Understanding the performance of roofing membranes

Iain Fairnington of A. Proctor Group provides an insight into the performance of roofing membranes and what this means for specifiers and contractors, in ensuring compliance with building standards and assured performance to reduce the risk of failure and minimise ongoing maintenance costs


or years the debate on how to achieve a true ventilation free cold-pitched roof has continued across the construction industry. Now architects and roofing contractors working on projects in both the

public and private sector are coming to the understanding that underlays can perform at an exceptional level, providing a failsafe option, without the need for additional ventilation.

IMPROVED THERMAL AND MOISTURE PERFORMANCE OF PITCHED ROOFS Te combination of cold-pitched roof construction and increasing thermal performance requirements has fuelled the tendency to place additional insulation above the ceiling joists, leading to an increased risk of condensation in the roof space. In an effort to combat this problem, the industry developed a series of breathable membranes, which were designed to be installed over the raſters as roof underlay, and allow the vapour to escape. Te majority of these membranes were vapour permeable, but airtight, rather

like a Gore-Tex jacket. While water-resistant, they did not completely prevent condensation within the roof space, meaning additional ventilation had to be introduced in order to allow air to circulate.

NOT ALL ROOFING UNDERLAYS ARE THE SAME Generally, two types of technological solutions have been presented to specifiers; vapour permeable, but airtight solutions, based on film laminated polypropylene technology, or a vapour and air permeable version, alleviating the need for additional ventilation.

“Roofing underlays come in all manner of different colours, but it’s what you can’t see that does most of the work – the middle layer” – Iain Fairnington, technical director, A. Proctor Group

Iain Fairnington, explains: “Roofing underlays come in all manner of different

colours, but it’s what you can’t see that does most of the work – the middle layer. While the colour of the top surface or underside is useful for identification, it is irrelevant in terms of performance. Tis middle layer can be likened to the same robustness as normal kitchen cling film and needs the protection of the sandwich construction to make this suitable and fit for purpose in a pitched roof. Taking a microscopic view of the middle layer clarifies the difference between an airtight and air permeable membrane. Te difference between air permeability and air tightness is fundamental to the products performance and use.”

BRITISH STANDARDS UPDATE – FEB 2018 CLEAR DEFINITION OF AIR PERMEABLE UNDERLAYS In February, a second amendment and updated version of the British Standard for Slating and Tiling BS5534 was introduced. For the very first time BS5534:2014 + A2:2018 now includes a clear industry definition of air permeable underlays. | HMM September 2018 | 43

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