By: Jörg Breuning
The main purpose of an extensive green roof is stormwater retention and
delaying stormwater runoff amongst many other added values.
Above: An Irrigated Green Roof
In the last decade, I've seen many green roofs where the intended plants never really flourished. I estimate that in the US at least 50% of the green roofs are not performing to their fullest potential. This can be observed by simply looking at the most obvious of indicators, the plants themselves, regardless if they were planted on purpose or somehow found the space to take root. Most of these less healthy extensive green roofs are pre-planted boxes, or commonly known as modular systems. Not only are these systems much more costly, the mid to long term results are often far below systems that are assembled in place and at the time of installation.
I know that the transition from being a common nursery-grown plant (including pre-planted boxes) to the extreme environment of a rooftop poses severe challenges. Green roof plant nurseries typically have "great" advice and recommend the installation of temporary or more permanent irrigation systems. This advice is defeating the purpose of an extensive green roof and shows that asking self-appointed experts can cause a spiral of failures. Since many green roof installers lack the proper horticultural knowledge, they may not be able to identify problems by simply looking at the indicators (the plants). This could cause the problem to gain momentum. In addition, the false conclusion that technology (Google search, Apps, synthetic growth media or sophisticated soil moisture control devices) can fix the problem supports my theory of less experienced or misinformed green roof professionals. They rely heavily on technology to fix any issue and miss the big picture. Nature has the ability to take care of itself, as long as the appropriate design, materials and plants are used in addition to being familiar with the immediate climate.
Green roof designers and green roof professionals must understand that less is often more when it comes to extensive green roofs. No building owner wants to irrigate their roof in short, mid or long terms. There is no need to make a green roof more complicated by using multiple synthetic or plastic layers. LEED™ certification supports using gray (recycled) water for green roof irrigation; but what is the point of watering an extensive system when it's main purpose is to retain stormwater? Common sense and experience are the only two things that will aid in planning the perfect green roof.
Above: An Unirrigated Green Roof
Implementing irrigation on an extensive green roof is a clear sign of not understanding the basic principles of horticultural techniques or the laws of nature. Irrigation reduces the water retention, increases the nutrient pollution in runoff and requires higher fertilization application. Irrigated extensive green roofs are not environmentally friendly, not economically feasible and have hardly any payback for the building owner.
I tell my clients if they have an offer or design for an extensive green roof that includes irrigation - be cautious!