Breathable roofing membrane is an underlay installed beneath tiles or slates on a pitched roof. It provides secondary weather protection while allowing water vapour to pass through the membrane.
It is not the finished waterproof roof covering
The tiles or slates are the primary weathering layer. The membrane sits below them as part of the roof build-up. That makes it fundamentally different from exposed shed roofing felt or an EPDM flat-roof membrane.
Breathable membrane and vapour control layer do opposite jobs
A breathable roof underlay is designed to allow vapour to pass outward through the roof build-up. A vapour control layer is normally positioned on the warm side of insulation to restrict moisture-laden indoor air entering the construction. They are not interchangeable.
“Breathable” does not automatically mean the roof needs no ventilation
NHBC distinguishes low-resistance (LR) and high-resistance (HR) underlays. Ventilation depends on the complete roof construction. For example, NHBC says an LR underlay fully supported on high-resistance sarking such as plywood, OSB or chipboard should be treated as an HR underlay for roof-ventilation purposes.
Install the membrane as part of the designed pitched-roof system
Orientation, overlaps, drape, support, counter-battens and ventilation can vary by membrane and roof design. Follow the membrane markings, product instructions and roof specification rather than copying a generic overlap measurement.
Do not assume stapling membrane beneath existing rafters recreates a proper underlay
A roofing underlay needs a drainage path and correct detailing beneath the roof covering. Retrofitting material from inside an existing roof is a different construction problem and should not be treated as equivalent to installing underlay correctly from above.
Shop our 135gsm Breathable Roofing Membrane in 1m and 1.5m × 50m rolls, then see Roofing Battens for the next layer of a tiled or slated roof.
