Construction
There are a range of techniques developed for modern green roofs. These include drainage materials, substrates (="soil" in a wider sense), choice of 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. The result will be so much better if you know what you want, and what is possible.
Aims and motives
As you saw from all the different advantages with green roofs, you get a range of benefits when you lay a green roof. Most of these can be combined in a single roof. However, certain materials chosen can enhance certain functions.
Aestetics
Your motives may be mainly aesthetical. This generally does not conflict with any other function of the green roof. You might have to consider if your notion of what is beautiful also requires a lot of maintenance, or is very expensive, but very attractive roofs can be achieved with very little extra cost, and no additional maintenance.
Recreation
The health aspects and recreational value of a roof garden is not directly dependent on the materials chosen, but consider using a more garden-like version, that can be achieved with thicker soil layers.
Storm water
A lot of green roof projects are motivated mainly as a solution to problems arising from rain. (Review the section on Storm Water) In order to hold as much water as possible, you might consider a thicker soil layer and drainage material that can store a lot of water, for example rockwool. This also increases (apart from the weight) the variety of plant species that can grow on the roof. This has effects on the maintenance needed, and also on the risk of fires, as you can read more about if you click here.
Ecology, Biodiversity and Enviroment
Since green roofs have great potential in providing habitats in an otherwise rather sterile city context, imagine the impact if large industrial complexes would contribute to alleviate the effects of a consumer society by laying eco-roofs especially designed for biodiversity and environmental benefits! In this case, choose materials that are as natural as possible, or at least recycled, and put some consideration that the plant species chosen are of local origin, or at least fits in the ecosystem. Use substrates favoured by invertebrates.
Physical Conditions
Once you’ve thought through your aims and motives, the first thing you will need to consider if you wish to build a green roof are the physical conditions of the building and the site. Whether your project is a completely new building, or you are fitting a green roof onto an existing building, here are some conditions to take into account:
Slope
Green roofs usually are made on roofs with between 1 and 30 degrees slope. Over 30 is possible, but more difficult to succeed with. The steeper the slope is, the more difficult it is to prevent erosion and slide. This can be done with nets etc that stabilize the soil. However, the more difficult problem, unless you are prepared to irrigate the roof, is that the steeper the roof the dryer the conditions become. This is both because water drains off fast, but also because the roof receives less rain per m2 surface: Figur Because of the differences in water retention and erosion, the choice of drainage material varies with the slope. A very flat roof risks getting waterlogged, and has other requirements than one that might instead need help to retain water.
Length
The length of the roof, from the ridge to the eaves, has an effect on the moisture content in the soil. Water gets drawn up by capillary forces into the soil, but only up a shorter distance. The conditions get drier the further towards the top of the roof you get.
Direction
The north-facing and south facing side of a roof get very different conditions of light and heat. Also if a part of the roof is shaded by trees or other buildings, or by an extending roof window the vegetation will develop differently there. There are also places where you might get more water, for example from an adjoining building. In other places you might get no water, if an edge is under an overhang. In such a case, it looks quite nice to have gravel there instead of dead vegetation, which would be the case unless you want to irrigate that part.
Wind
You might not have thought so, but a green roof is less susceptible to damage by storms than many other roofing materials! Experiences have shown that after a bad storm in Malmö, Sweden, newly laid green roofs on the school in Augustenborg were unharmed, whereas tin sheets on another roof at the school had blown off.
In very windy places, however, make sure to use some erosion protection such as a net inside the soil, and fasten it well to the roof - but never, ever, screw through the waterproofing membrane! Even using screws with rubber packing is not to be trusted in the long run.
Weight
One thing you have to know about your building before you decide to lay a green roof on it is how much extra weight the structure can bear. 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 rockwool 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, that weight has to be reckoned as well.
Materials
Waterproofing
People often ask us if having all that moisture on the roof won’t harm the building or the roof membrane. No. 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 (see Life expectancy of roof membrane). Unless the roof membrane is damaged while laying the green roof (don’t ever screw or nail through the membrane!), the soil and vegetation cover will protect the membrane from these types of injuries later. Of course the membrane has to be in good condition before you lay a green roof on it - ask a professional to look at it if you are uncertain. Starting with a newly laid bitumen roof under the vegetation, you should avoid leakage later.
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 ecolological consciusness, 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. The practice in Sweden is to use a rubber mat to protect the bitumen roof membrane when building for thicker soil layers.
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 steaper slope or other conditions on the roof.
It is not absolutely necessary that the draining function is performed by a separate layer. It is also common to use a substrate that is porous enough that excess water can drain effectively through it. This is called a single layer green roof.
Drainage materials used in green roofs include natural materials, recycled materials and manufactured drainage mats. |
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Gravel, crushed rock
To start with the simplest materials, natural gravels (though preferably not so round as to roll off the roof) and crushed rock are examples of drainage material that is a natural resourse, and that works well. It is cheap, and when local materials can be used, straight from the building site, this gives both economical benefits and a contribution to biodiversity, by "giving the habitats back" to the invertebrates that lived where there is now a building.To the disadvantage of these materials is that they are heavy. Neither of these materials can store a quantity of water or dissolved nutrients for the plants.
