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.
Four to five inch extensive green roof systems are well suited to most regions and do not require irrigation. (Depending on when the extensive green roof is installed, a temporary irrigation program may be necessary during the establishment phase until the cooler months.) Weight-saving systems less than 4 inches are typically designed with an irrigation system to compensate for their inability to retain sufficient water for the plants’ needs. If the purpose of an extensive green roof is to retain stormwater, then why design a system that does not retain water sufficiently and requires the use of additional potable water? It is again difficult to justify the need for an irrigation system on extensive green roofs.
- 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.