Flat roof + regs and height and architects argh!

We have a flatroof extension going on, it's about 6metres square, butting up to back of bungalow.
We then have dormer from roof finishing at gutter, flat roof will double up as terrace (large balcony) for loft room.

The structural engineer spec'd the 9x3 inch ceiling joists for extension in order to take the weight of being a terrace area, and on top of that the architect adding this celotext with ply bonded to it as the roofing, of course im very concerned about how much im going to bang my head stepping up from loft room onto balcony, i just assumed it would all be similar levels, i dont understand why you can't just cut the celotex and wack it between joists?

Any links pointers appreciated.

you can wack it betwixt but you will also need to under-draw with celotex too.

you may also have to fill it solid, ie. no air voids.