High performance roofing membranes - BS5534 update

In February 2018, 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.

Previously an underlay would be defined as type HR (high water vapour resistance) – an underlay with a vapour resistance greater than 0.25 MNs/g and type LR (low water vapour resistance) – underlay with a vapour resistance less than 0.25 MNs/g. The updated standard now also defines a low water vapour resistance (type LR) and air permeable underlay as “underlay that has a water vapour resistance not more than 0.25 MNs/g combined with an air permeability of not less than 20 m3 / m2 h at 50 Pa which allows for the transfer of both water vapour and air’.
High performance roofing membranes - BS5534 update cover image

What this means for contractors and developers: Roofshield is designed and manufactured to ensure contractors and developers are guaranteed the highest quality, providing a pitched roof underlay, which is both air and vapour permeable. Roofshield vapour resistance EN12572 = 0.065 MNs/g and air permeability EN12114 = 34.4m3/m2.h.50Pa (Mean test results quoted).

Updated shielding factor

The amendment to BS5534 includes an updated shielding factor in relation to wind uplift for air permeable underlays. Until further research and test evidence is made available for inclusion as a future update to the standard, the following applies A value of S1.5 for tiles and slates with unsealed joints with an air permeable underlay without counterbattens.

In relation to wind uplift on ridge and hips the standard now reads: The value of the substrate shielding factor, S, should be determined from the protection provided by the underlay as follows: a) Where the ridge is fully protected by the underlay (non-ventilating), S = 1.0; and b) On ventilated ridges and roofs without underlay, S = 1.75. Where the ridge is ventilated then a shielding factor of S = 1.75 will require more fixings and time on the ridge.

What this means for contractors and developers: Using Roofshield the whole of the ridge is covered by the underlay and so fully protected from the elements removing any risk of water penetration and ensuring that internal works can continue without delays even when all roofing works have not yet been completed.

NHBC technical guidance

In 2011 NHBC confirmed that it would be adopting guidance as outlined in BS5250 ‘ Code of practice for the control of condensation in buildings’, requiring a ventilation gap to be installed at high-level, equivalent to a 5mm continuous slot at, or near the ridge. The basis for this was that vapour permeable roof underlays permit the movement of vapour through the membrane, but generally do not permit the passage of air. Since 2012 NHBC issued technical guidance which acknowledged that there are some vapour permeable roof underlays that permit both vapour and air to pass through them. Where an underlay can be shown to provide suitable ventilation, i.e., at least the equivalent of a continuous 5mm high-level slot, NHBC will accept that underlay without the need to provide any further ventilation.

In line with the guidelines issued by the NHBC, independently certified air and vapour permeable underlays can be used without additional ridge ventilation in cold roofs.

What this means for contractors and developers: The use of Roofshield will lead to savings in labour and material costs, due to no VCL required, and its high-performance air permeability means that the roof space has similar air changes to that of a roof using traditional eaves/ridge ventilation.

Increasingly leading housing developers such as Bellway, Bovis Homes and Crest Nicholson, backed by the NHBC, and independent industry research, all agree that some underlays perform at an exceptional level, providing a failsafe option, without the need for additional ventilation. By specifying Roofshield developers are guaranteed the highest quality, pitched roof underlay, which is both air and vapour permeable.