Thickness of board under shower tray

[lostdiyer]

My bathroom floor consists of 2 layers, OSB board and T&G chipboard. I’m laying a 1200 x 760 shower tray down on plywood but was wondering which thickness plywood I should be using?

I was thinking 18mm but may need to go thinner as I need my floor tiles to cover the plywood. Not sure if there is a minimum thickness? Or a better material to use e.g. OSB, tile/hardie backer board.

 

[Madrab]

Depends on what the tray is sitting on - OSB3 or P5

If it's OSB3 then that's not so bad. If it's P5 then if you're worrying about height, cut out the chipboard and replace it with 22mm OSB3/WBP ply. That will also allow the joists to be supported, if needed, where the tray outside edges will sit, if they happen to be sitting between joists.

 

[lostdiyer]

Depends on what the tray is sitting on - OSB3 or P5

If it's OSB3 then that's not so bad. If it's P5 then if you're worrying about height, cut out the chipboard and replace it with 22mm OSB3/WBP ply. That will also allow the joists to be supported, if needed, where the tray outside edges will sit, if they happen to be sitting between joists.

The tray is sitting on P5. I’m thinking of laying some 12mm ply as this would be covered by the floor tiles.

Would 12mm be sufficient given that there is P5 and OSB underneath?

 

[Madrab]

If it's 18/22mm P5 then yes a 12mm on top would be fine. That being said I hate P5 flooring anywhere in a wet space, any water on it and it just turns to mush.

 

[lostdiyer]

If it's 18/22mm P5 then yes a 12mm on top would be fine. That being said I hate P5 flooring anywhere in a wet space, any water on it and it just turns to mush.

I’d prefer to replace the P5 but the way the joists are positioned makes it a little tricky to get the floor secure. If I cut the size of the tray I’m not sure how to secure the plywood I replace it with as there’s no joists near the dividing wall. Hopefully this makes sense.

 

[Madrab]

Ah ok - you have I joists - a little more tricky. If it was normal joists you could just drop the CLS support down level and screw from the back side but with I joists they don't have as much lateral support so not sure you could really do that. That and you'd also lose the support to the P5 on the other side of the wall. Only way would be ot cut the flooring on the side and drop in lateral supports.

OK, it's an ideal to add in extra support underneath but if it's a stone resin type then the board on top should be enough.

 

[lostdiyer]

I’ll add in some support across the joists and some plywood on top.

Thank you!