Our client GPE, formerly Great Portland Estates PLC, is a FTSE 250 London property development company. GPE commissioned sustainability experts Greengage to help set a biodiversity net gain and embodied carbon KPI, baseline, and strategy. You can read more about that here: https://www.greengage-env.com/case-studies/great-portland-estates-bng-and-embodied-carbon/
To implement this strategy GPE undertook a detailed review of its central London portfolio looking for ways in which to improve biodiversity and cut carbon. One of two sites we worked on with them in early 2023 is a two-year-old building consisting of Oxford St facing retail units with offices above, crowned with a large roof terrace with seating and ornamental planting for use by the buildings’ occupants.
Around the perimeter of the roof is a cradle track for the window cleaning gantry, inside which was around 150m2 of sedum roof. Despite substrate depth of 150mm, exceeding the GRO minimum recommendations for sedum by two and a half times, the growing medium appeared too free draining and, in the absence of irrigation, had struggled to support more than one or two species.
The brief therefore was to retrofit this area with a range of native wildflowers, as well as microhabitats of natural materials intended to provide basking and sheltering
space for invertebrates. Replacing the substrate was not part of the brief, so we sought to improve with plants more likely to find the conditions favourable.
What we did
In addition to a range of plugs suitable for sunny and exposed locations, we over-seeded with mixes of cornfield annuals and perennials. We also created habitat
features with banks of sand, sweeps of pebbles emulating dried riverbed, with varied depths to make the feel more naturalistic and climate resilient. To this we added a
number of creatively arranged log piles and log retainers for sand with coppiced sweet chestnut (supplied by W.L. West & Sons in West Sussex.)
We also added irrigation to the roof via a porous pipe set on a timer. All green roof projects should allow for a water supply during the initial couple of months while
plants are rooting.
Opinion is divided amongst green roof experts whether green roof irrigation should be a permanent feature but here at Organic Roofs we
recommend the compromise that it be installed and its use then discouraged. During watering-in its preferable to have surface spray – sprinkler or micro mist – but we
couldn’t risk water spraying over the roof to the busy shopping street below. The install was quite challenging; the short lead in time coincided with our busy
spring maintenance period, there were several weeks of early starts to avoid the morning rush hour traffic in central London, and we needed to provide a ‘white
glove’ service to bring so many tonnes of green roof components through the pristine lobby, lift, and communal roof terrace: there were at least 5m3 of natural
green roof habitat material that made the journey. We also need to remove 100 paving slabs. At Organic Roofs we are always looking for innovative ways to repurpose waste.
What did we do with 100 old paving stones? Lee has a canal boat and he advertised the concrete slabs to the London Boaters Facebook group; and boat owners at a permanent mooring next to
Walthamstow Marshes took almost all of them, to be used as ballast in the hulls.
This project stands out as an example of how a company can help nature recover in a heavily urbanised environment. By creating a buzzing meadow with food and
habitat for invertebrates, we help support bigger creatures that feed on them. GPE was able to upgrade an existing poor quality green roof, and deliver significant uplift
in biodiversity value, without any expensive changes to the structure. They were extremely happy with the outcome; the ecological benefits, the improved
aesthetics and the added social, health and wellbeing value of connecting their customers to nature.
LINK TO VIDEO on blog