Exploring Green Roofs: Living Surfaces for Sustainable Buildings

Many people don’t know what green roof is. So, what it is? A green roof is not a surface painted green, it is a living surface of plants growing in a soil layer on top of the roof. A thinner or thicker soil layer is spread over the roof membrane itself, sometimes with a protective root barrier, often with a drainage layer underneath. On the substrate, draught-tolerant species of plants are grown. Roof vegetation like this one doesn’t at all damage the waterproofing membrane of the roof. In fact it protects it from the uv-light which does damage the membrane over time.

There are two types of green roofs: extensive and intensive. There are some differences of its. An extensive roof has very thin soil layers, draught tolerant plants and requires little or almost no maintenance. The limitation here is in the choice of plants. Extensive roof can be established on a very thin layer of soil. An intensive roof garden has much thicker soil layers, and can look like an ordinary garden, with trees and shrubs. Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.

This of course needs as much maintenance as a garden does, and can only be constructed on a roof that can bear heavy loads. (http://en. wikipedia. org/wiki/Green_roof) It doesn’t matter what type roof is, but construction is a similar. Construction includes drainage materials, plant materials and establishment methods. For each green roof project, different site conditions and your aims and motives will govern the choice of materials and establishment methods. Therefore, if you are planning to build a green roof, consider the following two sections carefully.

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There are some roof sections which are important. First is a waterproofing. The membrane is made waterproof, and it will stay waterproof unless it gets physically damaged or aged. What ages a bitumen membrane is mainly uv-light and temperature extremes. Unless the roof membrane is damaged while laying the green roof, the soil and vegetation cover protects the membrane from these types of injuries later. (http://www. greenroof. se/? pid=32&sub=19) Second section is root barrier. The asphalt waterproofing itself is not classified as root resistant and had to be reinforced with a root barrier.

This barrier consists of rubberized asphalt enforced with polyester and treated eight a root-repelling agent. There are also synthetic waterproofing membranes (like PVC liners) available that are root resistant and do not require an additional root barrier. Different countries have different building practices and rules for green roofs - and many countries have no rules at all yet. In Germany, for example, a root barrier is always laid under a green roof. This is to ensure that no roots can damage the waterproofing.

A root barrier is sometimes incorporated in the drainage layer. It can be chemical and poisonous to plants, or it can be a thin copper sheet, or rubber sealed so as to avoid seams. If your motive for having a green roof is sustainability and ecological consciousness, it’s a good idea to ask your contractor about the method. In Sweden, the usual practice is that under extensive roof vegetation (2 - 5 cm soil), where only sedums and moss can survive, an extra root barrier is not considered necessary, since the sedums used have weak roots, and moss has no roots at all.

When the soil layer is thicker, and therefore can hold more water, other plants grow there, and there is a risk of getting weeds with more penetrative roots. Second layer is insulation. Insulation boards can be made from extruded polystyrene which has high water resistance and compressive strength. The main difference from the old roof is that the insulation layer must be places above the waterproofing membrane, but on the new roofs insulation must be located below the waterproofing. Second, but not less important, is drainage.

The main reason for using a drainage layer is that if a lot of water drains off along the surface of the soil, this will cause erosion. The second reason, is that for roofs which are almost flat, too much water would remain on the roof, forming puddles. Especially sedum vegetation is adapted to dry conditions, and tends to rot if submerged. Besides, too much water available can lead to unwanted plants establishing on the roof. Another function of the drainage materials that are often used is to store some amount of water.

This effect becomes more and more important the drier the conditions on the roof are, due to climate, a steeper slope or other conditions on the roof. Drainage materials used in green roofs include natural materials, recycled materials and manufactured drainage mats. (“Green roof systems” Susan K. Weiler, Katrin Scolz-Barth) And finally, the top layer is growing medium with plants. The growing medium provides nutrients and structure for the roots to anchor in. the proprietary mixes of the manufacturer consist of a lightweight aggregate, expanded shale, and some compost.

The organic components make up about three to six percent of the shallow planting areas. The mix for deeper planting areas has slightly more organics. This mix is also heavier and allows a higher water-holding capacity in order to support the larger plants of these areas. In conclusion, one has to realize that there is no cookie cutter approach when it comes to the design of a green roof section. Though certain layers are always present – waterproofing, root barrier, insulation, protection layer, drainage layer, growing medium – their actual composition varies widely responding to a particular situation. “Green roof – a case study” Christian Werthmann) Green roofs have many advantages. One is a cooling effect. It is very useful effect for individual houses at summer time. A black bitumen roof easily reaches temperatures of 80° Celsius in the summer. When a soil layer and the shading plants protect the roof, the surface temperature usually doesn’t rise above the surrounding air temperature. A study conducted by Environment Canada found a 26% reduction in summer cooling needs and a 26% reduction in winter heat losses when a green roof is used.

In addition to this, the plants and soil evaporate water, creating a cooling effect, and a moister air, more comfortable to breathe. Cooling effect is useful at summer, but at winter there is another green roof feature – insulation. The soil layer provides an additional insulation. In warmer climates a green roof especially the versions with thicker soil layer, can make a difference in the heating required in the building. Life expectancy of the root membrane is also important. The waterproofing of the roof, for example bitumen, has a normal life expectancy of 25 years.

