Leveling Second Floor room

Hi Everyone,

I'm a first time poster on this forum and i will say now i'm a keen DIYer but not a professional.

My bathroom (which is on the second floor) has a fairly uneven floor (i would estimate a dip of about 1inch in the middle) on which a so called professional tried to tile onto. They did fit plywood down but i'm unsure how thick or how well secured. As a result all the grouting has cracked on the floor looks a mess.

I need to level the floor so i can either re-tile or lay vinyl (i understand for vinyl a level floor is not a requirement but i'm a perfectionist)

I keep reading about self leveling compounds however i'm a little concerned are they suitable for second floors?

Do you need to cover over any gaps to stop the compound seeping into the void between the floor and ceiling below?

The other way i was considering which we have used in another room is to create a timber frame of varying thicknesses to create a level and lay ply or chipboard ontop.

Any help would be appreciated, I am keen to tackle this problem myself with help for family (father in law used to be in the building trade)

Tom

 

Flooring board which are not in the best state, on top of which bodger the bathroom fitter has put down ply unsure on thickness or how well secured as i was working away when this was completed