38 | OCTOBER 2019
Roof Technology
Read online at www.proinstaller.co.uk
ROOF VOID
VENTILATION
WITH SWISH
Dave Osborne, Technical Manager, Swish Building
Products, explains why roof ventilation is so
important in maintaining healthy building stock.
Modern homes generate a
huge amount of water vapour
– baths, showers, cooking
and general living all produce
moisture that’s held in the air
around us.
Warm air can hold much more
water vapour than cold but
when warm air comes into con-
tact with cold surfaces, it cools
and releases excess vapour in
the form of condensation.
Ventilating the living space is
relatively straightforward, either
with passive methods – opening
a door or window, or by using
mechanical ventilation systems
such as an extractor fan.
What is less straightforward is
the ventilation of the roof void
above.
Here, the temperature differ-
ence is often at its most extreme
and moisture is readily deposited
on cold surfaces much the same
as dew on the grass – hence the
term, dew point.
Once moisture has entered the
roof space, the most practical and
effective way of removing it is
through ventilation.
Unrestricted air
movement
“Building Regulations Ap-
proved Document F: 2011 and
BS 5250:2011 Code of Practice
for the Control of
Condensation in
Buildings” both
recommend ven-
tilation to control
the build-up of mois-
ture in the roof area.
This involves allowing
air to enter the loft on one
side of the roof and exit on
the opposite side.
Clear, continuous air paths at
the eaves are therefore crucial, as
without the through-flow of air,
the performance and condition
of the roof construction can be
compromised.
Design principles
The size of the air path across
the roof void is determined by
the style of roof construction:
Mono pitch, Duo pitch or Flat.
• Duo pitch roof, greater than
15 degree pitch but less
than 70 degrees, with insu-
lation at the ceiling level =
10mm.
• Duo pitch roof, greater than
15 degrees with the ceiling
following the plane of the
roof = 25mm at the roofline
and 5mm at the ridge.
• Mono pitch roof, greater
Once moisture
‘ has
entered the
roof space, the
most practical
and effective
way of removing
it is through
ventilation
’
than 15 degree pitch but
less than 70 degrees with
insulation at ceiling level =
10mm at the roofline and
5mm at the ridge.
• Flat roof less than 15 degree
pitch with insulation at ceil-
ing level = 25mm.
• Duo pitch roofs greater than
20 degrees pitch or great-
er than 10m span should
have additional ventila-
tion at the ridge to assist
airflow through the roof
void, equivalent to at least a
3mm wide continuous gap.
Additionally, if the span is
greater than 10m, the over-
all ventilation area should
TABLE A
ROOF PITCH PITCH FACTOR
10° 1.088
20° 1.182
30° 1.288
40° 1.419
be increased to at least 0.6%
of the total roof area.
• Where the duo pitch is greater
than 15 degrees with the ceil-
ing following the plane of the
roof or a flat roof, less than 15
degrees pitch with insulation at
ceiling level, a minimum 50mm
free air path should be main-
tained between the top of the
insulation and the underside
of the roof decking. Air paths
should not be obstructed.
Working Together
As a best practice solution that
combines adequate ventilation and
good insulation, Swish recom-
mends the installation of eaves
vent trays or roll-out eaves trays
in conjunction with soffit ventila-
tion. These sit between the rafters,
above the insulation, and force
the insulation away from the roof,
ensuring that the flow of air is
maintained.
www.swishbp.co.uk