For the second time in less than 10 years, I tore out the entire kitchen ceiling because of extensive water damage from an upstairs bathroom. The first time was due to a leaking drain line in the sink, so I had all the plumbing and waste lines replaced. The second because of a leaking wax ring on the toilet and a clogged toilet overflowing. The first time the ceiling was replaced, the contractor stapled thick plastic sheeting to the joists before installing the ceiling tile, but over time, water still leaked from above onto it. Is there any way to make the bathroom floor more waterproof so water does not seep below the floor and above the new kitchen ceiling? How do contractors make second floor bathrooms waterproof in new construction?