced it with DuRock cement board. I've got the tile all set, except for the top 2 or 3 rows. I'm wondering how to seal that 1/8" gap between the top of the new DuRock and the bottom of the old drywall. There's not only a gap between them, but they are also slightly different thicknesses. So, if I wanted to use slightly taller bullnose tiles for the very top row, which would extend them past that joint, the tiles wouldn't set flush because of the differences in thicknesses of the drywall & DuRock. Should I put an extra thick layer of Mastic in and around the joint and feather it out like you do with drywall joint compound, let it dry and try to sand it to smooth it out? Or maybe just use drywall compound itself and Mastic over it once it's dried and sanded?
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snipped-for-privacy@fastmail.fm wrote:

and tile also) the ceiling. Wondering how high is your shower head. The pipe on mine comes out of the wall at 6' 6" and nobody in the family is very tall.
Anyway, don't you think that moisture will get to the area left with Sheetrock and without tiles?
The different thickness between sheet rock and durock and that 1/8" gap are the consequence of poor design in my view. No offense meant, but I would reconsider those choices.
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My shower head also comes out of the wall at about the 6'6" mark. But the original tiles only go up to about 6', and the original drywall above it has never had a problem with moisture. It has several coats of oil-based enamel and that's kept it sealed well.
I don't want to run the tiles all the way up because based on the shower's FIRST 40 plus years of service it just isn't necessary. I live in the dry desert Southwest.....maybe that's a factor in moisture not lingering on the drywall above the tile line.
But back to my original question......what would be the best way to smooth out the joint between the original drywall and the new Durock???????
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I did a similar project in my house, though the durock and the sheetrock were the same thickness. A detail you left out: which one is thicker in your bathroom? If the durock is thicker, I guess I'd just keep tiling up and use some extra mastic behind the tiles at the top, and then seal off the top of the mastic with grout-colored caulk. If the sheetrock is thicker, then I am not so sure.
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The sheetrock is thicker, by just over 1/16". That extra thickness my be the result of several layers of paint over the last 46 years!
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snipped-for-privacy@fastmail.fm wrote:

You should have taken this into account before you started. Ya think?
Tile up *to* the sheetrock with a piece of surface bullnose without attempting to overlap it. Run a bead of silicone. Makin' the best of a bad situation.
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if you have a 1/16 gap, i would use a trowel to apply the glue on the drywall with notches 1/16 less deep than the trowel you used on the cement board so the glue surface is the same, and put the top layer of tile so it overlaps the drywall. bead of caulk to seal the top edge when done.
randy

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feather it out >> like you do with drywall joint compound, let it dry and try to sand it to smooth it out?

If the drywall is 1/16" thicker then the durock just sand the drywall down a bit with a palm sander to even out the transition. Apply some mastic to the joint and lay some mesh tape over that to make a stronger joint. Smooth a bit more mastic over the tape and let it dry.
Just decide before hand how far up you want the tile to go over the existing drywall, a couple of inches maybe is enough. If you go to far with the sander you will have to cover that up with paint or drywall compound if it is bad.
This isn't nearly as bad as everyone is making it out to be.
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Of all the suggestions, that one (sanding the drywall to smooth the transition) sounds the most promising.
Damn! Why didn't I think of that?
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snipped-for-privacy@fastmail.fm wrote:

You're not going to sand a 1/16" off of drywall without removing the paper. It's a stupid idea.
Maybe that's why you didn't think of it! Take the credit.
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Hence the mastic and the mesh tape to provide the needed strength on the joint... and covering that with tile... to hell with the paper.
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snipped-for-privacy@optonline.net wrote:

IMHO It's a shitty idea. He'll see. He deserves the mess it will be anyway. He had to have seen the potential problem before he started and chose to ignore it.
Simple way is to do as I said. Which he could have planned for. Cut a bullnose tile to fit up to the transition and forget about lapping it.
The OP's a moron.
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