High performance roofing
membranes - BS5534 update
With the recent changes to BS5534 – the British Standard for Slating and Tiling – it continues to strengthen
the case for A.Proctor Group’s Roofshield underlay membrane as a high-performance non-ventilated roofing
solution. The manufacturer explains more to RCI readers
In February this year, 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”.
What does this mean 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 counter battens.
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
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.
For contractors and developers using
Roofshield, it means that the whole of
the ridge is covered by the underlay, and
fully protected from the elements, thereby
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 the National House-Building
Council (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 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.
Savings in labour and
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.
22 www.rcimag.co.uk July 2018