Summer '22

The key feature of a blue roof is an
attenuation void in which water can
be contained. This is positioned above
a waterproof membrane in a warm
roof design and above the water flow
reducing layer on an inverted roof.
Blue roofs are also often combined
with green roofs due to the need for a
permeable roof surface to capture the
water.
When designing a blue roof, optimum
drainage and discharge should be
achieved for effective stormwater
attenuation to take place. Establishing
the site drainage rate and storage
volume capacity are key factors for this.
The overall site drainage rate, set by
the planning authority, is often defined
as the ‘greenfield runoff rate’ and is
the rate of discharge if the land was
undeveloped. Typically, this is between 5
to 8 litres per second per hectare.
The storage volume is how much
rainfall the site can hold and then
discharge. The standard commonly set
for SuDS is the ability to attenuate a
storm event with a return period of 1 in
100 years, meaning a storm intensity
with an annual probability of 1%.
However, the location of the site and
type of development may also affect
this calculation.
In addition, the site must be viewed
holistically as the maximum storage
volume will also often be influenced by
the size of the roof area and the roof’s
loading capacity. The NFRC guidance
states that for a void depth of 100mm,
the water can exert an additional load
of 1.0kN/m 2 on the roof structure, which
must be accounted for.
Number and position of outlets
The NFRC guidance also highlights
that stored water should be discharged
within 24 hours of rainfall. Therefore,
the number and position of roof outlets
should be calculated to ensure effective
drainage and achieve the required run
off rate. Some blue roof outlets, such
as Marley Alutec’s Elite outlets, feature
adjustable flow restrictors, allowing the
outlets to deliver various runoff rates
and water depths to meet whatever the
required rate might be – whether that’s
from 0.35 to 5.58 litres per second (l/s)
at 25 mm water depth to 0.58 to 9.31
l/s at 150mm depth. Only high-quality
outlets should be selected due to their
necessary exposure to high water
pressure for extended periods of time.
Marine grade aluminium outlets offer
specifiers increased natural strength,
corrosion resistance and overall
longevity.
The outlet and roof structure should
also have a secure connection.
Marley Alutec’s Elite outlets feature a
membrane compression clamp and
high performance butyl sealing ring
to ensure a durable, watertight seal. It
is recommended to use products that
have been tested beyond the predicted
requirements to guarantee that
unexpected conditions will not lead to a
system failure.
Finally, the discharge of excess water
should be considered. If overloaded, a
blue roof can cause water ingress that
will damage the roof structure and
building. To combat this, outlets can
The range of Elite Outlets from Marley Alutec.
be used to facilitate emergency water
discharge by integrating an overflow
into the outlet. However, measures
to prevent debris blocking the outlet
should also be considered and regular
inspections should be carried out to
ensure the system is operating as
intended.
Blue roofs are an effective way to
mitigate flooding in urban areas. With
the increased likelihood of severe
flooding in the coming years, there
are manufacturers that can provide
key technical support for specifying
drainage designs. These manufacturers
can also align with wider environmental
aims through responsible sourcing and
accreditations such as ISO 14001 for
environmental management.
Find out more about
Marley Alutec
www.marleyalutec.co.uk/
Tel: 01234 344108
www.greenscapemag.co.uk • GREENSCAPE SUMMER 2022 29