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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.

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    I would expect full 7 inch joists with a 15 foot open span to be bouncy. Knocking an inch and a half off in the middle can only make it worst. It would have been a bit better if he had drilled holes in the joists.
    – crip659
    Mar 12, 2023 at 16:18

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The best solution is to take measures such as sistering in new boards along the joists as you proposed and new tongue and groove sub-flooring.

With or without that done, it would benefit you to cover the shower walls in 1/2" cement board such as durock. Apply cement board rated tape and mud any seams. The seam between the acrylic pan and the wall can be sealed with a polyester caulk over the cement board rated mesh tape.

This will allow minor movement to the base from the walls yet keep the seal intact. Then cover all the walls with a waterproofing finish such as RedGuard or AquaDefense. This will make the entire shower waterproof and the products act as an uncoupling membrane as well.

Tile as you wish and you will be good for years.

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