The characteristics of vapour permeable underlays for both ventilated and non-ventilated pitched roofs vary according to manufacturer, and this must be taken into account when the roof is designed and constructed. The advantages of Roofshield vapour permeable underlay are described. The underlay consists of three layers, the centre being a fine fibrous melt-blown material, which contributes to the high permeability of the product, such that the underlay is classified as low resistance according to BS 3177. Tests have confirmed that the membrane is suitable for use without ventilation for a variety of roof constructions, as it has sufficiently high air permeability to prevent condensation in the loft space. Drumming is prevented in high winds by the membrane weight and the fact that it does not have to be pulled taut. The treatment applied to the membrane to provide external water penetration is compatible with wood preservatives.