A vapour control layer limits water-vapour movement within a designed construction. Its role differs from that of the external waterproof covering. Appropriate position and connections depend on the complete roof assembly and the conditions in which the building is used.
On existing roofs, its presence and continuity around junctions may be unknown. Ask which information comes from documents and what has actually been assessed. A visible membrane cannot automatically be identified as vapour control without further information about its material and function.
When insulation or use changes, building-physics relationships belong within the investigation question. This site prescribes no universal layer sequence and does not advise adding a membrane yourself. Have the actual construction and proposed solution assessed as a complete arrangement rather than isolated layers.
Worth asking
Frequently asked questions
Is vapour control the same as roof covering?
No. Their roles and positions within the assembly differ.
Can I add an arbitrary membrane?
This site does not advise that. Relationships with existing layers require appropriate assessment.