February 2024

How a history of innovation has helped develop the roof
membrane of the future
Specifiers and contractors want practical solutions for delivering resilient, sustainable
buildings designed with the future in mind. Considering the need for future-proofed solutions,
Iain Fairnington, Technical Director at A. Proctor Group, explains how the next generation of
air permeable roof membranes is set to help.
Pitched roof underlays are often described using the catch-all
term ‘breather membranes’. The generic description doesn’t
reflect the distinction between high-resistance (HR) and lowresistance
(LR) membranes. And LR membranes themselves
can be air tight or air permeable.
indoor environment.
Water resistance on the external side must not impact on the
vapour permeability – or air and vapour permeability, if both
are offered – from the internal side.
Roofing membranes must meet a host of performance criteria,
of which we’re going to focus on two: water resistance and
vapour/air permeability.
Ventilation in modern buildings, and especially dwellings, is
often not well aligned with the energy efficiency performance
Water resistance of membranes in a changing climate
When a roofing system is installed, there is an inevitable
period of time where the underlay will be in place without a
roof covering to protect it. The underlay might therefore have
to bear the brunt of any inclement weather. Good practice
will dictate that this period should however be kept to a
minimum and a temporary cover used if rain is expected.
The NFRC’s Technical Bulletin TB06 describes how
membranes should perform in terms of water resistance.
It says an underlay with “a water hold out over 1m when
tested to BS EN 20811 will be adequate.”
As the frequency and intensity of extreme weather increases
as a result of the changing climate, roof systems generally –
and membranes as a specific part of those systems – need
to be capable of withstanding more severe levels of rainfall.
An underlay’s ability to perform with a 1m head of water is
a minimum level of performance to look for during product
selection.
KEY FEATURES
• Vapour permeable
• Fully air permeable
• Highly water resistant -
hydrophobic additive in all 3 layers
• More uniform airflow than vents
• Wind Uplift Resistance Complies
with BS5534
• No VCL required
• 15 year warranty
Designing roof membranes to cope with modern living
Any manufacturer developing a pitched roof underlay
must balance the competing needs of resisting the external
elements, and contributing to the quality of the building’s