Green Roofs

Introduction

It may seem a strange idea to allow vegetation to grow on your roof. In fact many people invest time, money or both to eliminate the vegetation from wind blown and bird deposited seeds germinating and growing.

Roofs with plants or green roofs as they are more commonly known are becoming more and more popular with architects as well as the general public. An interesting fact is that such a low tech finish should be chosen in such a high tech age.

Sod roofs have been used in Europe for hundreds of years, especially in rural Scandinavia.

Modern green roof technology was born in Germany about 30 years ago, greenroofs have become quite popular throughout Europe, mainly due to their positive visual properties and environmental impact

Apart from the supposed existence of the Hanging Gardens of Babylon, the greening of flat roof areas is a relatively modern phenomena. Apart from one notable German advocate in the nineteenth century (an inventor of waterproof cement), it was not until the beginning of the twentieth century that the flat roof and in turn its potential for greening, that interest was shown by building clients and architects.

Green flat roof areas can of course occur naturally through poor maintenance where debris gets lodged and seeds subsequently germinate. Sometimes however, old structures are positively planted to develop such vegetation. 

Whilst there are a growing number of new buildings that have green roofs incorporated into them, their presence in either town or country locations in the UK is fairly insubstantial. In parts of Southern Germany for example, developers of flat roof commercial buildings are required to green them, as part of gaining planning consent. It seems doubtful that such policy requirements would be introduced within the UK at the present time, but as the sustainability issue moves up the political agenda, it may form part of local authority planning policy in the not too distant future.

A survey of English Planning authorities carried out by Nottingham Trent University’s Green Roof Research Team clearly underlined the little evidence of green roofs in practice or, very positive attitudes towards them by planning officers. London boroughs were however, something of an exception both in experience and attitude.

'INTENSIVE' roofs have deep soil profiles that can grow and support lawns, shrubs and trees.  These are more elaborate in design, and are intended for human use and interaction, and need to be engineered to conform to the load requirements. 

Intensive greenroofs are distinguished from the roof garden of container filled plants by the existence of a continuous underlying green roofing layer system.  These greenroofs generally have flat roof surfaces or shallow roof slopes of up to 3%. 

Different soil types and depths allow for a larger selection of plants, including flowering shrubs and trees. Typical soil depths are at least 150mm (6 inches).  Pathways, terraces, water fountains, ponds, and other architectural features enable the creation of dramatic spaces.  Depending on the plant selection, additional water collection cisterns, reservoir boards, irrigation, fertilisation and/or maintenance may be necessary, just as it would be.0

3.3.03 ecological benefits of intensive greenroofs remain the same or can be greater due to the greater natural processes utilised by larger and greater plant species diversity.

The alternative is the 'EXTENSIVE' type which is based on shallower soil profile roofs that are planted with mosses and sedums. These are more often non-access roofs as compared to the 'intensive' type.  Extensive greenroofs are lightweight veneer systems of thin layers of drought tolerant self-seeding vegetated roof covers using colourful sedums, grasses, mosses and meadow flowers requiring little or no irrigation, fertilisation or maintenance. 

Diagram of an extensive green roof system 

The types of plants suitable for extensive landscaping are those native mainly from locations with dry and semi-dry grass conditions or with rocky surfaces.  

As a method of surviving extreme conditions, the plants have developed  special mechanisms such as water storage organs, thicker leaf surface, silver surface hairs, narrow leaves, etc. (Zinco, 1998).  Soil depths that will support the smaller plant communities, range between 50 and 75mm (2 - 3 inches).

Generally, extensive greenroofs can be constructed on roofs with slopes up to 33%, and can be retrofitted onto existing structures with little, or most often, no additional structural support.  The average weight of a fully saturated extensive greenroofs is 17 pounds per square foot, which is comparable to the weight of gravel ballast placed on many conventional roofs.  These roofs are not intended for recreation, or to accommodate the weight of people, larger shrubs nor trees. Extensive greenroofs are much less costly than intensive green roofs.

What are the benefits of green roofs?

Extensive green roofs have been shown to have to have a wide range of environmental benefits such as moderation of urban climate, improvement in air quality and managing run-off after heavy rain. However the environmental benefits of are only apparent when roof greening is taken up on a large scale. There are many benefits that relate specifically to roof greening on a domestic scale, including:

• Aesthetics : although not usually considered a major benefit, the adding of visual interest to the roof structure can be considered a significant advantage.

• Increasing the opportunity for plant growing : horticultural interest is not normally considered a benefit however, in some gardens, roof planting represents a real opportunity to extend the scale of planting options while providing an alternative outlet for rockery and alpine plants.

