Hi, we volunteer with The Firs Chalet at Mount Baker, WA. The Chalet is a nonprofit retreat center that sleeps 100 people for weekend retreats both summer and winter. The Chalet is located 54 miles from Bellingham, WA near the Mount Baker Ski Area. The Chalet was hand built in the 1950s with huge beams and tongue-in-groove planks. Winter snow amounts are typically 660" per season. Summers are glorious! We are remodeling bathrooms on the third and fourth floor. Bathrooms are about 8 feet wide and 16 or 22' long including showers. There is a floor drain in each bathroom near the two toilets. We can slope the tiles slightly at the hall doorway to ensure any water that spills can be directed to the floor drain. Pics and building cross section follows. The kitchen is directly below the third floor bathroom. WE MUST HAVE A FLOOR AND SHOWERS THAT CANNOT LEAK! We are open to flooring solutions. In the showers, we are thinking a Schluter Kerdi waterproof solution covered with porcelain tiles. On the floors, one option is 3/4" plywood over the planking, followed by Schluter DITRA taped up the wall with Kerdi Band, covered with porcelain tiles. We are having a hard time verifying that the floor construction and loads will be strong enough and stiff enough for the porcelain tile. We are also considering some type of solid vinyl sheet goods. What kind of floor prep and waterproofing would be used in that case. Flooring must hold up to heavy weekend use by students. Any observations or input welcome!