Classifying Vegetated Roof Criteria

by Brad Garner on Thursday, December 6, 2018

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“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

Purple-Roof is a non-proprietary specification for the optimal green roof (vegetated roof).

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:

  1. What is an optimal green roof?
  2. To whom does it matter?

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.

Are Those Crop Circles?

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.

What is an optimal green roof?

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.

Biology: vegetation and soil

Vegetation is essential to any green roof (also known as living roof, ecoroof, vegetated roof). And soil is essential to supporting vegetation and allowing vegetation to sustain droughts, manage nutrient uptake, and form symbiotic relationships with microorganisms in the soil. Vegetation and soil are not only physical components; they are biological components.

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 a soil, per se, but is an engineered, man-made component. We want to re-introduce the term “soil,” emphasizing the importance of microbial life within the material. Plants and soil form symbiotic relationships via beneficial microbes. These symbiotic relationships are critical to the biological health and resilience of the green roof.

Read more about vegetation here, or green roof soil here. This is a blog post discussing vegetative and soil symbiosis.

Hydrology: retention and drainage

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. Clearly hydrology is always important, even if stormwater is not a concern, as hydrology and biology are interrelated.

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 best practices to make it happen… on actual projects? These are a sampling of topics relating to drainage that we will discuss in our blog.

Read more about retention here, or green roof drainage here. Or check out this post about how water drains through soil.

Putting it together: Resilience

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?

  • Ability to be sustained with minimal nutrient input
  • Ability for vegetation to thrive with minimal nutrient runoff
  • Ability to withstand drought
  • Ability to provide runoff characteristics similar to predevelopment conditions
  • Longevity
  • Healthy vegetation
  • Protection for underlying waterproofing layers

To whom does it matter?

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, policy is informed by research..., or so we hope. Policy determines what criteria some future project must comply with, or which incentives will be offered to accomplish some larger goal.

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. Design is the most general and encompasses aesthetic design, incorporation with other building systems. Obviously green roofs need to be coordinated with waterproofing and other building envelope systems. Energy consumption or energy savings in the form of heating and cooling is an increasing concern of design teams and developers, and energy production (such as via solar panels) is also gaining much attention. Green roof ecology - including biodiversity, urban habitat, etc. - is also a growing concern and an area of much research focus. Finally, the social aspect of green roof design is a public and private concern, encompassing biophilia (such as studies of how green roofs on hospitals may improve recovery rates) and other types of interaction (such as schools incorporating rooftop vegetable gardening in curricula).

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 policy being an intent to do something, which can include the recommendations of non-government organizations, non-profits, or planning departments, and which are generally recognized by government agencies as guiding principles and permitting as the nuts-and-bolts requirements that are enacted and enforced.

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.

Let’s optimize green roofs, together!

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.