Floor is higher at the edges of all external facing walls making the floor unlevel. Planning on laying 6mm ply, do I need to level the floor first?

[Lendios]

Subfloor/Joist is higher at the edges of all external facing walls making floor unlevel, rest of the room is flat. I was planning on laying 2.44m x 1.22m sheets of 5.5mm ply in preparation for engineered wood fooring being glued down, is this going to be an issue or do I need to level the subfloor first?

Really strange how it's like this on all the external facing walls, not sure if it's normal? The chipboard is 18mm, I'm thinking the sheets of ply are so big it will make the gap unnoticeable as the floor just has to be flat, not necessarily level for glued down engineering wood flooring?

Thanks

 

[Koolpc]

I would want to see what is going on underneath the floors first.

 

[Lendios]

I would want to see what is going on underneath the floors first.

I've been repairing the chipboard where it was cut up for electrical and plumbing works, adding in noggins and such, so I've seen underneath and there wasn't anything that looked different to the other joists, it just seems that joist sits higher than the rest.

Unfortunately I haven't taken any good photos and I've screwed and glued the repaired bits back down, but I've attached what I have. Please ignore the hole...my girlfriend tripped and went through the ceiling (thankfully not all the way through!)

What would you be looking for?

Thanks