Constructed Rootzones for Green Roof Systems

Green roofs are considered among the best technological solutions for greening existing urban landscapes characterized by the lack of open and green spaces. The selection of the appropriate rootzone (vegetation layer) is of major importance since it needs to comply with several criteria such as: (a) providing sustainable growth of the selected plant material; (b) exercising limited weight on the building framework; (c) providing adequate anchorage depending on the type of the green roof (extensive-intensive); (d) consisting of environmentally friendly materials; (e) prohibiting any environmental hazards such as nutrient or agrochemical leaching; (f) quickly draining excess water yet retaining increased moisture.

Up to date the green roof industry has mainly been following the German guidelines (FLL) for green roofs while other countries have completely or partially accepted them. However the FLL guidelines have been formulated for northern climates and demand alterations for being applicable to semi-arid or Mediterranean type climatic conditions. In addition the formulaic categorization of green roofs as extensive, semi-intensive and intensive has recently received criticisms in an effort to proceed in an adaptive green roof approach that would depend on the local conditions of each urban environment.

Rootzone type, substrate depth and plant species selection are the most important factors contributing to the success and sustainability of a green roof system. The green roof rootzones are mainly constituted by inorganic and at a much lesser degree by organic materials. The most commonly utilized inorganic materials as rootzone constituents are pumice, crashed tile or brick, expanded shale or clay, sand, and zeolite whereas for the organic materials peat and composts.

There is a worldwide research that has provided significant information concerning the effects of different types of green roof rootzones combined with different substrate depths and with various plant species. Several plant species with C₃, C₄ or CAM metabolic pathways have been evaluated and the conditions of their sustainable growth have been determined. It has been acknowledged that rootzone depth has a significant role in green roof flora sustainability since in most cases increasing the rootzone depth has resulted in increasing plant survival and sustainability and contributed to water inputs reduction. Conversely plants have differentiated reactions in regards to rootzone type since they seem to have different inherited preferences for inorganic constituents, organic content and in several cases their behavior has been altered between growing periods (water stressed and unstressed conditions). So far the effort has been focused on utilizing native and endemic plant species to reintroduce the lost flora and fauna in contemporary cities. However there is also an effort to increase the selection palette of plant species by utilizing an adaptive method for green roof construction. In the adaptive approach several plant species categories could be utilized such as turfgrasses, exotic species and other plant species with minimal inputs of natural resources such as water.

The interest of future works on green roof rootzones is to define new materials with a small CO₂ footprint, preferably recycled to be utilized as constituents. In addition the determination of the necessary inputs of natural resources, especially water in the adaptive green roof approach, is imperative in order to support and facilitate decision making processes throughout the world.