I would certainly insist the contractor replace that board before installing the tile. A very minor replacement now --- very major once the tile is installed.

That's some pretty serious structural settling before the house is even dried in. I'd want reassurances that it's not symptomatic of a larger structural issue. As in an inspection by a structural engineer.

Interesting point, live_wire_oak. My husband was thinking along those lines, but in terms of something being wrong with the area behind the board. The crack had to have appeared immediately or within a day or two of the board going up, since I'm over there almost every day. There's also one very small and not deep crack in the downstairs slab, but I know that's common here, so I'm going to keep my eyes out for any other signs that we might have a deeper issue.

EmmJay, a crack like that is unacceptable. If anyone calls you "too picky" ask them if they'll be picking up the bill when subsequent damage occurs from such a defect down the road.

Is it cement board - or drywall? It may have no structural repercussions, and no structural function, but sealing issues are obvious.

Sealing against water intrusion is done on many (somewhat redundant) levels: tile and grout, mud, backer board, plastic sheeting, and so on. The cement board not only seals but is structural as well, and it's failed. "seal with a joint membrane" can mean pretty much anything.

There are three ways cement board can crack. My suspicion is that the piecework tile guy did it and was fully aware of it and expected to just cover it over with mud and tile later. The reason I don't think this was what happened is even a hack would still be aware of things enough to smear on some mud, with or without cement board tape, to cover it.

Secondly, it could be from being stressed by being bent under heavy fastening to a shabby understructure (assuming warped wood?). The third is via the stuff it's fastened to moving. These later two can happen after the installer has packed up and left. None are acceptable as a tile base on an overhead or a vertical shower wall. They all smack of middle finger workmanship.

It would take a very few minutes for the worker to come back and rework that spot at this stage, and a huge number of minutes to do it later when you contact the builders to remind them of the concerns about excessive movement and shabby installation that you pointed out in the registered letter you sent them today. Make sure the repair guy doesn't just fill in the crack.

It's the waterproof backing board, not just drywall. I'm shocked they left it like that (if it cracked before they left) since they saw me at the house several times while they were working. Do they really think I'd just shrug and ignore a 26 inch crack in a water area?

I haven't sent anything formal to the builder, but I have tons of photos and our whole correspondence is documented in emails. I'll take it to another level if needed, but I'm waiting for the response to my "Um, no" reply to the "It's fine, that crack is acceptable" email. I figure that until they take care of it, I'll have to loudly discuss it with the hubby whenever I'm in the model and potential customers are there. I work at home, so I can make it my business to be there even more often than I am already. This is a top builder with a good reputation, so I just don't get the resistance on something so obviously screwed up as this.

The point of the above is that you will be sending them a registered letter today...

And, it isn't waterproof anything. It is just cement backerboard. Once it is repaired, it will be covered by something waterproof (hydroban, redgard, kerdi) before it is tiled. Or, your shower walls will not last.

Good luck. (And, maybe hang out in/read up on tiling showers in the bathroom forum).

Great suggestion on reading that forum! Building a new home is definitely turning into an education. I'm just glad there are so many resources and helpful people here. My last home building experience was both pre-internet and long distance, and the house literally almost had a fire due to what must have been drunk electricians.

I definitely would not allow that to go with out being replaced immediately, do not go one step farther in the shower till it is all repaired and can be properly sealed for water tightness.
That is ridiculous that anyone would tell you it is ok. I would be really suspicious of any other work if they think that is ok.

Shower leaks can be nasty! You usually do not realize the leak is happening until the studs are rotted and the floor joists start to show signs of rot too. There is only one way to repair studs and joists in this situation...

Cracks in multiple units is a big red flag on the quality of the overall project. It's not just about the shower being watertight. I'd be concerned with the overall structural integrity of the building.

I thought I had posted a follow-up on this, but apparently not. They replaced the board, and I've been poking around the adjacent units to make sure there are no cracks or other bad signs in any of them. The way the crack was near the screws makes me lean toward sloppiness as the cause in my unit.