Not exactly the best scenario. Being that it's a patch, you obviously can't put in a proper vapor barrier behind the cement board. . I would say mix up some super stiff thinset, fill that gap between the bottom of the cement board and the tub's gunnel, and then waterproof it right up to the top of the exposed cement board. Not really by the book, but unless you want to take down ALL the walls, it's about as good as it's going to get.

Forgive my ignorance...so the cement board is not sufficient? It should have had a vapor barrier behind the cement board, but because that was not done, you are suggesting waterproofing it up to the top of the exposed cement board?? Products you recommend?

dorry,

As you know, water can get past the tile and grout. Once it does, it'll hit the tile backer board. Cement board is an excellent tile backer board because it is not negatively affected by water. Wet it, dry it, repeat the cycle...and in 50 years the cement board will still look like cement board. Unlike a gypsum board, which will slowly turn to mush with repeated wetting and drying cycles.

Now cement board is not waterproof. Wet it and water can pass through it. That's why you need some sort of drainage barrier in the wall.

It's often done by stapling 6-mil polyethylene plastic to the studs, then lapping the bottom edge of the poly over the tub flange. Any water that gets past the tile/grout and through the cement board will hit the poly. The poly prevents the water from getting into the framing bays.

Most of the time all will be well. The tile and grout shed 99.99% of the water down the drain. Any minor imperfections in the wall, the cement board might get a little bit wet. But think of the cement board as a sponge, it can hold a bit of moisture in it's pores. When the shower is not in use, any moisture that got into the wall usually evaporates back out of the wall. No big deal, no damage.

Now if you had a damaged wall...bad grout, a cracked tile, etc...then you could get more water into the wall. If thee is more water getting in than can get out via evaporation, then the poly will stop the moisture from getting to the house's framing. It'll hit the poly and flow downward, over the tub flange, and back into the tub if weepholes were left in place.

Poly is one type of membrane.

Anther is a topical membrane that can be used on the face of the cement board. RedGard, Hydroban, etc. They get "painted" on the cement board, then you tile on the membrane.

But as Bill wrote, any partial repairs leave you in an conundrum of sorts.

Not sure if this is worth anything, but you mentioned greenboard. Greenboard as a tile backer in a shower has been a known poor building practice for decades. It has been an outright code violation since January of 2006. That's no help to you now, I know that.

The big concern to address with your worker is the top horizontal seam of the repair. You'll have a seam in the tile backer, topped by a "seam" or grout line in the tile layout.

A seam over a seam is an excellent point for flexure failure (cracks in the grout) and eventual water intrusion into the wall. Any issues will be compounded by there being nothing behind the tile backer to contain any water that gets back there.

At a minimum, I'd remove the cement board and try to slide poly up behind the bottom edge of the existing tile backer, between the bottom of the existing tile backer and the wall studs. Even an inch will help. Enough to get some protection behind the eventual double seam. Then drape the poly down over the tub flange. Reset the cement board over the poly.

I just got done with installing a new shower floor base or pan and the new backer/cement board installation is okay! The tiles are placed in front of the backer board bottom past the bottom edge of the backer board to the bottom of the shower floor edge - about 1 1/2"-2" so that it will be sealed by the tile and grout.