An economy breathable roof underlay for cold and warm roof applications which give major benefits over traditional roof underlays. Vent 3 Light is a triple layer, high performance breathable membrane, made from two high tensile spun bonded polypropylene layers, thermally calendared to a micro porous polypropylene film. Designed for use in fully supported or unsupported tiled, slated or metal roof systems. Vent 3 Light has full moisture vapour permeability, whilst maintaining the highest levels of water resistance. Vent 3 Light has a printed grey upper surface and plain grey underface.