The waterproofing can be behind the cement board in the form of tar/felt paper or sheet plastic. We can't see if that was done. What we can see is that not nearly enough screws were used to secure the Durock to the studs (8" OC for walls, 6"OC for ceilings) and we can see that mesh tape and thinset were not used on the seams and corners.
Absent the waterproofing behind the cement board, a newer method is to apply 2 coats of RedGuard or similar liquid waterproofing membrane to the cement board. (After the seams are taped with alkali resistant mesh tape and thinset.) Obviously, there was no membrane applied to the Durock.
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Master Bath Plan
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