Cromar Vent 3 Blue Breathable Roofing Membrane 1m x 50m is a vapour-permeable roofing underlay designed for use in pitched roof construction. Installed beneath the primary roof covering, the membrane provides a secondary protective layer while allowing water vapour to pass through the roof construction. The 1 metre wide roll provides 50 metres of membrane, making it suitable for new roofing projects, extensions, renovations and roof repairs.