I'm trying to repair the seam between a whirlpool tube and the cultured marble wall. I think the floor joist under the tub were not spaced correctly for a large fiberglass whirlpool tub. I'm guessing that with about 20 gals of water ( 160 lbs ) and an adult male ( 210 lbs ) is adding so much weight that the tub is sagging away from the wall and pulling the caulk loose. This creates a leak to the kitchen ceiling. I haven't ran a string across the ceiling downstairs but I'm sure its sags from the weight and the dissection of the floor joist done buy the plumber that installed the drain. I used bags of sand to weight down the tub today and used silicone caulk generously in the void. When it dries I'll remove the 350 lbs of sand and see if the seal holds when the tub is used. Ultimately I know that dropping the kitchen ceiling and doubling the joist will stop the sag but what is the proper way to seal the walls to the tub. Before I caulked today I could see the bare studs and fiberglass insulation through the crack between the wall and tub. It sags about 3/16 to 1/4 inches with the weight. Shouldn't there be some type of waterproof membrane at this point just like you use behind tile ? I'm familiar with the lip of a steel tub that the wall can overlap but this fiberglass unit doesn't have that. Thanks for any thoughts.