There are two types of green roofs: intensive and extensive. Intensive green roofs can accommodate large trees, shrubs, and well maintained gardens. They can be regularly accessed and use is encouraged. The intensive roof garden is designed with a minimum of a foot of soil depth, which can add 80 – 150 pounds per square foot of load to the building structure. The design also includes complex irrigation and drainage systems because annual precipitation can not feed the more intensive plant life. Regular maintenance for an intensive roof garden is required. The extensive green roof is more low key. It can accommodate many kinds of vegetative ground cover and grasses. Plants from the Sedum genus are usually used because they are hardy and colorful. Access and use of the roof by the public is generally restricted for an extensive roof garden. The extensive roof garden is designed with only one to five inches of soil depth, which can add 12 – 50 pounds per square foot (dry weight) depending on soil characteristics and the type of substrate. The design also includes a simple irrigation and drainage system. Maintenance on an extensive roof garden is minimal.
The construction of roof gardens can be difficult, due to the many layers involved. See figure 1 below.
The bottom layer is the roof construction. The roof construction must have a waterproofing layer that is durable enough to safeguard the structure over time with minimal maintenance. An example of a commonly used waterproofing agent is a fluid-applied rubberized asphalt waterproofing membrane. Above the waterproofing layer is the moisture retention protection mat that retains a portion of the precipitation for future plant usage. Contained within the moisture retention protection mat is a root retardant that prevents plant roots from penetrating. Root barriers often contain copper sulfate to retard plant growth. The next layer is the drainage layer. Various kinds of drainage layers are used by different contractors. Some layers have drainage channels that allow excess precipitation to collect and drain. Others contain small cups that collect excess precipitation that can be absorbed into the soil medium for plant use in the future. The water contained within the cups provides a moist, beneficial subsoil environment for the plants, without allowing fungus or root rot. Next a filter fabric mat is installed to prevent soil particles from entering the drainage layer.
The final layer consists of the soil medium. Ordinarily, good soil is very heavy due to its high clay content (Wood 2004). Because of the loads already associated with green roofs, lighter soil mediums are required. Soil medium used for green roofs is a combination of shale, pumice, sand, and organic matter. Care must be taken when preparing the soil mixture to prevent the export of pollutants. Mixes with large quantities of compost have been shown to export nitrogen and phosphorus. The USDA-Agricultural Research Service suggests providing the mix with a quality mature compost manufactured using industrial byproducts high in iron and manganese to reduce phosphorus solubility and increase heavy metal adsorption. The mix must promote hydrated plant life, but prevent over-saturation. The depth of the soil medium depends on the type of green roof under construction.
The final step of the whole process is the selection and installation of the vegetation. Vegetation should be chosen for its ability to thrive in the local climate, withstand the harsh conditions of a roof, and imitate the surrounding landscape’s diversity. It also needs to withstand direct radiation, drought, frost, and strong wind conditions. For an intensive green roof, trees, bushes, and other large plants are suitable. For an extensive green roof, smaller plants and grasses are more appropriate.
