Summer '22

Optigrun FKD 25 drainage board - for use on
standard sedum roof system build-up.
consequent root damage and potential
failure of the vegetation. A good green
roof system will provide this at different
layers within the roof make up, one of
which is within the drainage layers.
The drainage layers on a vegetated
green roof will typically consist of a
protection fleece – unless the roof
is of an inverted design – drainage
and reservoir board and filter layer.
With regard to the drainage and
reservoir board, this will typically be
manufactured from a recycled plastic,
and it’s important that it has good
reservoir function on its upper face with
individual storage cells or chambers
with overflow provision so that excess
water can drain away below the board.
The underside of the board should
facilitate good drainage so that water
cannot back up through the board and
risk waterlogging of the substrate layer
above. For example, at Optigrun we
have different drainage and reservoir
boards within our systems, ranging
from 20mm to 60mm in depth, with
different performance characteristics
so that the correct product can be
chosen for each project. The filter
fleece that’s laid above it should have
good mechanical filtration properties
but also allow a sufficient rate of
water flow. The current Green Roof
Organisation (GRO) Green Roof Code
(2021) sets out minimum performance
characteristics for this layer.
Pitched roofs
Pitched green roofs are being specified
in ever greater numbers and these
present their own set of requirements
when it comes to product selection. The
general rule here is that it’s best not to
improvise with the specification, but
instead to use a system designed for
the purpose. The “system approach”,
mentioned previously, definitely
applies where pitched green roofs are
concerned.
Generally, a pitched green roof is
considered to be one that has a pitch
in excess of 5 degrees. Beyond this
figure the green roof specification will
need to be completely different as the
physics of the roof change. For example,
transferred shear load and slippage risk
have to be considered and the method
of accommodating this will vary on
each roof depending on, for example,
pitch, slope length, waterproofing
membrane being used and roof design
– i.e., mono or duo pitch, perimeter
detailing etc. The method of providing
water storage and drainage will also
change. I have surveyed several failed
pitched green roofs in the past where
an improvised anti-slip arrangement
has been used, or in one case none at
all, creating costly mistakes which could
have been avoided if the correct system
had been selected originally
For pitched green roofs, therefore,
a range of solutions is needed, and
an engineered solution is required to
provide a bespoke specification suited
to the individual project.
Substrate
For the substrate or growing medium
for a green roof, topsoil or recovered
site spoil are not suitable products.
Instead, a blended and engineered soil
replacement should be used with a
specified organic content. In this way
the performance characteristics of the
material, such as weight, water storage
capacity, organic content and porosity
are known.
The GRO Green Roof Code sets out
performance characteristics and also
recommends that substrates selected
Optigrun substrates are specifically
engineered to provide the correct organic
content needed for each green roof system.
should have been tested according
to BS 8616:2019 Specification for
performance parameters and test
methods for green roof substrates.
Technical support
Finally, it may seem obvious but make
use of the technical services that
your green roof supplier provides.
They should be able to provide a
bespoke specification for your project
which should include technical data
for the products included. If you are
a contractor tendering for a green
roof, this is particularly important as
the specification that came with the
enquiry may have been cut and pasted
from a previous project and be entirely
unsuitable for the project in hand.
There is also useful independent advice
available from GRO who are the body
that represents the green roof industry
in the UK. The latest edition of the
Green Roof Code, introduced in 2021,
and mentioned in this article, is full of
useful information and design guidance
for green roofs. You can download a
free copy at:
www.greenrooforganisation.org
Contact
Optigrun
W: www.optigruen.com
E: info@optigreen.co.uk
T: 0207 5899 400
www.greenscapemag.co.uk • GREENSCAPE SUMMER 2022 21