I live in an apartment building. The resident upstairs has overflowed her bathtub 3 times in 2 years. The water damaged my ceiling and has been repaired twice. Now the ceiling will be replaced at her expense. What is the best material to use for this -- knowing that it seems likely an overflow could easily happen again with this resident. MY building super advised that it's better to leave the current plaster ceiling (built 1953) because it has some small rocks and gravel that are intended to absorb small amounts water. He recommends putting a "water resistant" drywall over the current ceiling. What is the best long term solution for me ? Is there a waterproof drywall? If waterproof drywall would have water spilled from upstairs, would it become watersogged in time? Or would the water sit in the ceiling cavity, possibly collecting mold ? What is the best long-term solution for me?
While there are solutions to overflowing tubs or washers and dryers, they all need to be fashioned upward, and you are downward of the leaking fixture.
Some locales now require a flood pan to be placed under washers and dryers, and a bathroom could be constructed with a flood pan under all fixtures. It is basically a waterproof surface that rises up at the sides with no seams. This serves to hold in any spilled water until it can be removed. This obviously wont work indefinitely if the leak is not stopped within a short period of time.
Trying to waterproof from below cannot rise up at the sides. Therefore, all you would do is either soak up the water above the ceiling (leading to mold and deterioration) or channel the water to the edges where it will run down inside the walls creating damage elsewhere.
You might want to consider a no-paper drywall (often colored purple, instead of white or green). This has no material that can support mold and it would slightly reduce the risk in the event of a very minor leak.
If there is a significant leak, nothing will help.
The better solution is to fix your neighbor's overflow drain so it doesn't happen again. I can't imagine that any of these solutions isn't going to result in a mold problem if they're allowed to get very wet in an already damp environment and then not be removed and replaced.