The main purpose of an extensive Green Roof is stormwater retention and delaying stormwater runoff amongst many other added values.
Above: An Irrigated thin layered Green Roof System – without irrigation it would be dead.
In the last decade in North America, we'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 thin-layered systems, pre-planted boxes (often called modular systems or hybrid systems), Green Roof Systems from roofing manufacturers with ineffective components, single source warranty-systems, or design intents without consideration for the needs of plants. Typically these systems much more costly (whether thin or thicker) and the mid to long term results are often far below systems that are assembled in place with common sense.
Thin-Layered system with irrigation - doesn't work and doesn't retain much water
We all 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 permanent irrigation systems. In some areas this might be helpful when operated correctly but in all cases this advice is defeating the purpose of an extensive Green Roof (water retention). In almost all cases any type of irrigation caused a spiral of failures and it is wrong to irrigate extensive Green Roofs in areas with limited water for the cause of just having a green roof (Green Washing). Most company sponsored experts don’t see the entire picture or they only see the short term profit and commission (“Mow-Blow-Go”).
If a problem occurs and since many Green Roof system developer lack the proper and specific green roof 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 hardly experienced or misinformed green roof professionals. They rely heavily on technology to fix any issues 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 location and climate.
Thin layered, super lightweight systems or systems with “innovative” components are just by physics not meant to retain “more” stormwater for the plants or they retain too much water for specific plants. As a matter of fact the costs for retaining stormwater in thin layered systems are extremely high comparing to a standardized extensive Green Roof with a thickness of 4-6 inch (depending on location). Besides the initial costs the lack of water retention (don’t forget all studies are done by the companies!) won’t support any significant bio diversity plus the operational costs (because of the required irrigation) are tremendous over time. Typically these Green Roofs never pay back with they are not utilized for “Green Washing” purposes in advertising.
Irrigated extensive Green Roofs and/or thin layered, super lightweight systems (below 25 lbs/sf) will increase the nutrient pollution in runoff, require higher fertilization application, have hardly any tendency for self-healing, and may increase the pressure of unwanted plants and have a high risk for failure. Irrigated extensive Green Roofs are not environmentally friendly in most loactions!
Green roof designers and green roof professionals must understand that certain advertising, studies, White Papers or project references are all assembled by specific manufacturers and their goal is to make money – unfortunately on the back of a very advanced technology and on the back of their clients.
When we advise our clients they get the full picture because we have seem all solutions in the last 35 years, we have tested them and we are currently working millions of square feet where these systems require upgrades, reinstallation or massive repair (around 20 million in damage so far).
We also help our clients doing it as efficient as possible and to get their money back from prior failures through warranty claims and sophisticated lawsuits. This might help to standardize the function of Green Roofs and helps to get the black sheep out of the market. The building owner has a right that things are done right from the beginning.
Never irrigated in the last 10 years.