Transactions on Ecology and the Environment vol 16, 1997 WIT Press, ISSN

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1 Green roof systems J. Dominguez-Hernandez*, A. Lozano-Martinez-Luengas ^Area de Ingenieria de la Construction. Universidad de Zaragoza, c./maria de Luna, Zaragoza Spain ^Area de Ingenieria de la Construction. Universidad de Oviedo, Crta. De Castiello, s/n Gijon - Asturias Spain Abstract The idea of using a vegetal covering is not new. Since the beginning of time it has been known that this kind of construction existed. In spite of its great advantages, through the years they have not been very popular. Their difficult construction, complicated technical solutions and the high building cost and maintenance, made the project unsuitable. With the progress of the technique and the new materials appearing, these problems have been solved, and today the weight of the garden roofs is greater each day. Flat and slightly sloping roofs offer an ideal opportunity to develop new green zones in our living areas. The ecological benefits (microclimate improvement, important retention of rainwater, reduction of noise level due to the additional sound insulation, dust particle retention, etc.), economic benefits (increase in life expectancy of waterproofing, energy saving because of the use of thermal insulation ) and also the possibility of using recycled materials, have meant that lately, in some areas of central Europe, construction using garden roofs is a priority when buildings are in the design phase. Otherwise,

2 616 Ecosystems and Sustainable Development Transactions on Ecology and the Environment vol 16, 1997 WIT Press, ISSN architects have a powerful tool in order to diminish the landscape and environmental impact, and improve the building appearance too. This paper will try to explain the benefits mentioned, the different kinds of green roofs, technical solutions employed, and the practically nil maintenance, always using renewable energies and recycling material when it is possible. 1. Introduction Green or Garden Roofs in recent years have gained greater importance, although really it is not a completely new concept. Remember, The Hanging Gardens of Babylon, the Orangerie in Versailles Palace or the Isola-Bella's Gardens? However, garden roofs do not always mean the same as ecology. The plants, bushes or even trees that integrate the roof, and at the same time, the whole building, in the landscape, are very important, but also the rest of the materials that blend in the roof as well. The development of new materials and recycling are the weapons we use today to overcome the difficulties that this kind of covering had years ago. The heavy weight, maintenance and its behaviour against meteorological actions are beaten using new products which we will explain later, acting just as a covering that is not harmful to the environment. Figure 1. wm

3 Ecosystems and Sustainable Development Benefits It is clear that our living areas today are to a great extent built up, and this means a massive use of steel and concrete. In other ways, green spaces or gardens are on the decrease, due to the increasing demand for building land. For this, flat roofs offer an ideal opportunity for creating new green areas, also using recycled materials, that involve additional benefits, such as: 2.1. Retention of rainwater When it rains, especially in torrential form, local drainage systems suffer problems, producing floods. In this way, garden roofs act as a sponge, retaining rainwater (above 70%, depending on the systems) and subsequently transfer it for its self maintenance. Hence, the amount of water that reaches the local drainage system is much less Improvement of climate Following the previous point, in the dry periods, this water that is little by little transferred to the soil, enables the humidity grade to diminish Retention of dust Pollution affects us every day. In general, flat roofs (for example, gravels, fibercements, etc.) contribute to the increase of this polluted cloud. Plants and bushes help us to clean the atmosphere, because they are able to filter up to 85% of the particles, producing pure oxygen at the same time.

4 618 Ecosystems and Sustainable Development 2.4. Increase in life expectancy of the roof In summer, roofs can easily suffer temperatures of about 80 C, and these temperatures turn to 0 C after a heavy hailstone. This temperature fluctuation manifests itself in contractions and enlargements of the covering elements, promoting its destruction. With the soil, these fluctuations are just about 35 C, even when the sun shines strongly. The whole roof lives longer Reduction of noise level Sounds coming from the town (men at work, traffic, etc.), or the air (planes, storms, etc.), reverberate round the common materials, increasing the sound wave. However, plants and ground absorb a great part of this sound, acting as an additional sound insulation Improve thermal insulation This thickness of the substrate layer saves energy (up to 50%), warming and cooling are much slower. in addition 2.7. Better look Of course, every garden looks better than gravel roofs or terraces, which are often dirty and dark.

