I am renovating an old bathroom that is circa 1910. I took out the old tub, and noticed that the tub and tile sit on a mud bed that is about 4-5 inches deep. A lot of people have told me to just float mortar over the old mud bed and then lay down tile. However I had noticed that there is a slight dip in the floor. I can see that a drain pipe is running through the old mud base. My concern is that moisture from that drain or some other source (tub overflow, etc) has rotted out a floor joist. The house is otherwise square. I do not think this is a foundation issue because this is on the 2nd floor, and the 1st floor is level. My question is how I take out this stuff without seriously damaging the ceiling beneath, or the floor joists. All the other posts I have found on this subject relate to mortoring over the existing base.