Plumber didn't have enough. Don't let him tile until it's corrected.

Questions like this should be directed to your builder.

Ran short and did not want to buy more to finish the project would be my guess. But I would definitely question your "builder".

Where's Waldo? There's more than just the board shortage that's an issue in that pic.

My board went out to cover the entire walls of the tub properly. Ask, er, tell, your contractor to do the same.

Source

Your contractor undoubtedly knows more than Durock about the installation of its product.

("We always do it this way. No problems.")

Interesting that nowhere in the above Durock manufacturer drawing do they specify additional waterproofing.

As to Panda Bear's concern, it doesn't look to me to be anything to worry about.

Who needs waterproofing all the way 'round a tub or shower...! Hint: everyone.

...nowhere in the above Durock manufacturer drawing do they specify additional waterproofing.

Good point!

Durock does note that TCNA guidelines recommend a waterproof membrane.

Or, to save a step, you can use a waterproof cement board.

Is there a shower head above the tub?

@ Mark Bischak, Architect If this is not a soaking tub it will be installed after the tile is done :-)

What did the builder say about the cement board?

I suspect that one small panel was installed with the rough (logo) side facing the wall so the exposed face is smooth which is not a problem.

Wonder Board is now about 50 years old with many imitators. In the mid to late 70's the Tile Council of North America added a hand stamped note diagonally across their drywall shower wall detail saying "NOT RECOMMENDED" but it took a long time for backer boards to be accepted as standard.

I ran a 13 story condominium project in Minneapolis and specified Wonder Board in the showers but the Construction Manager changed it to MR gypsum board to save money however the tile low bidder included Wonder Board in his proposal even though he was not expected to install a substrate. In a team meeting the sub was asked why he included Wonder Board and he said he had found that to be less expensive than than replacing the shower walls later.

But waterproofing was not recommended by manufacturers except behind the backer board in moisture sensitive areas. Waterproofing the face of the backer board came much later.