“To understand the whole it is necessary to understand the parts. To understand the parts, it is necessary to understand the whole. Such is the circle of understanding.”
— Ken Wilber
So what do we mean by “optimal”? How do we understand “optimal”?
To understand green roofs (the whole), we are first looking at the parts from two key perspectives:
We think this is in direct contrast with another common method: focusing on individual components, such as a filter fabric, or drainage layer, or edging. Focusing on individual components is convenient for manufacturers who want to position their products as superior, but is not well suited to a holistic understanding of green roofs.
No, these diagrams are the four quadrants of an optimal green roof and an illustration of to whom this matters. Our blog is organized around these concepts, and these diagrams can be used to find blog posts about each topic or combinations of topics.
An optimal green roof balances biological components and hydrological functions. Biological components are vegetation and soil. Hydrological functions are retention and drainage. Only when these four aspects are in balance, do we attain a resilient green roof.
Vegetation is essential to any green roof (also known as living roof,
The term “soil” is not common within the green roof industry. The term “green roof media” is used much more commonly, recognizing that the combination of aggregates and organic matter typically used in green roof installation is not
Stormwater management is the most common reason for installing a green roof. But even green roofs installed for purely aesthetic or energy-related or other concerns require highly functioning green roof hydrology.
Retention is the process of absorbing water and only releasing that water through evaporative processes. The term evapotranspiration refers to the combined the processes of evaporation (from soil or other absorptive materials) and transpiration (from plants). Transpiration is essential to plant survival, thus retention is essential to plant survival. The volume of water retained is always equal to the volume of water converted to vapor via evapotranspiration.
Drainage is the process of releasing water gravitationally. Drainage is also very important to green roofs. These might seem like statements made by Captain Obvious, but we find that green roof drainage is often quite misunderstood.
Should a green roof drain very rapidly? Should it drain slowly? What is the quality of the water that drains off? Once we understand what should happen, what are
In reading the two prior paragraphs, you might have noticed that vegetation, soil, drainage, and retention are all related to each other. This is why all four aspects (two major components, and two major processes) must be balanced to create a resilient green roof.
What do we mean by resilience?
The other organizing diagram we use is a spiral. The spiral is all about you, and your interests, your perspectives. Green roofs matter in different ways to different people and different disciplines. Are you interested in ecology? Or energy? Or scientific research? From the blog home page, click those circles to filter posts by those topics.
At the very center of the spiral are research and policy, which generally come very early in the life cycle of a green roof, long before a specific project. Research may be centered around hydrologic or biologic processes and assemblies or practices that address those phenomena. Hopefully,
Moving outward in the spiral are six topics generally intended to encompass the design phase of a specific project. Hydrology is listed first, recognizing that civil engineers often deal with stormwater design as part of the site plan approval process, before any other disciplines do much work.
Just outward from the design disciplines are permitting and construction. These collectively represent the end of the design process, via approvals and implementation. Permitting obviously begins very early in the design process, and is closely related to policy. We chose to list policy early and permitting separately and later to emphasize that policy and actual permit approvals are not always the same (a topic for discussion) and that policy and permitting basically bookend the design process.
We distinguish these two as
Construction is the physical implementation of the design, and construction of a green roof can have a significant impact on whether the design goals are actually met. Not only do construction practices, such as material handling, matter, but construction presents the potential for design changes, such as material substitutions.
At the far outer edge of the spiral are stewardship and observation. Stewardship is just a term for maintenance, but with a twist. Maintenance is "maintaining" something as it originally was. Stewardship recognizes that the object of that action is a living, evolving resource, that should be cared for, but which will change. Observation encompasses long-term monitoring of installed green roofs, including all manner of performance criteria, such as vegetative health, stormwater performance, energy performance, etc. These two collectively represent the full lifespan of a green roof after it is installed.
But notice in this diagram that research is highlighted as well. We chose the spiral, in part, because it represents an iterative process. The body of green roof research should incorporate observations of completed installations so that each generation of green roofs is better than the last, and so that each generation of green roof professionals is more informed than the last.
One final item worth mentioning is that the spiral represents the interdisciplinary nature of the green roof life cycle. Though not directly adjacent, any one of these topics may relate directly or indirectly to another. For example, ecology and hydrology may both be of interest to you, particularly how one relates to another. If you click on these two topics on the blog home page, you will see all blog posts that address both topics, with those that address all topics selected ranked highest.
Hopefully, the categories and perspectives above are a good framework for conceptually dissecting green roofs, understanding why they matter, and to whom, and putting them back together in a more resilient manner.
We are interested in collaborating with others. Purple-Roof is a non-proprietary performance specification for the optimal green roof, not a specific product, and the Purple-Roof blog focuses on concepts and topics, not specific products. If you would like to co-author an article, click "Contact Us" in the footer. Just as green roofs are stronger and more viable when all important criteria are balanced, our industry is stronger when we work together.