COLD OR WARM ROOF DESIGN. WHAT IS THE DIFFERENCE? 

1. Cold roof construction
Cold roof construction is probably the most commonly known type of flat roof, however most new builds are opting now for warm roof construction, primarily because the insulation value  is greater. There is nothing wrong with "cold roof construction" as long as it is correctly constructed and insulated and adequate ventilation is provided.
The advantage of cold roof construction is that the overall depth of the roof can be kept to a minimum which may be necessary to allow for window openings, Fascia depth, etc. The roof surface is more solid and less likely to be damaged by foot traffic or falling objects and is much cheaper to install. Properly constructed roofs will last a lifetime if correctly installed from the start.

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2. Warm roof construction.​
Warm roof construction is becoming more popular in new build and refurbishment projects, mainly due to its greater insulation value.  With a warm roof, the insulation covering is continuous across the roof, which means there is no thermal bridging (no heat loss through the roof joists etc.) It also eliminates the need for ventilation in the roof space which can be one of the main causes of roof breakdown.
The insulation can be installed by the roofing contractor in a range of thicknesses to suit a client's needs. This system can be used on an existing timber roof to upgrade the insulation value as long as the decking is in good condition and there is no trapped moisture below the existing membrane.
This system is also suitable for installation on concrete, steel or asphalt roofs to upgrade the insulation value.