Building Regulations set standards for design and construction which apply to most new buildings and many alterations to existing buildings in England.
Part L1B Conservation of Fuel and Power in (Existing Dwellings)
Part L1A is slightly more stringent than part L1b this is because is considered easer to build the insulation in when the building is being built
If you have an old flat roof and you are striping the old roof and replacing it with a new roof covering then you will trigger part L1B
If you are “capping” the roof you will not trigger the building regulations.
Because of the amount of confusion over when the regulation is triggered and when it’s not I produced a video where I explain the situation, this has now been updated with a second video I link to both below.