Green roofs are essentially a growing medium and vegetation surface applied to a
waterproofing layer of a suitable conventional roof build-up. Generally
speaking, there are three main types of green roofs: intensive, semi-intensive and
extensive (including biodiverse). These definitions denote the amount of input
required in terms of irrigation, nutrients and maintenance to keep the green
roof flourishing.
Intensive green roofs, therefore,
tend to be well manicured and need significant input, whereas extensive roofs
need only minimal input.
Water and sustainable drainage
In countries such as Austria,
Switzerland and Germany, green roofs are recognised as a significant
source-control component,contributing to storm water management and pollution
control. For example, in many German cities businesses receive a rebate in
their storm water charges if they have a green roof
Green roofs not only store water at
roof level, but also reduce the run-off rate from the roof, which in turn
reduces the underground drainage requirements. It is also possible to use or
harvest rainfall from a green roof, although the amount of rainwater that can
be used may be reduced depending on the type of green roof implemented. Water
can also appear discoloured at first, though there are established methods of
dealing with this discolouration
Thermal performance
Green roofs also significantly
enhance the thermal performance and comfort of buildings, particularly in terms
of summer cooling. Studies in North America have shown that an average of
4.15kWhr/m2 can be saved during the summer months. This impressive performance
can also reduce the need for expensive air conditioning equipment and
associated energy costs. A prime example of this is a small London café – Paradise
Park – where it was calculated that no air conditioning units were needed for
the offices beneath a green roof, providing an annual saving of 3,800kWhrs per
year and significant financial savings as a result.
Green roofs do provide some
additional insulation; however, this varies year-on-year depending on how dry
or wet a given winter is. In Germany though, recent studies have shown that a
100mm green roof system can lead to savings of between 2-6kW/ m2 per year on
winter energy bills. This variance is dependent on the amount of rainfall
throughout the winter in any given year
Biodiversity
Any green space will provide
intrinsic value to biodiversity and, as such, increasing biodiversity has been
a key driver for green roofs in parts of the UK, especially London and
Sheffield. To achieve optimum biodiversity there is a need to look at the type
of green roof build-up (the substrates) and vegetation or flowers to be used.
Research in both Switzerland and the UK has demonstrated that using various
substrate depths and planting with sedums and herbs can increase the benefit of
green roofs to rare invertebrate populations. What is more, the use of
semi-intensive and intensive green roofs can provide an array of advantages for
many common – and in some cases rare – species within the national, regional
and local biodiversity action plans. Greater substrate depths will also aid
storm water management and the building’s thermal performance
Sound insulation
A further advantage of green roofs is
that they provide superior sound insulation compared to more traditional roof
coverings. While there is no research in the UK or Europe at present to support
this, recent studies in North America demonstrate the increased sound
insulation they offer and, therefore, the decreased need for sound insulation
or soundproofing on ceilings to reduce the amount of sound penetrating a
building. With soundproofing an increasingly important design consideration in
a school’s construction, as well as a key stipulation in the DCSF’s guidance
document ‘Standard Specifications, Layouts and Dimensions’ (SSLD document
five), this is clearly a major advantage
Health and safety
Ensuring safe access to the roof, as
well as safety whilst on it, are also key issues that need to be addressed at
design stage. In particular, consideration should be given to which areas of
the roof need to be accessed by students and teachers and where the green roof
is situated in relation to other surrounding roof areas. Other considerations
will be dictated by the age of the students accessing the roof. However, by
meeting these health and safety requirements, schools often find a green roof
brings unexpected health and safety benefits – for example, a reduction in
insurance costs for field trips together with a reduction in associated
parental-approval paperwork.
Additional benefits of Green Roofs
Other key benefits occur when green
roofs are applied to waterproofing, providing protection from frost, UV light
and other climatic factors, and thus increasing the life of a building’s
membranes. The provision of appropriately designed landscape at roof level can
also create valuable amenity areas within buildings, providing space for
outdoor classrooms and other educational activities – a benefit that is largely
ignored at present.
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