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Designing a Roof Garden - Eco Systems

Designing a Roof Garden - A Designers Guide

There are locations where roof gardens can form part of a policy to recreate valuable ecosystems. In a highly developed urban location there is little that can be done to restore the environment which once existed in the area. A small area of planting can provide a home for a few birds but can never be called a stable environment. However, in the context of a greenfield or low density site it is feasible to create a completely natural environment with ponds, wild flower meadows and forest to support mammals and birds as well as providing a soothing environment for people.

There are very few places in the world that have not been modified by man. In Western Europe and North America most areas have been farmed at some time. There are examples where farmland is of ecological importance but in the main cultivated fields are of little ecological value. Hedgerows were once an important habitat but the current policy of trimming has rendered them of no value to wild life.

There is therefore a sound case to be made for building large developments in rural areas providing that they are low, say up to three stories high, and completely covered with natural plant communities. It would then be very difficult to argue that the development was inappropriate when a field of say barley, which has no ecological value, is replaced with woodland albeit that there are buildings located within and under it.

The features of such a policy would be as follows:

1:		The buildings need to be low. A two storey building is relatively easy to hide particularly if it has a flat roof. With a storey height of 3.5m the building could be as little as 7m tall. Trees of this height can easily be transplanted to provide as instant screen. Fast growing trees, such as poplar and willow, which can grow at a rate of more than one meter a year, can be used to provide a good screen in as little as five years.
2:		Car parks need to be covered with structures which can carry at least 600mm of soil. Such a depth is sufficient to sustain a stable ecosystem. Undercroft parking can be used providing that the covered car park extends well beyond the outside of the building so that the bulk of the development is lost through planted terraces.
3:		Windows can be placed around the outside of all buildings but light wells offer a good way to hide windows from external view.
4:		Any structures need to be finished in a dark non-reflective material so that they do not show through the planting.
5:		Spaces between buildings can be used to create wooded valleys.
6:		Watercourses can run across roofs from the highest structure and into the valleys.
7:		Pools and water courses will need to be re-circulated to prevent stagnation but can be largely supplied with recycled rainwater which is stored in large balancing ponds at the boundary of the site.
8:		Planting can consist of a mixture of semi-mature trees to give an instant effect, small trees which grow rapidly to provide volume, and a herbaceous layer of native species to provide an under-storey.
9:		Growth will be optimised by using good quality topsoil. However, wild flower meadows will need impoverished topsoil or subsoil to prevent competition between the flowers and more vigorous grasses.

The policy outlined above will create a built environment which is of considerably more environmental value than farmland. An initial planting structure is needed which wild plants and animals can invade. Within a few years a stable ecosystem will develop which resembles the world which existed before mankind appeared.

On a more practical note the presence of a tree canopy in Summer will prevent overheating of the buildings. Similarly the loss of moisture from the plants will provide cooling. The volume of soil and the presence of ponds will reduce runoff from the site to almost nothing.

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