After that it needs replacing. The reasons are that the uv-light from the sun makes the surface brittle, and then the expansion and shrinking caused by the fluctuations in temperature, together with the loss of elasticity, causes cracks to form in the membrane. The high temperatures in themselves, above 60° Celsius, age the membrane. The green roof protects the waterproofing from both the uv-light and the temperature extremes, increasing the life expectancy of the membrane to at least 60 years. (http://www. greenroof. se/? pid=28&sub=19)

One of the most important effects of green roofs in the cities is their potential for retaining and delaying rainwater during storms. The gutters and sewers in the city have to be big enough to cope with the amounts of water that falls during extreme storm events. As more and more surfaces in the built up areas are made hard and impermeable, less water can percolate naturally into the soil. This means that the sewers and treatment plants get higher and higher loads. For the water treatment plant this can be a great problem, since they take care of sewage as well as rainwater.

If the sewage gets very diluted by a heavy rain, the sensitive chemical and microbiological processes are disturbed. Getting the balance back costs time and money, and in the meantime, sewage is less completely cleaned. Also, the treatment plants cannot store unlimited amounts of water, so during extreme rains, they are forced to let completely untreated water to the natural waters. The more rain water that is taken care of locally, the less often we will have to face these polluting events. (http://progressivetimes. wordpress. om/2010/10/04/one-roof-two-roofs-green-roofs-blue-roofs/) There are some effects which are interesting. The heat island effect. This is the effect that makes cities reach a higher average temperature than the countryside around them. The large amount of stone, asphalt and concrete in the towns and cities that absorb the heat from the sun in the daytime, and release it at night. An additional cause is the lack of trees and other vegetation. The night time temperature rises, and in the summer people suffers from sleeplessness. The cost of air condition goes up.

Green roofs can protect from noise and electromagnetic radiation. Thick layers and plants can reduce noise in the outdoor environment and green roofs can reduce the electromagnetic radiation that enters a building to a great extent. (http://www. greenroof. se/? pid=28&sub=19) The most important problem in big cities is pollution. Green roof vegetation, as well as other vegetation, use CO2 for their respiration, and therefore reduce the negative effects of pollution. Best are highly productive plants, which in a year can produce a lot of biomass.

An extensive green roof does not produce very much, but intensive ones could. In this case, it might be a question of a small extra advantage, when large surfaces of the cities rooftops are used for vegetation. This is not a method that can be used as an only solution to the problems of pollution and global warming, but together with all the other beneficial effects of green roofs, it is a small step in the right direction. (http://www. greenroof. se/? pid=28&sub=19) Green roofs also provide habitats for plants, insects, and animals that otherwise have limited natural space in cities.

In modern city planning, "green corridors" are planned into the cities, where parks and waterways can connect the city with the surrounding countryside, and get humans in closer contact with nature. In the places where there isn’t enough ground space for green space, the green corridors, and the habitats for animals don’t have to be discontinued, if the flat roofs are used. (http://www. greenroof. se/? pid=28&sub=19) Thus green roof save materials, energy and money, produce less waste, can protect from negative cities impact and what is the most important - is environmental friendly. However, green roof has some disadvantages.

Weight is problematical field. A thin extensive green roof of the type widely used weighs about 50 kg/m2 when saturated with water. Lighter versions are available, that uses rock wool as substrate instead of soil, which gives a weight of about 35 kg/m2. Compare that to tiles that weigh 33 - 37 kg /m2, depending on the type. If you live in a region where you can expect snow, then weight has to be reckoned as well. So some buildings, especially old buildings, cannot be retrofitted with certain kinds of green roof because of the weight load of the substrate and vegetation exceeds permitted static loading.

Depending on what kind of green roof it is, the maintenance costs could be higher. Cost is the main reason why people didn’t want green roofs. Properly designed and -installed systems include root barriers. It is true that installing adequate waterproofing systems and root barriers automatically can increase the initial cost of the roof. A properly designed and installed green-roof system can cost 15 to 20 dollars per square foot as a total cost, not including the roof's waterproof layers.

In Europe, a well-designed and professionally installed fully integrated green roof can cost anywhere between 100 to 200 euros per square meter, depending on the kind of roof, the building structure, and which plants are used. Despite of that, I think that green roofs have more advantages than disadvantages. (http://en. wikipedia. org/wiki/Green_roof) It is clear that living green roofs and landscapes over structure are not a panacea for ameliorating the negative environmental impacts resulting from increased development or the loss of open space.

They cannot and will not replace our forests and prairies, will nor remediate the degradation of all stream corridors, and will not stop global warming by themselves. However, green roofs and landscapes over structure can act buffers to mitigate the impacts of unbridled and unplanned urban growth and development. Reducing building roofs generates less storm water runoff, reduces the heat gain that affects our indoor and outdoor environments, and mitigates the continued degradation of air and water quality. Green roofs provide additional usable, confortable open space.

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