The ceiling joists effectively tie together the feet of the rafters on opposite sides of the roof to make a triangular structure.
You can replace the ceiling joists with deeper 8x2s which - as well as tying the rafter feet - would also form your new floor. BUT, if the new joists are split by the 152 x 89, they will not be as effective at tying the rafter feet together and the roof might spread long-term.View attachment 191142
Thanks for the sketch. Your option would make sense but does not work on my case as I would have to expose a beam below the ceiling and wife is completely against it.
I understand the joist are part of the structure and let leave them alone. Don't want mess around with that. What about my proposal below?
Double the joist number to reinforce/help the old twisted ones and will put 50x150mm blinders instead of the old 50x100 with 400mm center to match beam height to have a flat flooring. Would this make a stronger flooring? The new blinder will have a span of 3.5mm and set to the existing ceiling joist which is set on top of a wall bearing wall. Any Thought?