The overdeck system gives an insulating exterior skin to the building thereby protecting it from daily expansion-contractions due to day-night temperature variance and thus protecting the underlying layers (waterproofing & the concrete deck) from thermal stress cracks due to the same. Moreover, it uses the underlying high mass (high thermal inertia) concrete layer as a storage for ambience (heat/cold) thus improving the insulation effect. KOOLSHEETS made of High Density Polystyrene foams or Extruded Polystyrene (XPS) sheets are laid over the waterproofing layer on RCC Slab to provide a thermally insulating skin to the roof. The compressive strength of the insulation layer should be sufficient to not compress by more than a milli meter under a man’s weight. XPS sheets of density 34 Kgs/m3 or Polystyrene Foam Koolsheets of density > 24 Kgs/m3 have such a strength.