Insulation must resist water absorption, frost damage, this cuts down the choice that is available. Waterproof membrane is protected from damage. Separate vapour control is not required. A form of slab is required to hold down the insulation, this adds extra loads. Access for maintenance can be difficult.
Structure becomes a loadbearing structure and roof deck.
Laying a screed creates a fall, this also acts as a smooth base for the insulation and waterproof membrane above. Insulation material can be cut to falls if required. Most likely to be found on blocks of flats or commercial buildings. These are not normally as economical as the other forms of construction. Simple sheeting is not possible due to the risk of weather penetration.