My house was built in 1925, it has plaster walls. The area around the bathtub/shower is only tiled halfway up the wall. We recently hired someone to finish the tiling up to the showerhead, and he placed the tiles directly on the wall. I'd assumed he would cut out the wall and put in cement board or something. It appeared that he did no waterproofing before adhering the tiles. Is this ok? Is it ok to install tile directly on plaster walls in a shower? He also cut a hole into the wall to build a box for shampoo and whatnot, and he did this with wood. I am worried about having moisture problems as a result of this. I hired this tiler in a hurry, and did not do much research. I'm not sure what I was thinking.
I have repaired tile jobs that were over 20 years old that the tile was applied directly to regular sheetrock and plaster walls. This was back in the 80's. It used to be very common to put tile up on whatever the wall was and seal the the tile. More recently I have seen some contractors still put tile up with out a backer board to save $ for the owner especially for flipping a property. Is it the best way? NO. Will it work? Yes. Maybe several decades if well sealed. All the frame work around showers and tubs I have installed has been wood, again if sealed it will last for quite a while if there is a slight angle to prevent water from pooling on the ledge or box. You should have received a quote prior to the start of work that you agreed to (the law in my state) it should define the scope of work and materials. For what it is worth if cement backer is not included the contractor may be matching the existing tile job.
no plaster is NOT waterproof I've seen problems with it exposed to water, see:
not nearly as bad as wood though.
also the recess should have been waterproofed.
trouble mostly proves where water consistently contacts or pools.
I've needed to fix bad damage where waterproofing was left off of shower walls. but damage is typically much worse on horizontal surfaces with poor waterproofing construction.
though it's reasonable to believe you won't have problems at all as this work seems to have been done at an elevated level. but absolutely keep water out of that recess.
In this case, problematic areas are in the corners.
In an old construction, or wooden construction, walls can move ever so slightly relative to each other. Tile grout is not flexible, and a crack can form in the joint in the corner, which lets water through. Joints on flat surfaces are much safer.
So you should check your corners every few months, just look at them up close and run a fingernail along the joint to check for cracks. The bottom of the recess for the shampoo bottle is a potential problem area too.
It's a pity the guy didn't waterproof, it isn't that expensive or time consuming...