12mm hardie backer on uneven floor

[Jason85]

Hi,

I have an uneven timber floor and one section I will be sitting my shower tray.

I have used 18mm inbetween my joists and a 6mm screwed over the top.
I have then layed a 12mm hardie backer board over that using 12mm notched trowel with non flex adhesive. I have only used this to get the hardie backer level. Would I still need to screw it to the timber underneath once the adhesive has gone off or could I get away with it as it is