December 2022

Ask A. Proctor Group
Continued from page 40
High-performance membranes
Roofshield was developed in Scotland, where it is
common practice to use timber sarking over the
whole roof area and then apply the membrane
over this. Softwood sarking allows the roof to
have extra racking strength and wind resistance,
which is beneficial in the harsher climates
experienced in Scotland. As natural slates can be
fixed directly into the sarking, there is no risk of
water ponding behind battens. As well as being
highly permeable to water vapour, Roofshield is
also air permeable. Its air permeable design
means that there would be airflow through the
roofspace comparable to that of traditional
ventilation, so a cold pitched roof (with the
insulation at ceiling level) would function in the
same way as a typical ventilated roof. If the roof
is sarked in this manner, clearly the insulation
could be installed against the underside of the
boarding without any cavity, and the roof would
still comply with BS 5250. Therefore, a vapourpermeable
(whether air permeable or airtight)
roofing underlay could be used in these
circumstances to negate the requirement for a
cavity below the membrane.
Wraptite is one such vapour-permeable and
airtight membrane. When Wraptite is applied over
a boarded roof, the insulation should be pressed
hard up against the underside of the sarking, so
there is no need for an air gap. The same
principles apply to roofs without sarking; if there
is no air gap between a vapour-permeable roofing
underlay and the insulation, the roof performs
well hygrothermally.
Tiling or slating battens are typically applied when
slates are not being nailed directly into sarking
boards. Counterbattens are needed above the
underlay, which raise the tiling battens so that
water can flow below them. This prevents water
from ponding below the battens.
Roofshield and Wraptite are each BBA certified for
non-ventilated warm pitched roofs and can be
used directly over insulation without a cavity.
Images show A. Proctor Group’s Roofshield underlay system.
Roofshield is additionally BBA certified for cold
unventilated roofs. This warm roof certification
allows the rafters to be fully filled with insulation,
maximising the thermal value without reducing
headroom internally.
The more insulation you use in a construction like
this, the further out the dewpoint is moved. This
has benefits for reducing the risk of
condensation, especially if this moves the
dewpoint outside the underlay.
Since 2012 the NHBC has issued technical
guidance that acknowledges that some vapourpermeable
roof underlays permit both vapour and
air to pass through them. Therefore, where an
underlay can be shown to provide suitable
ventilation in a cold pitched roof, NHBC will
accept that underlay without needing any further
ventilation. This means that Roofshield can
therefore be installed without high level vents.
“Roofshield and
Wraptite are each BBA
certified for nonventilated
warm
pitched roofs and can
be used directly over
insulation without a
cavity”
The BBA certificate for Cold Non-Ventilated Roofs
now includes confirmation of this.
New Condensation Risk and U-value
Assessment Tool
At the A. Proctor Group we have introduced
dedicated U-value and Condensation Risk
Analysis software that will help contractors
accurately carry out assessments to ensure
compliance with the latest requirements of
BS 5250. The software is an essential tool for
meeting building regulations and improving the
performance of new and existing buildings, and is
now available on the members area of our
website.
Contact A. Proctor Group
www.proctorgroup.com
01250 872 261
42 TC DECEMBER 2022