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CowboyAndy 12-09-2008 11:23 AM

Cement board meets drywall?
First, a little backstory: I have had to rip out the tub surround twice now. The first because we noticed a little mold, and after investigating it revealed that the caulking kept pulling away between the tub and the surround, and the drywall was black as night with mold. So we contacted the contractor who installed the tub/surround a few months earlier and he basiclly flat out refused to do anything about it. After settling with his insurance, we replaced the drywall and installed a new surround. After the caulking cured for several days, it pulled away between the tub and surround almost imediatly. We hired someone to come in and find the problem and they told us that the floor was unlevel, and the contractor had leveled the tub (which is good) but failed to support the underside, so everytime my fat arse stepped in it the whole thing flexed enough to pull the caulking away.

This weekend I am going to tackle this project. I have the drywall cut out basiclly the shape of the tub surround, but an inch or so short so the surround goes up and over the seam.

I want to put in cement board instead of drywall for the water restistance factor, and down the road we sould like to do tile.

Do I need to mud/morter the seams of cement board to cement board? what about the seams where the cement board meets the drywall? Tape? what material do I use?

AtlanticWBConst. 12-09-2008 05:39 PM


Originally Posted by CowboyAndy (Post 195977)
Do I need to mud/morter the seams of cement board to cement board?

Yes. You should use fibergalss mesh tape and thinset.


Originally Posted by CowboyAndy (Post 195977)
what about the seams where the cement board meets the drywall?

Those seams may be treated as standard drywall seams. You can coat over the cement board portion of the seam, just as you would normal sheetrock.


Originally Posted by CowboyAndy (Post 195977)
Tape? what material do I use?

You can use standard paper tape and joint compound.

CowboyAndy 12-10-2008 06:36 AM

Thanks man, appriciate it.

CowboyAndy 12-11-2008 10:06 AM

One thing I forgot to ask, do I need to use different screws for the cement board?

angus242 12-11-2008 10:48 AM

If you are installing tile over the cement board, the CBU should be installed with alkali-resistant screws and then an alkali-resistant mesh tape be used over the seams. A coat of thinset would be used over the tape.
If you go to a big box store, the screws and tape are usually in the tiling section. Typical screws are called Hardi Screws and the tape should be near by.
You may want to venture over to the tiling section of this forum to get some really good advise on water proofing! When a surround is water proofed, no chance of leaks and therefore, no mold!

Good luck!

trav2001 12-25-2010 01:30 PM

I have a friend getting ready to do this same project. I'm going to go over there today to take a look at what he's got going. I think he too wants to use cement board around the tub and then tile it. But I think he is okay to use the greenboard for that plus tile later. I'll be helping him with the project, so we'll see how it goes :)

oh'mike 12-25-2010 05:58 PM

Green board is not to be used in a wet area----No longer allowed by code in most places--to many failures.

epson 12-25-2010 06:14 PM

Using green board drywall in a wet location is a recipe for disaster. At the very least, severe mold growth will occur, if not outright failure of the product and green board drywall's paper covering is water resistant, it is not waterproof. Not only that, but the brittle gypsum core is not suitable for wet applications. A cement based product like Wonderboard or Hardibacker is suitable for tile covered wet applications like showers and tub surrounds.

trav2001 12-25-2010 09:29 PM

Wonderboard was the final descision. Will be starting that in a day or two

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