Joist and plywood inlay advice

[john2k]

I have a 1700x800x25mm Mira Flight Safe stone resin tray that I need to install as low as possible for a wet room look. My initial plan was to cut out the chipboard floor where the tray will sit, then use 38x63mm CLS battens screwed to the joists slightly lower, followed by a 20mm plywood layer laid in between the joists.

However, now that I’ve lifted the chipboard floor to check the joists, I’ve encountered an issue. The house was built around 15 years ago, and the joists used aren’t the standard type. I’ve attached photos to show what I mean. On the external block wall, there are metal joist hangers. But instead of a timber piece exactly the height of the hanger, there’s timber at the bottom of the hanger, followed by what looks like OSB board, and then another timber piece at the top of the hanger.

I’m not sure how to proceed with installing the plywood in between the joists to make everything level.

The current chipboard is 23mm thick, and the tray is 25mm. I’ll be adding 9mm tiles plus a decoupling membrane, which will raise the surrounding level. I’m wondering if it would be possible to use a thinner material over the existing joists instead of doing all the infilling. What’s the thinnest plywood that could work as a structural layer if I add extra timber support across the joists?

Photos attached for reference. Any advice would be greatly appreciated!

 

[thomp1983]

Those are i joists, perfectly normal for the age of your house.

You can still so what you originally planned it's just a bit more work, if you use pu glue and pilot hole the hangers you can still fix your cls to the joists, the 2 pieces running between the joists are probably there to prevent them twisting so lowering the top one shouldn't cause an issue especially as your fitting some ply in between which will have the same effect.

 

[foxhole]

Just lay you ply and lay your tray on top of, with tile, cement , mat,etc, over board entire floor with cement board , should only leave a few mm above the tile surface which you will need for a water proof seal to tray.
So raise floor level around the tray instead of trying to lower it.

 

[john2k]

Just lay you ply and lay your tray on top of, with tile, cement , mat,etc, over board entire floor with cement board , should only leave a few mm above the tile surface which you will need for a water proof seal to tray.
So raise floor level around the tray instead of trying to lower it.

I’m leaning towards this of just putting plywood on the existing joists. So the current chipboard sits 23mm above the joist level. What’s the lowest thickness ply I can go with

 

[foxhole]

22mm ply , well supported .

 

[noseall]

I’m leaning towards this of just putting plywood on the existing joists. So the current chipboard sits 23mm above the joist level. What’s the lowest thickness ply I can go with

You want the ply to finish level with the existing, surely?