A U-value is the measure of heat loss through a building element (walls, floors or roofs). U-value calculations take into consideration all components of a building element (i.e. U-value calculations for flat roofs typically take the following into consideration; outer & inner surface resistance, waterproofing system, insulation, vapour control layer, deck type, cavities, plasterboard ceiling and plaster skim).
The figures and information quoted are for guidance only. Official Building Regulation / Standard documentation should always be consulted.