A green roof allows vegetation to grow in a growing medium and usually includes the following elements: waterproof membrane, a root repellent system, a drainage system, a filtering layer, a growing medium and of course plants. Here are three main types of green roof systems as explained by the city of Toronto: These roof types can be divided into seven categories: intensive, extensive, new, retrofit, complete, modular, or pre-cultivated.
Intensive or active green roofs have a deep growing medium that supports a variety of landscape design and growth. They are accessible and used as recreational space. Intensive green roofs are essentially parks in the sky. They provide a peaceful retreat, full of trees, walkways, and seating areas. In dense urban environments, intensive green roofs can create a garden and yard for an entire apartment building.
An extensive green roof has a shallow growing medium and the landscaping is designed to be more self-sustaining, requiring little maintenance. Extensive green roofs are less expensive than intensive systems, since they are lighter and require less structural support and need less frequent maintenance. Extensive green roofs were previously used solely for environmental benefits. Increasingly people are planting crops and vegetables so that these extensive roofs can serve a second sustainable purpose.
New or Retrofit
Green roofs can be designed to be an integral part of a new building, or can be installed later on an existing building. When a building is designed with a green roof system, there can be several benefits. For example, the building is designed to provide the necessary structural support, and won’t require reinforcement later. Also, the building can be designed to take advantage of the aesthetic value that a green roof can offer by providing viewing areas.
Complete systems imply that all the components including roof membrane are an integral part of the whole system. In a complete green roof system, all parts of the roof are designed to support vegetation growth. These systems provide the most flexibility in terms of the type and nature of growing medium, drainage and protection layers and type of vegetation. Complete systems vary in thickness and weight from as low as 50mm to 75mm (2 to 3 inches) in depth and 60 to 90 kg per sq. m, (12 to 18 lbs per sq. ft.) in weight. They can be installed with a variety of waterproofing membrane types.
Modular systems that are positioned above the existing roofing system. Modular systems are essentially trays of vegetation in a growing medium that are grown off-site and simply placed on the roof to achieve complete coverage. They are available in different depths of growing medium typically ranging from 75mm to 300mm (3 to 12 inches). The variety of vegetation is typically more limited.
Pre-cultivated vegetation blanket
Pre-cultivated vegetation blankets consist of a growing medium and plants that are rolled onto the existing roofing system with drainage mats and root barriers. A pre-cultivated vegetation blanket is a pre-grown interlocking green roof tile. The blanket shown below is available in a thickness of about 45mm (1.75 inches). Blanket systems are available in a variety of system designs. The most versatile system contains 25 mm (1 inch) of planting substrate. The result is a lightweight system ranging in weight from 40 to 60 kg per sq. meter. The majority of the vegetation is made up of several varieties of Sedum, a succulent plant (8.0 to 13.0lbs per sq. ft.) tolerant to extremes in temperature that survives with little or no irrigation while requiring very little maintenance. They are cultivated at ground level, then rolled and transported as a complete system on pallets or by crane.
For a more comprehensive description of each roof type see the consultant’s report, Environmental Benefits and Costs of Green Roof Technology for the City of Toronto.