February 2022

Blue/Green Roofs
Continued from page 36
In direct response to this, blue roofs are typically
designed to use a single roof waterproofing layer
that incorporates a modified drainage system,
which aims to accommodate both drainage
extremes. This approach has an inherent problem
however, in that it involves a degree of
compromise, and therefore also requires an
acceptance of increased and managed risk.
Failures in blue roofs won’t just create a leak –
but it will likely lead to damaging flooding. Clearly,
for habitable and sensitive buildings, there are
serious implications if this happens,
and it often leads to a reluctance to
plan such systems in these
locations. The possibility of
flooding may inhibit the number
of blue roofs that can be
specified and installed in a city
such as London, which has a high
density of buildings that would fall into
the category of ‘habitable or sensitive’.
This approach to blue roof design is not
appropriate given the drive to increase the number
of blue roofs in urban areas. Instead, a de-risking
approach is required – one that accounts for the
key water management factors, without impacting
the ability of a roof to perform its function.
Separating the roofing systems
The most effective way to deliver a blue roof
solution that minimises the flooding risk is to split
the roof into two distinct systems. This involves a
specifically engineered blue roof attenuation
system, such as ACO’s RoofBloxx, that sits above
a standard flat roof which has been designed and
installed in the normal way.
The result is an attenuation system that delivers
slower drainage and holds water in instances of
high rainfall, working as a separate element to
the main roofing system. In other words, this
ensures roofers can adhere to BS EN 12056-
3:2000 and other planning regulations, even
when working with two systems that seemingly
contradict each other.
Left: RoofBloxx. Above: Blue/green roof completed with RoofBloxx; Below: ACO’s blue/green roof build-up.
There are other advantages
to this approach beyond
just compliance with
regulations. First, the roof
membrane is not compromised,
which means decreased risk and no
warranty issues. It also means that the system
can operate on half empty in a 24-hour time
period, so it is compatible with standard civils
hydraulic calculations.
Along with the above, inverted roof designs –
which are the most common build up with blue
roofs – can be achieved without reduction in
U-values or potential buoyancy issues. Crucially,
this makes them suitable for use above habitable
areas due to the increased resilience of separate
waterproofing and attenuation layers.
While it may seem counter-intuitive, creating an
overall design that separates the roof and the
attenuation system greatly reduces risk and
actually improves hydraulic performance. This is
key in delivering blue roofs at scale, as well as
improving perceptions of their suitability across a
range of building types.
It’s crucial to collaborate as early as
possible
From low drainage flow rates through to differing
structural load, fire break and wind uplift
considerations, the design requirements of a blue
roof are myriad. With so many factors in play,
collaboration is critical to their successful
implementation. As the prevalence of blue/green
roofs increase, so too does the importance of
getting them right in a time-efficient manner.
With roofing contractors at the heart of
installation, and invariably the most
knowledgeable party of any project, it is critical
that they are engaged as early as possible in any
blue roof design process. Equally, drainage
experts such as ACO Building Drainage have a
part to play, given the innovative yet logical and
proven approach that has been outlined above.
By adopting this approach, the industry can
address the installation dilemma associated with
blue roofs, and ensure that we provide solutions
that meet the sustainability needs of tomorrow’s
urban environments.
Contact ACO
www.aco.co.uk/blue_green_roofs
@ACO_BD
38 TC FEBRUARY 2022