Intensive and Extensive
Basically there are two different types of green roof systems, either intensive or extensive. You will know the two types by their structures. An intensive roof normally requires a very massive structure to support it. If for example we wanted to have trees growing on top of our roof, it would be necessary to provide soil approximately three feet deep to sustain the desired vegetation. As you can imagine when that soil is wet, it will present a massive dead load on the structure. You will typically find an intensive system on parking decks, massive structures, and things like that where you have the structural capacity to support that kind of load. An extensive green roof system on the other hand is a very lightweight system that will impose a minimal dead load on the structure of the building. Typically with an extensive roof system, the entire area of the roof is green. The final roof covering is intended to make the entire roof appear like a meadow or field in a natural state. There are a variety of different elements involved in extensive and intensive roof systems. Each one of these roof systems will consist of a structural deck with insulation to comply with local energy codes on top of that. One questions often asked is; will the green roof itself add r-value to the roof system and to the building? The answer is yes; but only at certain times of the year. We are going to try to keep these roofs moist and anything that is wet becomes a thermal conductor, not an insulator. When you are calculating the r-value for a roof you should not include any additional r-value that will be generated by a green roof system. Always include the amount of insulation that will necessary to give the proper r-value required for the building.
The next component is the waterproofing membrane or roofing membrane. Absolutely crucial to the performance of these roofs is a good sound waterproof membrane, after all, we are now going to be allowing water to stay these roofs. In the past it has been industry practice to remove the water from a roofing system. Green roof technology requires a system that is more resistant to constant moisture exposure. In addition to the waterproof membrane, a green roof requires a root resistant layer to prevent the plant roots from penetrating the water proof membrane. Next we will have to include some type of water retention system which will hold the necessary moisture on the roof to sustain the plant life. Here is an egg crate type of design for a water retention system. Each one of the voids where an egg would sit is designed to retain water. On top of that is a filter cloth that keeps the soil from filling up the cavities where the egg would sit. This is only one type of water retention system. Another type is a vegetation mat. It looks a lot like a carpet pad and is made of 100% recycled materials. It actually has nutrients built into it that will help promote plant growth. This can be placed directly on top of the waterproofing membrane. A filter cloth is not necessary with this product. There are a couple of other options to consider as well.
Soil substrate is placed on top of the filter cloth or matt. The type of soil and the amount of the soil will depend on the type of plant that the landscape architect or botanist would recommend for your particular area. Most commonly the type of plants used for an extensive green roof system are known as sedums. A sedum is a rock garden variety plant.
To reiterate, there are two different types. With an intensive roof system you will find a greater amount of soil necessary to sustain trees, shrubs or large plants that need a much deeper root base to survive. An extensive roof system requires a minimal amount of soil. Typically two inches is all that is required to sustain the sedum. The soil mixture used will consist a lot of clay and gravel and not as much topsoil. Some people however have chosen to use a manicured lawn for the ground cover of the roof, which is fine. Understand , however, that if you incorporate a lawn on top of the roof it will require maintenance and a deeper root base. Typically about 6 inches, to sustain that type of lawn.