A vapour-permeable membrane allows water vapour to pass relatively readily within its intended application. The term alone does not establish its correct position in every roof.
What does this mean on your roof?
On a tiled roof, a membrane may form part of the underlay. Its properties need consideration alongside insulation, internal layers and junctions.
Vapour-permeable and vapour-retarding are not interchangeable labels. Colour or thickness alone is also insufficient to identify an unknown existing product reliably or establish its intended use.
During renovation, ask which membrane is proposed and why it suits the construction already present. Connections around roof windows, eaves and walls belong in that explanation. Any retained layer needs inclusion in the assessment. Adding another membrane without a coordinated design can create further uncertainty about how moisture moves through the assembly rather than resolving an existing problem.
Frequently asked questions
Does vapour-permeable mean an open hole?
No. It concerns material properties and vapour movement, not the absence of a protective layer.
Can any membrane be used beneath tiles?
No. Intended application and the complete roof build-up determine suitability.
Consulted 19 September 2026. Check requirements for your own address and selected system.