By Andrew Yanders
Here at Green Roof Technology we are often confronted with a perplexing industry occurrence. Frequently we visit extensive green roof sites or monitor extensive green roof projects online that have installed an irrigation system. It is difficult to justify the necessity of an irrigation system for extensive green roofs. Sedums, other low growing succulent species, many grasses, and herbaceous plants require no irrigation. The proper selection of plant species is crucial and species not suited for extensive green roof purposes should not be used. An irrigation system wastes potable water on plants that do not need it. This excess water only serves to promote the growth of unwanted plants from foreign origins.
Green Roof Technology has recommended that local governments establish a clear consumer-oriented guideline for the best-practice in extensive green roof construction in their communities. This document would clear-up any misunderstandings among potential green roof investors and serve well to educate the wider public on the functions of the various layers involved in green roof systems.
- In extensive roof planting a minimal depth of soil is used (generally less than 15 cm), with the objective of being able to leave the planting to its own devices after an initial establishment phase. Plants selected for such conditions are able to survive on a long term basis with minimal reserves of moisture and nutrients.
- The construction of an extensive roof garden is the same irrespective of the system that is employed. The first layer, a root protection membrane, is laid directly on top of the waterproof roof surface. Above this comes a drainage layer, then filter mat and on top of this the planting medium.
The City of Stuttgart’s green roof handbook is an excellent source of information on extensive green roofs and provides critical information for anyone looking to invest in a green roof. Green Roof Technology has work with the City of New York and the City of Lancaster to develop similar books on green roofs. We believe it would benefit everyone if local governments across North America adopted a similar program and published a how-to-green roof document for their citizens. Green Roof Technology would be pleased to work with your community to create a suitable green roof handbook for your citizens.