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There are several ways to install green roof systems, largely depending on where you want to place the insulation layer. The most important thing for any green roof is that it is waterproof and well drained; beyond that there are several different configurations that you can use.
The three main types of green roof system are inverted, warm and cold. An inverted roof system is one where the insulation layer is above the waterproof membrane; a warm roof system is one where it is below; a cold roof system is one without an insulation layer at all.
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Installing an Inverted Roof System
In an inverted roof system the the waterproof layer is right at the bottom and so this should be the first thing that you lay down. You can use asphalt or plastic sheeting but one of the best methods is to go for a liquid-applied membrane, which solidifies after spraying to form a water-tight skin. Whichever you choose, make sure that the layer is applied across the whole roof as water damage is the number one reason for green roofs failing.
Next you'll want to add an insulation layer – mineral wool, fibreglass and polystyrene are all possible but synthetic board foam is perhaps the most commonly used material. Remember that in an inverted system the insulation layer is going to get wet so pay attention to durability and the thermal rating when both wet and dry. The insulation layer also acts as a root barrier, stopping the plants from breaking up the roof beneath them looking for nutrients.
On top of that you need a drainage layer – gravel and pumice are popular here – to ensure that water is carried away to the gutter. The growing medium (usually soil) can be placed on top of this, along with the plants. Wind-blankets are also often laid to protect from erosion.
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Installing a Warm Roof System
Warm roof systems are similar to the above except that the insulation layer is placed beneath the waterproof layer. This has the advantage that the insulation layer stays dry and thus you can use a wider range of materials and also maintain a static thermal rating. The disadvantage of this type of system is that you will often have to add an extra layer on top of the waterproofing anyway to act as a root barrier. Otherwise, aggressive plants can poke holes in the waterproof membrane and ultimately lead to the failure of the roof.
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Installing a Cold Roof System
You don't have to use insulation at all — green roof systems are themselves insulating because of the soil layer. Alternatively, you can pack fibreglass or some other insulating inside your loft i.e. inside the roof itself. Bypassing the insulation layer can make the roofs easier to install but can often make the roofs less effective in the winter, particularly during damp periods, as cold, damp soil can suck warmth from the house.
Green roof systems comprise several layers, some of which are essential and others of which are optional. All roofs should have a waterproof membrane and a good irrigation system but there are several ways to install green roof systems. Insulation can be placed either above or below the membrane or within the roof itself. If you place the insulation above the membrane then there is no need for an additional root barrier; otherwise, you should add some other material to ensure that plants don't damage the roof with their roots.
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eHow, "How to install a green roof," http://www.ehow.com/how_2101500_install-green-roof.html
NBS, "Green roofs: an introduction," http://www.thenbs.com/topics/Environment/articles/greenRoof.asp
House Energy, "Green Roof Installation," http://www.house-energy.com/Roof/Green-roof-installation.htm