5 Ecosystems and Sustainable Development 619 Furthermore, the freedom in design that these green roofs permit, enables one to create small lakes, ponds, and every idea that the architects wants. It is a very powerful tool to have in mind during planning of a building, Figure Types Depending on the landscape, there are two different kinds of garden roofs: extensive and intensive ones. The first ones are those which have a thick layer of substrate soil (approx. 8 cm), using suitable plants, used to extreme weather conditions. Water and nutrients are supplied by natural processes. Construction and maintenance are cheap, and structural loads would be between 70 and 150 kg/m. Intensive landscaping demands normal maintenance (cutting grass, mowing) substrate soil layer thickness would be more than 20 cm, and structural loads would depend on the plants, but would always be > 150 kg/nf. The characteristics are summarised in the following Table. Thickness Structural loads Maintenance Extensive landscaping 8 cm. 70 kg./nf 1 or 2 inspect./year Intensive landscaping >20cm > 150 kg/m' Regular

6 620 Ecosystems and Sustainable Development Combinations of these two types are also possile In Figure 3 an extensive landscaping roof is shown, and in Figure 4 an intensive landscaping roof that has existed in Madrid since 1993.

7 Ecosystems and Sustainable Development Elements Concerning the waterproofing, and moving from below the surface to above it, the structure of the garden roof is the following Root Barrier Whenever it is possible to dispense with chemical substances to kill the roots (which could be transported by the rainwater), a seamless root barrier sheet made of unplasticized polyethylene is recommended. The type and thickness of the material depend on the type of landscape planned and the shape of the roof Protection Mats These are made of non-rotting recycled fibers, and are biologically neutral. Their task consists of protecting the root barrier sheet against penetration and the waterproofing system; at the same time it works retaining water and nutrients Drainage elements These elements retain part of the rainwater, even with dense root growth, and also ensure that excess water is drained off. Ventilation and evaporation must be guaranteeed. Made of several materials, such as recycled polyetheylene, currently, using recycled materials increases the cost, usually, 7 litres are necessary to produce 1 litre of new polyethylene.

8 622 Ecosystems and Sustainable Development 4.4. Filter System This membrane prevents fine particles from being washed out of the substrate soil, ensuring the efficency of the drainage elements Substrate soil The soils must be resistant to sparks, radiated heat, erosion, and have low weight, at the same time. Values of ph, porosity, nutrients, and vapour permeability are also important. All these reasons enable compost, mineral materials such as clay bricks or other materials coming from demolition to be used. Also grounds of vermiculite or Montmonilonite have good properties. Components must not be harmful to the enviroment, including in its production process Landscapes As we have said above, we can choose between extensive or intensive landscapes, using low-maintenance suitable plants, self-regenerating and resistant plants; grass, bushes or even trees can be used with intensive landscaping Auxiliary elements In addition to all these basic components, today we have specific instruments, that using non-contaminant energies, enable retention of rainwater, and its subsequent use. According to this, solar cells, automatically regulated, permit mowing of the substrate soil layer when it is more than 40 cm thick (see Figure 5).

9 Ecosystems and Sustainable Development 623 The complete section of these roofs is represented in Figure 6. Plants Roof planting soil N Filter Drainage "xvj^:-:-:^?:':-:':';*:!:^ M' ' I"'^^^*TT~ Protection mat :>m:#:#wm##w# " Roof construction with waterproofing Figure 6

10 624 Ecosystems and Sustainable Development 5. Conclusion This technology, without being new, is now developing in all its power. Architecture is integrated in the environment, as never before. We must consider the possibilities that nature offers trying to better our cities, and improving the quality of life. It is not only an aesthetic reason, it is a necessity. Bibliography " ABC der schall und warmeschutztechnik" (Hans W. Bodran) "Vom Flachdach zum Dachgartem" (DDV) "Kiesdacher weden griindacher"