The roofing works to be carried out will need to comply with the requirements of Building Regulations 2000, Part L (Revision October 2010) in England & Wales, or The Building (Scotland) Regulations 2004, Section 6 of the Building Standards Technical Handbook in Scotland.
The calculation of thermal transmittance, or U-Values, for a roof is controlled by the above mentioned regulations. There are different procedures according to whether the roofing work is for new-build or refurbishment. Where the refurbishment works is part of a change of use of the building, the works should comply with the latest Building Regulations and any revision thereof.
In all circumstances, it is recommended that advice be sought from your local Building Control Office, as to the compliance requirements for this particular project. The following links offer outline guidance in specific areas: