It is a hot tar roof.
The joists etc are the same as the floor to the upstairs room. It is almost as though the end wall of the room was put up short so as to leave a outside area rather than the roof being an extention
we at present have a flat roof which is paved and used as terrace. It meets building regs etc. Unfortunately the paving slabs are breaking up. Can we put a decking floor on top? If Yes HOW and if No WHAT should we use?