Suspended ceiling replacement

[scottpetrie]

Hi
I have recently purchased a building which has an old dated polystyrene suspended ceiling. Sort of thing you would see in an office type environment. How would I go about replacing it for a plasterboard ceiling? Ive had a look up behind one of the tiles and it looks as if theres no wooden beams.

 

[freddiemercurystwin]

What is the soffit construction/material above the ceiling then?

 

[scottpetrie]

Its open to a metal roof with the metal frame work for the ceiling suspended by threaded rod