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I have an area of drywall that is next to my bath tub that has gotten soft and crumbly. Mostly held together by paint & caulk now.

I'm reasonably sure I understand why. We're getting some water splashing back from the shower and coming down the side of the tub/wall. I know we used the "right" drywall (green/blue), but I'm sure it can only withstand so much abuse (It's not meant for lining your pool).

I'm thinking my best solution is to cut out that section of dry wall and replace it with a chunk of cement board.

My questions now being (which are all inter related):
- Is this the right solution?
- If so, can I do the transition between the cement board and drywall with drywall compound, or do I have crack open a bag of thin-set? - If so, how do I make its finish nice (just run a skiff of drywall compound over it)?

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You should figure out how to prevent water from coming in contact with the drywall, before you even think about repairing the damaged drywall. Tiles/tub surrounds commonly extend beyond the tub, to prevent this exact problem. This answer might be useful in helping you keep the water in the tub. –  Tester101 Jul 23 '12 at 18:45

2 Answers 2

I don't have experience with it, but there are fiberglass-covered drywall products available. If that is not available, I would recommend using 1/2" James Hardie Tile Backer Board. The smooth side is smooth enough to be painted. As far as taping, I would use a fiber glass mesh tape and a joint compound mix like Sheetrock 90, which hardens better than pre-mix joint compound.

But to really fix the problem permanently, I would pull cut away the bottom 18" of drywall all the way around the tub, replace it with Hardie Tile Backer Board, cover it with Schuter KERDI and finish with tile. Google or Bing to get more information about those products and how to use them.

Tip I picked up from Mike Holmes: don't run the tile backer board over the tub flange that goes up the studs. Stop the tile backer board above the flange, fill with thinset and cover with KERDI. Use KERDI-FIX to adhere an 1/8" fold of the KERDI to the tub deck.

This all assumes this is a tub only with no shower.

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Drywall compound is most often water soluble.

The method of repair will depend on your use case and the type of finish desired. If you remove the drywall you could replace it with a cement board which can be taped with special tape and compound. The concrete board will take a texture just as well as drywall (but note that the texture chosen might be made of water-soluble mud too).

Consider that if you have that much water damage to the drywall you may have moisture in the cavity as well.

Greenboard, while still commonly used, it's really a good moisture-sealed substrate. Consider whether or not you need to put some sort of vapor barrier behind the wall board.

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As far as finishing goes, I'd just like it to be smooth like the rest of the wall. Of course the cement board is no where as smooth as the drywall. Is there a different product available that will do the same job as drywall compound, that is more water resistant? –  BIBD Jul 23 '12 at 18:32
    
@CodeSlave the backside of something like hardibacker is just as smooth as drywall. What finish are your existing walls? –  Matthew Jul 23 '12 at 21:14

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