Crushed lava, crushed Leca and crushed bricks
Crushed lava is as the materials mentioned above, a natural resourse. Leca is manufactured, and so is brick, although when used in this way, it is recycled. Despite their different origins, these materials have a lot of similarities. They are porous, and can store water and nutrients to a greater extent than rock. Materials that can store more nutrients will help to ensure that there is minimal leakage of nutrients from the green roofs to the waterways, where they act as pollutants.
The porous drainage materials are relatively light weight, and can be used by invertebrates as a habitat. Experiences from London showed that sedum mats on a layer of crushed brick had much higher diversity of invertebrates (spiders, insects) than an identical mat on rockwool.
Rockwool
This is a material manufactured from rock minerals, often used in insulation, or as a growing medium in greenhouse crops. The roots of the roof vegetation can penetrate it, and the friction between the rockwool and the bitumen roof makes it lay still in place. Rockwool can hold a lot of water, and release it to the plants over a longer period, but also drains off any excess water.
In very light weight solutions, rockwool is used as a substitute for the substrate, plants grow directly in it, and gravel is sprincled on top to stop the mats from blowing off. This solution doesn`t hold any long term nutrients for the plants, and so has to be fertilized.
For environmental reasons, consider the energy consumption needed to turn rock into wool.
Foam materials
These can be made from recycled car seats and the like. They drain off water, but don’t store a great amount, and also not a lot of nutrients.
Plastic sheets with cups
There are many different brands and versions of drainage sheets for green roofs. Some are made for intensive, thick soil roofs, and some for extensive roof vegetations that are only a few centimetres thick.
The idea is that the water that fills up in these cups is evenly spread over the surface (compared to rockwool, where gravity will pull water to the lowest edge, leaving the crown of the roof much drier). Once the plastic cups are full, any excess water will drain off effectively through holes. These drainage materials are designed for an even performance of the green roof, and are very easy to transport and install. To their disadvantage can be said that once they are dry, they are very dry. The porous materials will dry out slower, giving the plants a "warning" and a chance to set their water consumption in proportion to availability. Plastic sheets store no nutrients, and can be a more expensive alternative, especially if energy consumption in manufacture is calculated, and the fact that plastic comes from a non-renewable resource.
In addition to the specific properties of the drainage materials, of course price, availability, weight, installation costs etc makes a difference when you make your choice.
Erosion control and Filter layer
Above a drainage layer, there is often a filter layer, in the form of a fibre cloth, to stop the substrate from disturbing the drainage function. When you buy a prefabricated sedum mat for a roof, this filter layer is included, and attached to that is a three-dimensional nylon netting that serves as an erosion control that holds the substrate in place. You can also buy this fibre cloth with the nylon netting separately, and rake soil into it after it has been rolled onto the roof.
Using this erosion control becomes more and more necessary the steeper the slope is on your roof, but on rather flat roofs it can be left out.
Substrate and Fire resistance
You cannot use any garden soil on your roof and expect good results. In short, a soil containing a lot of peat or other organic substances is liable to shrinking, and can be a fire hazard.
In prefabricated mats, a high organic content results in gaps between the mats, leaving the roof looking like a tiled floor.
There are solutions with peat in big bags with loose peat in between, used for green roofs with thicker soil layers, where the shrinking probably will not show, since most of the shrinking will probably be in deapth.
A second problem with soils of a high organic content, as well as thick soils, where grass and other plants accumulate dry biomass, is that it will burn. A fire in peat can glow and spread unnoticed under the surface for a long time.
Fire is however not a problem with all systems of green roofs. Sedums are succulents that do not burn easily, and neither does the moss in moss-sedum mats. A mineral soil is fire-proof, and so sedum roof vegetation with a suitable substrate and fire proof drainage, such as rockwool, will stand up to the tests of the fire department.
A good roof soil is therefore mainly a mineral soil, not containing clay, which is too impervious. To make the soil lighter, crushed brick or lava can be mixed in. Usually, to get a good plant cover fast, the soil is fertilized with slow releasing granules, type Osmocote or Multicote.
Plant species and establishment
It is necessary to choose plants suitable for each climate, whether the conditions are periodically very dry, very hot, very cold, or very variable. For an extensive roof vegetation, of course the most important thing is that the plants are draught tolerant, since dry conditions are the rule on these roofs in most climates. Succulent plants are plants that can store water in their tissues for long periods. Among these are the sedums, and a few related genuses within the plant family Crassulaceae. There are quite a few species of Sedum that are suitable for green roofs, and they come in different colours and growth patterns. In combination with mosses, these are the most common plant communities used on extensive roofs.
In parts of the world where sedums are not naturally occurring, a developing green roof market will have an exiting challenge in finding draught tolerant species that are suitable for the local climate, and that come from the local flora. This is an important aspect, since green roof technology is very much a part of a development towards a sustainable community. Introduced foreign species can not perform the same ecological role for the local wildlife as a native plant.
Here in Scandinavia, we mainly use local strains of sedums, or, on the thicker roofs, local dry grassland flowers and grasses.
The plants can be sowed, planted or propagated with cuttings directly on the roof, or established with prefabricated vegetation mats.
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