Flat roof insulation

[foxy]

I am looking for advice on how to insulate an existing uninsulated flat roofed extension. I'm almost certain there is nothing between the plasterboard and the exterior material.

From what I can see the only options seem to be to take down the ceiling or remove the outside skin.

 

[z102]

If you use hard polystyrene or hard polyurethane foam sheets on top of the existing roof cover (presumable bitumen felt ) than you have a better insulation for less money compared to just filling the space between the rafters. Both in combination would of course be even better.But if you need a new cover for the roof sooner or later than the insulation would be best on top of the roof. Covered with the skin of course.
If you take of a ceiling rosette (fuse!) you might be able to peep into the structure to see if there is already some insulation.