The insulation is placed immediately above the ceiling and between the joists. The void above the insulation must be well ventilated so as to prevent moisture condensing on the cold timbers. A continuous ventilation gap is required at the eaves. Wider choice of insulation as it does not have to carry loads. Waterproof membrane is easily accessible for repair.
Thermal movement is more likely as the roof warms with the sun.
If condensation occurs there may be fungal decay of the timbers and staining of the ceiling. Where the insulation is positioned above the deck. Temperature of the deck and structure are closer to the inside of the building. Protected from the extremes of weather, therefore the potential for damage through movement is reduced. A ventilation void is not required.