April 2018

What type of green roof?
There are three main classifications of green roof: semi-intensive,
intensive and extensive. Semi-intensive roofs will typically require a
deeper level of substrate, but as a result a wider range of plants can be
incorporated into the design. Intensive green roofs are predominantly
built for recreational use – such as providing additional outside space –
and often require regular maintenance and irrigation. Extensive green
roofs provide environmental benefits to society, such as reducing CO2
levels, and reduce whole lifecycle costs to the end client.
Logistics, logistics, logistics
Contractors who have installed green roofing systems before will be
well aware that careful planning and close co-ordination between
all parties is absolutely paramount before work begins to ensure
that the materials can be delivered to site, off-loaded and
transported to the end location exactly when needed. Some
installations, such as sedum green roofing, ideally needs to be
installed within 24 hours of delivery to prevent damage to the
vegetation.
Challenges of installation
Green roof systems can present a particular challenge when it comes to
the transfer of materials for installation. The roofing area is often large
and therefore the volume of substrate material means that equipment
must be utilised to ensure the safe delivery of the system. Installed green
roofs are difficult to physically protect from a live construction
environment and are therefore often installed towards the end of the
construction programme. Early engagement with experienced contractors
is essential in overcoming the logistical and physical challenges.
Carry that weight
Green roof systems are also heavier than traditional roofs and
installing a green roof will add between 50kg – 200kg/m². In
retrofitted applications, it is crucial that contractors are aware of
the load-bearing capabilities of the building’s structure. In
addition, the roof should account for the saturated weight of the
green roof, snow loads and any imposed service loads such as
pedestrian access loads and point loads from water features and
large trees.
Consider windloads
Contractors should ensure that wind loads are calculated according to
BS EN 1991-1-4: 2005+A1:2010. The system should resist uplift from
wind forces either by anchoring the installation to the main structure, or
by having sufficient self-weight to prevent uplifting and displacement in
windy conditions.
Check the pitch
Depending on the pitch, the green roof build-up may be different. Pitches
of 10° or more will have different requirements to flat and low-pitched
roofs. On roofs above 10°, there is a risk of the substrates being exposed
due to excessive shear forces. Measures such as retention baffle
systems and slip barriers should be considered at the design phase.
Insulation and waterproofing
It is crucial to ensure that the roofing system works hand in hand with
the green roofing system for both the application and longevity of the
building. Engaging with a specialist contractor through the initial design
and specification process is key to ensuring that the roof membrane
design and green roofing systems are compatible for the application.
Selecting a membrane that protects the roof’s waterproof covering by
preventing plant roots from growing through it will ensure the longevity
of the system and reduce the need for costly remedial work.
Consider the cost
Long-term and regular maintenance of a green roof should be
factored into the specification process. Whilst extensive green roofs
are generally robust and considered low maintenance, the type of
installation can affect the level of maintenance required. For
example, installing an intensive or semi-intensive classified roof
will often require sophisticated irrigation and regular maintenance
cycles.
The Solution
Specifying and installing a green roof can be an extremely complicated process. As such, early engagement
is recommended to ensure that the designs are in line with the client and end user requirements in terms of
aesthetics, technical performance, sustainability and ecology. By considering all factors prior to installation,
contractors can ensure delivery to the highest possible standard.
Contact Prater
01737 772 331
www.prater.co.uk
@praterltd
APRIL 2018 TC 45