Breather Membranes (or Breathable Membranes) are the most common form of underlay used in pitched roofs. Breather membranes allow water vapour to escape from the building, therefore preventing condensation from forming mould, mildew or wet rot on timbers etc.

At the same time Breather Membranes will not allow water droplets to pass through its uppermost side, providing a waterproof layer underneath the roof covering. Breathable Membrane A modern alternative to the traditional ventilation of the roof space is to use a breathable membrane that allows water vapour to escape out, but does not let water back in.

Typically, a roofing felt / underlay / membrane must perform four functions: 1. Secondary drainage layer. Wind-blown rain and snow which finds its way below any tiles and slates will drip onto the continuous underlay beneath it and drain safely down into the gutter outside the building. 2. Temporary waterproofing. During construction, and in the event of future damage to any slates or tiles, the underlay protects the insulation and fabric of the building below it. 3. Condensation control. Warm, moist air which rises up from within the building and into the roof space should be able to pass freely through the breathing underlay. Any condensation forming on the underside of the slates or tiles will then drop onto the underlay to run down into the gutter. 4. Wind Loading. The roofing underlay should provide a barrier to minimise the wind load acting on the slates and tiles, thereby helping to prevent wind driven snow or dust entering the roof space.