A breathable roofing membrane — sometimes called a breathable underlay — is the layer installed between the roof timbers and the tiles or slates. It replaces the older bituminous felt that was standard for decades. "Breathable" means it allows water vapour to pass through while keeping liquid water out, which helps manage condensation in the roof space.
Why condensation matters
Warm, moist air from the living space below rises into the roof. If it cannot escape, it condenses on cold timbers and the underside of the covering, leading to damp, rot and a shortened roof life. A breathable membrane lets that vapour pass through while still shedding any wind-blown rain that gets past the tiles.
Breathable vs traditional felt
Traditional bituminous felt is vapour-resistant, so a felt-lined roof relies on high- and low-level ventilation gaps to manage moisture. A breathable membrane reduces the reliance on those gaps, though ventilation still matters. We install the right membrane and ventilation combination for your roof as part of a re-roof.
Do I need one if my roof is sound?
If you are having a full re-roof, a breathable membrane is standard practice and a worthwhile upgrade over like-for-like felt. If you are only repairing, the membrane is not usually touched. We advise on the right approach during the inspection.
Part of a properly built roof
A breathable membrane is one element of a roof built to current standards, alongside correct battens, ventilation, leadwork and ridge detailing. See our new roofs page for the full picture, or arrange a survey through our quote form.