On Aug 16, 6:38 pm, "S H O P D O G" wrote:
Oh yeah, I'm getting it nice and clean. You can actually watch the tub
"sink" as it fills with water, that in turn pulls on the caulk rippingit
away from the surround.

SD

If you see flex like that then something is wrong, it would be helpful
to know if the joists are flexing or if the underlayment is
compressing...

In the kitchen under the tub hang a plumb bob from the joists while
tub is full so the bob is touching the floor, then pull the drain plug
and run downstairs to see if the plum bob rises off the floor with the
joists. If it does then the joists are flexing... so sister the floor
joists under the tub with more wood (while tub full) till no flex.

If the joists are not flexing then the underlayment is, in that case
you will probably have to live with it and cauck the tub with butyl
rubber window caulk instead, that stuff has an enormous amount of flex
and adhesion.