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 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
Flat Green Roofs
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.
More recently RMC specified a green flat roof to the extension of its company headquarters in Surrey. This helped gain a planning consent given its green belt location.
UK - EUROPEAN COMPARISONS
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.
RATIONALE FOR GREEN ROOFS
What are the drivers for encouraging the increased use of such roofs?
| Technically | - | They can reduce the expansion and contraction of roof membranes that historically have helped cause roof failure |
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They can help insulate buildings against extremes of heat and cold |
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| - | They can act as temporary ‘sponges’ in periods of heavy rainfall and thus reduce the pressure on drainage systems | |
| Functionally | - | They can add additional useable space for relaxation, informal meetings, events and games ~ and even as a smoking area in smoking free buildings! |
| Visually | - | They can provide a softer perspective when viewed from higher buildings and in greenfield settings, help hide and disguise a building’s presence |
| Environmentally | - | They can help reduce dust and carbon dioxide in built up areas |
| - | They can help retain a higher humidity level in city areas | |
| - | They can provide a habitat and ‘stopping off place’ for birds and insects |
All in all, these attributes can bring real added valued to both building owners and users, the community and the environment generally. There is even a choice in types: extensive and intensive green roofs.
'INTENSIVE' AND 'EXTENSIVE' GREEN ROOFS
'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 greenroofing 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 for a traditional garden. Although a more refined application and therefore more expensive, the 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. 
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 greenroof 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.
Why are green roofs not more common?
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 POTENTIAL
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.
So to recap the things to think about are:
Structure
Before any undertaking of this sort of work it is always recommended that the structure of the roof is checked to ensure that it is capable of taking the expected load the weight of any stored water should also be taken into consideration.
Slope
A continuous slope of at least 1:50, (this equates to 20mm fall for every 1m travelled) towards the water outlet or guttering. This is recommended as an absolute minimum, a greater fall would be desirable.
Drainage
After the roof is installed as indicated a drainage layer of clay granulates, rounded washed gravel (river gravel) or similar should be laid.
Filter
After the drainage layer a filter layer should be laid to stop the drainage layer from silting up. Rot resistant PES or PP fleeces with a high tear strength and perforation resistance are suitable for this application.
Soil
Soil thickness depends on the use that will be made of it i.e. low or no maintenance garden thin soil higher maintenance thicker soil