Designing a Roof Garden
Designing a Roof Garden - A Designers Guide
Most of the views that are expressed on roof gardens are not based on scientific principles but rather on perceived logic. Because a garden was once built in a particular way it does not mean that the designers knew what they were doing, or that their approach is worth copying. No statement is made on this site unless it has been tested against good horticultural practice, documented trials, or observations made in the field.
There is a great deal of mysticism about growing plants on buildings as if all the rules of nature change as soon as a plant is grown in an environment which is created by man rather than nature. Plants have been growing in precarious locations ever since they first evolved. They should grow rapidly in a roof garden. In what other landscape situation are plants provided with good quality improved topsoil, regular irrigation, high root temperatures and a high level of maintenance?
Roof gardens are more common in some countries than in others. The main reason is a combination of planning laws and cost benefit. In any major city the value of the land upon which a building stands can represent a significant part of the total cost of the project. The consequence is that most buildings are built as cheaply as possible which in turn means that little money is spent on landscape, unless planning law requires it.
To build on a greenfield site in Switzerland and Germany there has to be a gesture towards restoring the status quo. Under German law development above ground is strictly controlled but there is much more flexibility when developing below ground. As a result it is often possible to develop right up to the boundary of a site. It means that more extensive activities such as car parks are often hidden from view which can be very useful in an urban context.
An additional advantage claimed for roof gardens is that, by absorbing water, they damped peaks in run off during heavy rain. This buffering is restricted as the drainage system still has to cope with peak flows. Although green roofs were initially required by law there is now a general acceptance, on the part of businesses and developers in Germany, that greening the environment is a socially desirable objective.
In most countries roof gardens are only provided by clients who wish to offer something special. A roof garden with paving and large trees will cost double or even treble that of a conventional garden at ground level. Even a minimal green roof is expensive. Few clients elect to spend money in this way unless there is a legal requirement or a financial benefit. Clients who have commissioned a building for their own use are another matter and, in the main, these are the only people who fund such features.
Plants and gardens certainly make life more pleasant for people. If cities are congested then the only areas which are left for plants are roofs. However, it is difficult to make a financial case for having a roof garden at high level in view of the cost. Strengthening the structure plus the need for additional services is an additional expense.
If the site is within a major city then the cost of importing materials further increases the cost. The ongoing cost of maintenance over the life of the building is also considerable. However, in more urban areas and in particular green field sites low level roof gardens can be a useful way of restoring the environmental character of an area.
Traditionally a roof garden was considered to be any collection of plants raised up on a structure. Today the term Green Roof has been adopted to refer to those systems that produce a sward, supported on a relatively shallow substrate. The green roof philosophy is a badly thought through idea which equates a thin layer of soil which supports small plants to ecology. For much of the year the plants are in a dormant state.
It may be argued that an area of dead grass or sedums is better than a bare roof, but few people will accept that this constitutes landscape. If this approach is considered acceptable then it follows a long tradition of putting turf on roofs, as practised in the Middle East, Scandinavia and North America. If year round greenness is required then all but the deepest roof gardens must be irrigated.
Landscape is usually expected to endure for generations. In some cases this might be desirable but there are many examples where such a rigid approach is not necessary. Most shrub plantings require a major 'overhaul' after 10 years, during which untidy or worn out plants are replaced. It may be reasonable to plan roof gardens with a life expectancy of as little as a decade.
In theory the whole scheme could be designed so that it can be removed and replaced within a few days. Replanting a roof garden could then be compared with, say, re-carpeting an office. Such planned obsolescence is common in commercial horticulture although almost unheard of in landscape. Such an approach would allow tired soil, worn out irrigation and over mature shrubs to be replaced quickly and efficiently.
An early decision in any design process is whether to follow a formal or an informal route. The approach is normally governed by the setting. A formal building with a rigid geometry usually attracts a formal solution. However, formal solutions usually reflect the fashion of the day and so tend to date. An informal solution can be more work than a formal solution but if it appears soft and natural its appeal will last forever. Also an informal solution helps to disguise any problems which the plants experience due to the environment being harsh.
This site offers guidance to designers and end-users to help them achieve the best results when designing roof gardens. This a broad subject but this site seeks to answer the more important questions in a concise way. If you have more specific questions then please contact us by:
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