There are now DIY kits for green roofs. These can be reasonable value for a fixed area of green roof – normally 9 sq m per pack. But there are a range of issues that need to be addressed before getting too carried away:
Type of green roof – there are a wide range of options here from using extensive systems (the more common sedum roofs) to intensive ones (turf roofs that can support a wider range of larger plants). The extensive ones are much easier to install, but harbour less wild life.
Weight – green roofs vary between types, but the extensive sedum roofs are by far the lightest ones. This is important as they joists / rafters in your roof will not have been designed to take the extra weight of the green roof and so you will probably need a structural engineer to sign off any designs (if you want to have an ‘official’ green roof. Remember it is not just the weight of the materials, it is the weight of the water retained within the structure as well. With water weighing 1 tonne per cubic metre this can add significantly to the load requirements of your roof.
Objective of the green roof. Is it there to look nice, to slow down water run-off, to help cool the urban environment etc. For slowing rainwater run-off significantly you need to have a larger, deeper and heavier roof, so this affects weight.
Aspect of the roof. Is the roof heavily exposed (south or west facing), it is overshaded? This will effect the type of planting that will survive on the roof and hence needs to be explored.
Slope of roof. Green roofs can be applied to quite steep roofs. Up to 30 degrees can be achieved, but you do need retaining bars in the roof to stop slippage. So this requires more thought and infrastructure to get it to work properly.
State of existing roof. All green roofs need to go over working watertight roofs. It is not a sticky plaster. So many people find that they need to replace their existing roof anyway before embarking on a green roof.
Overhanging fauna. If you have a range of larger fauna (trees, shrubs etc) overhanging the roof then seeds are likely to be deposited and hence take hold. Even though green roofs are designed to only support certain sizes of plants this does not stop the seeds from germinating and forming seedlings. The draft plants that this produces might well need weeding out, so this gets onto:
Access to the roof. Green roofs are not fit and forget. They do need some maintenance even if well planned. So you will need to allow for this and for the additional weight associated with standing on the roof.
Style of the roof. Most green roofs have a range of system edges. Where water enters the roof (run-off from other roofs / gutters etc.) you need to have a buffer to slow the water’s speed so that it does not wash away the plants. Also the edges of green roofs are generally outlined by river pebbles. These stop / slow the plants from growing over and into the guttering and potentially causing problems with the rainwater. So you need to think about what it is going to look like and how you are to retain the green roof / pebbles.
Green roof systems are therefore more complicated than just going out and getting a kit. However, there are major advantages of having green roofs especially in the urban environment and so as long as they are well thought out and planned they can extend the life of your roof considerably and also bring fauna, flora and cooling to our towns and cities.