A green roof, often referred to as a vegetative roof, is a roof composed of plants (sedums), a layer of growing medium, root protection barrier, membrane, insulation and drainage system.
There are three types of green roofs. The difference is mostly determined by soil depth and irrigation requirements.
• An extensive green roof is characterized by soil depth of less than 6”, has low maintenance and does not require an irrigation system. There are a number of nurseries that offer extensive green roofs in interlocking tray systems that weigh
between 10 and 35 pounds per square foot. Generally extensive green roofs are the lowest cost of the three systems and support the least variety of plant life.
• An intensive green roof has over 6” of soil, supports a large variety of plant life and weighs from 35 to 300 pounds per square foot. An intensive green roof is the most expensive of the three types of green roofs and will always require an irrigation system.
• A semi-intensive green roof has soil depth of 6” plus or minus 25% and is generally a lighter version of an intensive green roof weighing 35 to 50 pounds per square foot. The variety of plant life is somewhat restricted and a semi-intensive system may or may not have an irrigation system.
Green roof design and installation is still in its infancy in North America, although well established in some European countries like Germany.
Key Considerations
• The weight of a green roof precludes its use on many existing buildings.
• Fire ratings have not been determined due to the variety of plants available. A proposed rule requires a 6’ border around any green roof section greater than 15,200 square feet and that the longest side of any green roof be only 125 feet.
• As leaks might be hard to find under a layer of dirt, root protection barrier, membrane and in many installations, insulation and a vapor barrier, designers often include an automated leak detection “grid” adding another cost to an already expensive roof system.
• Plants on Intensive and semi-intensive green roofs are typically grown on the roof from seeds or sedums and need constant “gardening” during the initial growth period. Plants used on extensive green roofs are typically full grown and need little attention.
• Metal terminations must be compatible with fertilizers that will be used on the roof. Aluminum terminations are generally not acceptable whereas stainless steel and copper are good choices.
• Green roofs are often frequented by visitors so fall protection is a must consideration.
• Many proponents of green roofs state that longer roof life offsets the high initial cost of a green roof. However the initial cost of a green roof can be two to four times the cost of a traditional roof and this cost does not include the ongoing maintenance required by an extensive or semi-intensive green roof.
