As part of the building regulations application for your extension, you’ll need to include full details of the proposed roof – type, dimensions, performance properties and the materials you’ll be using.
The regulations are concerned with:
Ventilation: as well as keeping out the weather, ventilation may be needed depending on how your roof is designed and how the insulation is installed to reduce the risk of condensation. If insulation is immediately below the weatherproofing layer (warm roof construction) ventilation will not be required. Insulation immediately above the ceiling will require a ventilated void above.
Loading: the load your roof will be able to support is important. The roof needs to support the materials it’s made from and the extra load put on by the weather (snow and wind, for example) and maintenance activity.
Insulation: has to be of sufficient thickness, performance and quality to prevent heat loss through the roof.