At the moment, the evidence is definitely pointing
towards the latter. In Germany alone, it’s estimated that
more than 30 million square metres of living roofs have
been installed since 2000. A driving force behind their
use in the UK is London Mayor, Ken Livingstone, who has fully
endorsed living roofs as part of his London Plan and the Greater
London Authority Environment Team is promoting them as a
way to enhance London’s biodiversity and improve
building performance.
Some of the reasons for the UK lagging behind its European
counterparts are a lack of understanding of the benefits and
types of living roofs available and planning constraints.
Here, we will look at the benefits that living roofs can bring
to the urban environment and look at some of the more
sustainable options available.
Living roofs can be used on all sorts of buildings from factories,
hospitals, schools, offices and housing developments, to garages
and balconies, helping to transform the urban landscape and
make a positive contribution to the environment.
There are three types of living roofs – extensive, semi-extensive
and intensive, relating to the amount of maintenance each requires,
depth of growing medium and the type of plants the area will support.
Extensive green roofs form the most lightweight of all the roofs
and are, therefore, suitable for small buildings or those which
require low maintenance. They have a thin growing media,
often a sedum mat, which is wind, frost and drought resistant.
Semi-extensive roofs will have deeper soils and can therefore
grow a more diverse selection of plants, but can require
greater maintenance. Intensive roofs are suitable for
more commercial uses as they have deep substrates, which
can support lawns, shrubs and trees and require
complex irrigation systems.
Whatever type of living roof you’re considering installing,
they all offer some good performance benefits for the environment.
Water retention
This summer’s devastating flash floods have shown that
our drainage systems are unable to cope with such an
unprecedented level of rainfall over a short period of time.
One of the main benefits of a living roof is the fact it can
act as a Secondary Urban Drainage System (SUDS), helping to
delay storm surface run off, with the plants and growing medium
soaking up large percentages of rainwater. A study in Germany
has shown that during a 10mm rainstorm, 200 litres of rainwater
fell on an 18m2 extensive green roof and only 15 litres
actually passed from the roof to the ground. It is also
estimated by Livingroofs.org, the independent resource on
living roofs, that under average conditions green roofs can retain
between 70 to 80 per cent of summer rainfall and in winter
they can retain between 25 to 40 per cent.
Sustainability
Sustainability is an important consideration in any project
with both business and public increasingly focused on reducing
their impact on the environment. Living roofs provide an
excellent opportunity for landscapers to make use of a range of
sustainable materials including recycled aggregate. Aggregate is
used within the growing medium to add bulk and certain
materials, such as sintered pulverised fuel ash (PFA), will
assist with the drainage of the living roof and retain water
helping to keep the living roof green and nourished during
dry periods.
Bourne Amenity Ltd, a leading supplier of Landscaping & Sports
Amenity Products to landscapers, across the UK, has developed a
green roof soil following requests from a number of architects
soil have been developed to suit different requirements from
growth a variety of seeds, plants, flowers or turf. Sustainability has
been an important criterion for clients and therefore the company uses LYTAG,
a lightweight PFA aggregate, which is a by-product of coal fired power stations.
Bourne Amenity Ltd uses LYTAG within the soil to provide the ideal
bulk density ratio, but one of the key benefits it offers is the ability
to retain moisture, stopping the soil from drying out in high winds and
periods of dry weather. Trials are being conducted into the use of LYTAG
as a growing media, reducing the need for compost or soil which will
further help to reduce the weight of the living roof.
Other benefits that are widely associated with living roofs are
improving a building’s insulation, keeping the building cool in
the summer and warm in the winter; reducing the heat