When constructing a warm roof, the ridged insulation is laid above timber joist and between the sub deck and roof deck. Ventilation is not acquired as there is no air space between the roof and the warm air this prevents any build-up of condensation. The simplicity of the warm roof makes it the most popular design for affectively insulating a flat roof. Through this method there is little or no cold bridging while ensuring there is no unnecessary heat loss. The advantage to this is it saves money on heating costs. This is the best and most comonly used flat roof design for Irish climate.
Cold roofs are the traditional or old method of building flat roofs in this case the insulation is placed between the joist, therefore leaves the roof decking and joist cold, causing thermal bridging. Cold roofs require a void between the top of the insulation and the roof deck. This void must be ventilated to prevent the build up of condensation which could lead to mould or damp. This roof design is generally not recommended as a design for new build flat roofs by architects and engineers.