Floor preparation in a bathroom

[Terry7556]

Evening folks .
I am getting ready to start preparing the floor in my daughters bathroom
It is currently 22mm floorboards and the room is 3 mts long x 2 mts wide
In the 3 mt direction the levels are almost bang on and in the 2 mt direction there is a difference of 12 mm
The wall behind the towel rail is in the centre of the lounge below .
The lounge is 18 feet wide and I knew there would be a sag in these floors on the first floor .i was not expecting 12 mm though but it is what it is in a 1960 s house .
It was my intention to use 6 mm hard backer board under the tiles I will be laying .
Can I have some advice about what to lay over the floorboards first of all

 

[johnny2007]

You need to use self leveling compound after the cement board.
Make sure your floor boards are solid before doing anything else.