• Wildlife benefit : green roofs are particularly suited to creating diverse, meadow-like vegetation providing habitat, food sources and shelter for insects and birds. In small gardens the planting may represent a considerable contribution to the wildlife value and in dense urban areas, roof planting may be the only green in the area.

• Protecting the roof and insulating : a vegetated layer protects the roof from the extremes of weather and can provide insulation from the summer heat and also retain the buildings heat in the winter.

Well, perhaps like proverbial double glazing they cost more initially than standard roofing; and the payback is not normally, in financial terms anyway, immediate. Capital costs of laying green roofs are higher than other coverings and of course there are different and sometimes increased maintenance costs. But what else can you do even on the best single ply membrane? ~ not much. And in any case, should the cost and value of landscaping a roof not be compared with the cost and value of landscaping on the ground?

The most straightforward way to install a green roof is on a new-build structure as it is possible to design in appropriate supporting mechanisms. In other cases, it  is essential that the roof on surface is able to support the weight of vegetation and substrates. This is not a problem if the commercially available systems is being installed-- these are generally sufficiently lightweight. However, if any type of more adventurous structure is attempted it is essential consult a structural engineer if there is any doubt about loading. It may be possible to increase load-bearing capacity of an existing flat roof by adding additional rafters between the existing roof,  this will effectively double the load-bearing capacity.  Similarly, larger or deeper beams are used in construction, increasing the standard 75-100mm beams to 200-250mm.

Commercial extensive green roof systems are supplied with vegetation layers. The plants are established as cuttings or plug plants, on a  pre-seeded or planted mats, or - over large are hydro-seeded. Sedums are attractive and retain their colour and interest year-round. A mixed species, randomly planted, is generally used. However, there is room for experimentations of a random mix of sedums why not work with single colours? For example, Sedum albums are  very effective red-foliage species, with dramatic white flowers in June. Where the substrate is between 6-10cm it is possible to grow a wider range of plants, but they will still have to be  tolerant of drought and exposure. Alpine and rockery plants are especially suitable. Specially thyme, armeria and dianthus are commonly grown, but it is worth experimenting with other alpines. Low-growing annuals and bulbs from desert habitats (such as alliums and tulips) are worth trying, along with compact, clump forming grasses such as festucas.

While shallow extensive roofs offer limited planting opportunities, a slight increase in depth of 15cm offers many more possibilites. The most appropriate species for semi-extensive roofs are those that grow naturally on free-draining soils or substances (for example sand dunes or limestone grasslands), in addition to the alpine plants already mentioned. It is also possible for wildflower meadow mixes (again it is best to use chalk or limestone grassland mixes). At the Department of Landscape, University of Sheffield, trails of a range of suitable grasses, per    and alpines for use on green roof systems have been carried out. The results show that plants normally grown in sunny, free-draining positions will also grow in a semi-extensive roof plant. Such plants tend to have grey or sliver foliage and blue white or pink flowers, resulting in harmonious colour and texture combinations. Using a balanced mix of plants, a continuous flowering from May until November can be obtained. Slightly taller herbaceous species such as Nepeta x faassenii, Origanum laevigatum OHerrenhausenO, Salvia nemorosa and Calami  nepeta can be mixed with alpines and creeping plants---such as Dianthus deltoides and Gypsophila repens--- together with clump forming grasses. Scattered upright species such as dwarf kniphofias can provide structural diversity. Such plantings performed well in depths  of substrate on a roof with no irrigation, over the entire summer of 2001 in the trials. Periods of watering ( when there had been no rainfall for seven days) produced longer flowering and great growth, but did not significantly enhance the visual effect. 

If initial costs may be one prohibitor of their increased use, the other arguably is the simple proposition that the solution does not immediately come to mind as option when developing a design brief and specifications for new buildings ~ let alone refurbishment projects. Against this the potential for greening roofs is enormous. One has only to look at aerial photographs of cityscapes and wonder at the number of wasted flat roof areas that already exist. The challenge of potentially having to build some of the proposed 5 million new houses in the countryside might in part be answered by an increased adoption of such visually unintrusive and well insulated dwellings The opportunity now lies less with technical capability. This has increasingly proved to be assured.  The challenge lies much more in recognising and promulgating the wider and longer term potential of green roofs as compared with simple watertight and well insulated systems. Green roofs offer that much more. All building owners and users and their professional advisors should arguably, start to think more positively about the opportunities offered by green roofs. Many of the flat roofs on commercial and industrial buildings built in the 1970's are coming to the end of their life cycle. If only some of these buildings opted for a green roof solution, a step would be taken in the right direction. 

Although these pages are checked regularly and every care is taken with the information within. Delston can not accept any responsibility or liability for information or results displayed within due to third party interference. All information was deemed correct at time of publication.

Author Myrddin
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