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Ehren
Mudding between drywall and cement board

First time mudding a room. I have a perfect seam between each piece of drywall, but when the drywall hits the cement board for the shower area, I dont know what do to. Should I tape and mud it like I would between two pieces of drywall? The cement board will of course be tiled over.

Thanks.

Timothy Miller
Re: Mudding between drywall and cement board

Howdy , could use the modified cement board cement and fiberglass mesh tape& top coat of drywall mud for top coat finishing.

AskMrJay
Re: Mudding between drywall and cement board

You would use the same thinset that you'd be setting the tile with, or a modified thinset bag like mentioned above.

You'll trowel it on and lay the boards out, and then tape them with an alkyline resistant tape and mud it just like you would drywall, but using thinset where you would mud.

Add in the appropriate cement board screws also for a better grip to the walls and stud and then you can tile over it.

Ehren
Re: Mudding between drywall and cement board

Okay, just to clarify:

1. I have an 1/8" gap between greenboard and cement board.
2. Begin by filling in 1/8" crack with a modified cement board cement.
3. When that dries, tape with green tape.
4. Now, instead of mud to smooth the surface, use thinset -- a proper job would look fine when painted (since the greenboard gets painted and the cementboard gets tiled.

Thank you.

AskMrJay
Re: Mudding between drywall and cement board

A bit confused on where you're getting the 1/8th from...

I'd be using a larger trowel size for putting up the backerboard, something around what I'd use for regular tile at about 1/4 x 3/8. 1/8 would barely be anything to hold up your 1/2 cement board.

When the boards are up, place some of the thinset over the joints and then apply the tape over those same joints. When it's on, go over it once again with the thinset and let it dry.

Once that's cured, you can thinset over that and begin your tiling.

Ehren
Re: Mudding between drywall and cement board

Okay, perhaps I am not explaining my issue properly.

My drywall goes to the end of the tub. When we get to the tub, I drilled in the cement board. Too late to thinset the joists, since the cement board is already up.

Due to the nature of butting things up together, there is a barely noticeable crack, maybe 1/16" - 1/8". That crack needs to be painted, so how would I mud that area? Are you saying I should mud it with thinset?

My plan was to tape and mud like I were doing regular drywall. Would it be all right for me to do that?

Thanks.

HoustonRemodeler
Re: Mudding between drywall and cement board

By the way,

'Green board' aka the green colored drywall which is billed as "water resistant" :confused: is no longer approved as a substrate for tile in wet areas. If your local code requires it, then by all means use it. Outside the wet areas the green board gives you no better protection than the plain old white drywall does. Before painting, primer the bathroom walls with an oil based primer such as the Zinser that dries in an hour and can be top coated with latex.

As far as the cement board areas, what is your waterproofing?

Ehren
Re: Mudding between drywall and cement board

Thanks Housten. 6 mil vapor barrier on the studs, cement board, and eventually that red waterproofing membrane is my setup.

The greenboard is outside of the shower area, and will run up to the cementboard. My concern, is how to finish off that transition. Because the cementboard will have tile, and the greenboard will have paint.

Thanks for the tip on the primer. That step should be coming up fairly soon. I hope.

HoustonRemodeler
Re: Mudding between drywall and cement board

If you are doing the red guard over the CBU then don't use plastic behind it. That will give you the dreaded 'moisture sandwich' Either the plastic behind (tucked onto the tub and sealed to the tub) the CBU or the redguard on the CBU with the joint to the tub flange sealed with red guard.

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