Warm roof
On flat roofs, a 'warm roof' is one in which the insulation layer is laid on top of roof structure. This results in the structural deck and its supports being at a temperature closer to that of the building’s interior. This is as opposed to a 'cold roof' in which the insulation is below, or within the roof structure, and so the roof structure is closer to the outside temperature.
An 'inverted roof' is a form of warm roof in which the waterproofing layer is beneath the thermal insulation rather than above it.
On pitched roofs, the definition of warm or cold can relate to the entire space below the pitch of the roof, but above the ceiling. This means that a warm roof might be one in which the insulation is installed in line with the rafters (rather than above the roof structure), so that the space under the pitch, ie a loft, attic or other space is insulated and warm. A cold roof in this case would typically have insulation laid above the ceiling so that the loft space below the pitched roof is cold relative to the rest of the property.
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