The context: The upstairs bathroom is being made larger. Separate tub, shower and other bits and pieces. An acrylic shower pan will be moved over an internal brick wall (below), and just into a room that is over the kitchen. The joists that span over the kitchen below are 4.5 meters (5 yards) long. A year ago an electrician notched the underside of all the joists in the kitchen - midway - to get a low amp mains cable from one side of the room to the other (LED lights for the kitchen cabinets - two AC/DC drivers at opposite sides of the room. The notches were 40 mm (give or take; 1.5") deep into a 180 mm (7") joist (each 40 mm - 1 3/4" wide).
The problem: Everything bounces now when walking upstairs. Mid-room upstairs, that's a 3 mm (3/32") bounce. One edge of the shower pan would be moving 0.5 mm (1/64"). The retaining stud wall for that has yet to be built and could reduce the bounce. I'm guessing the range of the movement, but you can see a glass of water wobble significantly, when it does not in similar upstairs rooms.
Question: Is there a way of tiling down to the acrylic shower pan up against a new stud wall (or will be), that will not open up and be a water egress route? Or is there something that can be epoxy-glued onto the edge of the shower-pan that'll ensure water out through a small gap only goes back into the sower pan. Assume 10+ years for that shower.
Is my only choice to open the floor-void that has the notched joists and steel-reinforce the notched areas. See https://www.joist-repair.co.uk/notches.htm. And do that to many if not all of the joists that were notched.
If I remove all of the old floorboards I could put new joist next to all of the existing ones (screw them sideways into the old) which could be cheaper/better than the metalwork needed. If I replace all the older floor boards, I would put down newer four-way tongue and groove boards, but I'm not sure if that would make any difference to the bounce or not.