Roofing Underlay / Breathable Membrane

A flexible sheet laid over the rafters and beneath the roof battens to provide a secondary line of defence against wind-driven rain and to allow moisture vapour from the warm interior to escape (breathable membranes). Modern roofing underlays are typically reinforced polypropylene, highly vapour-permeable, and installed with a specified drape for ventilation. Non-breathable underlays require a ventilation gap above and below.