Good to grow
Up on the roof
by Xanthe White
“Green” roofs have the potential to reduce stormwater run-off and better insulate buildings – good news for people as well as the planet.
Since my early twenties I have carried around a sketch of my dream house. I’ve even formed an idea of the fictitious topography within which it will be built. It encloses a garden court that leads out to the main sloping gardens with surrounding bush and is built of rammed earth. My favourite part, though, is up an external spiral staircase to the roof. Here, on the second floor between the master bedroom and a studio, is a magnificent roof garden that brings the house alive.
The plan on my crumpled, stained piece of paper is a long way from becoming a reality, but my determination for this dream was reignited when I met Zoe Zimmerman. By day, she is a town planner, but her spare time is dedicated to promoting roof gardens. She spent two years with the Environment Agency in London where she worked alongside developers to ensure their practices and buildings were sustainable. During this time she was involved in the planning of 22ha of green roofs. The social, economic and environmental effect of this is inspiring. After returning to New Zealand, she started the website [[www.livingroofs.co.nz[[, a resource guide that promotes green roofs.
Roof gardens are not new, even in New Zealand. Zimmerman’s ancestors, the Coopers, arrived on the third ship of settlers to live in a sod house near Christchurch. Maori commonly lived in these (whare paru paru) and, of course, the Austrian artist Friedensreich Hundertwasser has made Kawakawa famous with his avant-garde architecture and “growing” buildings.
In the past they were regarded as outdated, quirky, hippie or just odd. Now, they are considered internationally to be one of the leading solutions to many of the environmental issues society faces, particularly in cities, where global warming will have much of its impact. Central Auckland is already on average two degrees hotter than elsewhere in the region, and each year has 21 hot days (where the temperature exceeds 25°C). Niwa’s moderate prediction is that this will increase by an extra 40 days by the turn of the century. These predicted heatwaves will make the city increasingly inhospitable for the elderly and the young. One of the major contributing factors is our lack of green spaces and vegetation within the cityscape.
Green roofs have the potential to help limit temperature increases, while also substantially reducing stormwater run-off. The University of Auckland is running trials investigating stormwater attenuation on a retrofitted green roof on the School of Engineering building. Conservatively, the figures are showing about 85% of the rainwater on the roofs stays within the natural system, being retained and transpired by plants. The remaining 15% has the potential to be collected for use within the buildings, for flushing toilets and the like. Green roofs also help to insulate buildings for both cooling and heating. As gardens, they invite us to venture upwards to make the most of the views, particularly the urban landscape of our cities, which will be more stunning when tiled with green rooftops.
Green roofs can offer the same benefits in our suburban environment. Consider the environmental outcome if, rather than covering our gardens with more and more impermeable surfaces, we instead reverse this process and begin covering our impermeable surfaces, buildings in particular, with living ones.
Green roofs can be fitted onto existing structures as well as new buildings, and although professional advice should be sought before you begin planting on your roof, garden shed or garage, it can be a viable project if properly researched.
First, look at the load-bearing potential of your roof. To plant the shallowest roof garden, your structure needs to be capable of holding a concrete paver, as the lightest green roofs when wet weigh slightly less than this. The minimum planting depth to sustain plants in this environment is 50mm. At this depth shallow-rooted plants such as sedums, ice plant and echeverias will grow. Although they are yet to be trialled, shallow-rooting natives such as Pimelea prostrata, Scleranthus biflorus and even the small native geranium Geranium traversii var elegans could perform well.
To keep the load light, the basic planting medium should be around 60-70% pumice and 40-30% hummus. Native grasses require a soil depth of around 100mm. With the medium at a 150mm depth, the potential plant palette becomes a lot broader, and suitable shrubs can be included. You can, of course, have varied heights of plants and even trees across a roof if it is appropriately engineered to carry a greater load.
To prepare your roof for planting, first ensure it is appropriately waterproofed. And mesh will need to be laid to stop the roots penetrating. There are companies that specialise in the application of green roofs and they will generally use modular systems made from recycled plastics designed to optimise water usage in dry periods. Then the growing medium is laid and the plants are plugged in and left to acclimatise.
A wide range of plants have been grown in roof gardens around the world, but anyone planning a roof garden should first look to nature for inspiration. Coastal and alpine environments are probably the closest natural ecology to that of an exposed roof so are the ideal starting point. The low roof of a shed or garage that is shaded by large trees may be more suited to lichens and mosses that would naturally occur on rock shelves on a forest floor.
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