fixing shower tray

[baj25]

I'm fixing down a 1200x760 cast resin shower tray. It has 5 screw legs. I've put 18mm WBP ply down under the whole tray area onto the existing T&G floorboards, and screwed through into the joists. But, if I stand in one corner of the tray, I still get about 0.5mm deflection- it looks like an edge of a floorboard is slightly proud of the others and is bending. So, to the questions- 1. what can I use between ply and floorboards to spread the load better? (cement/builders sand screed? self levelling compound?) 2. what can I use to stick the adjustable feet to the ply, to stop future side to side movement?
TIA, Brian

 

[Fluffster]

Have you screwed down every 100mm in a square pattern into the t/g floorboards?
Pete

 

[baj25]

No, I've not seen that mentioned anywhere I've looked. I have screwed every floorboard to each of the 3 joists under the area, and then screwed the ply on about 300mm centres to the joists & floorboards.

 

[Fluffster]

Whenever we lay marine ply say for tiling on etc we always screw 100mm squares to ensure a good mating of materials especially as most boards are not level flat.
It may assist you in stopping the deflection, unless the t/g boards pull away too!
Pete