Help!: Problem with wet room floor

[cruddles]

1st post and it's long winded so bare with me.
Problem with a wet room shower approx 80cm x 160 cm. Tiles in central area started flexing and grout breaking up. I removed tiles, adhesive & tanking membrane. I've discovered that floor is 18mm ply directly onto joists which are 40-45cm apart (across the room). I assume that there is enough flex in this to cause the tile lift etc.
I cannot access the trap so it's therefore not possible to simply add another layer of ply or wedi board etc as I cannot bring the level of the trap up.
The solution that I am thinking of is to cut out the plasterboard ceiling below and put install noggins to support the floor. I have other plastering work on the agenda so plaster patching isn't a big deal.
3 questions - 1) is there an easier option that I am not seeing? 2) Will I have to dismantle the trap to re-tank under it's rim to get a good seal? 3) Can you recommend a good flexible adhesive for this situation?
Thanks in advance for any advice.
Cruddles

[Dave]

Hello and welcome.. Is this a project you undertook..?

I think you do need to brace the floor to remove deflection if that is the issue.

[whitebeam]

Is it 18mm ply in the actual shower base???

[Sean fsy]

I would start again using a propper shower base like impey aqua dec.

[Sean fsy]

How big were the tiles? I will only ever tile using a mosaic kind of tile in a wet room floor. If you lifted the tiles easily I would say there might be a problem in the installation, wrong adhesive or something. The floor might have shrank, is there any joins in the ply?

[cruddles]

Thanks for the responses, I'll answer in some kind of order.

1. We moved into the house a year ago and the wetroom was already built. I am trying to repair the floor after the tiles started moving. I'm discovering how it was built as I go along.
2. 18mm ply is the actual shower base.
3. Don't really want to start again as the walls are fine and intact.
4. Tiles were 30cmx30cm. Smaller tiles would make sense. Generally, the tiles were well fixed, there didn't appear to be a problem with the adhesive. There are no joins in the ply.

Cruddles

[Scott]

Is the adhesive stuck to the back of the tile or the floor?

You could brace the floor from below, which way to do the joists run in the shower, long ways or across the short side?
Is the shower against an external wall(s)? or is it stuck in the middle of the house?

[cruddles]

Adhesive was stuck to the floor rather than the tiles.
The joists run accross the short side of the shower room which is situated in the middle of the house.

[Scott]

What are the tiles? Ceramic, porcelain, stone?

You could add noggins from underneath, you could also add steel plates to the length of the joists, depends on how the joists are with the tray. These are bolted through the joists to remove deflection along each joist individually, however adding noggins isnt too easy with these.