What is a Sustainable Green Roof?

by Anna Zakrisson on Thursday, December 6, 2018

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Is your vegetated sustainable? A fair question to ask

Green roofs offer a dizzying array of positive environmental benefits, such as stormwater management, urban heat island mitigation, urban habitat creation, energy reduction, and building envelope protection. Green roofs also contribute to the urban biota by introducing vegetation to the urban environment. The greening of our cities improves not only physical but also mental health.

Those are all wonderful benefits! Those benefits can be measured and quantified.

The benefits just listed get a lot of attention. However, notice how all those benefits either improve the immediate environment of that property or improve the environment on a local or regional scale. As great as those benefits are, they only consider a very small portion of the life cycle assessment, specifically operations/use.

The US Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) rating system encourages the use of life-cycle assessments to assess a project’s offsite impacts. These impacts include raw material extraction, manufacturing materials, and products, construction, operations, as well as use and demolition. Energy inputs, transportation inputs, and waste materials are also considered.

Why does green roof sustainability matter

Life cycle assessments are just a way to quantify on-site and off-site environmental impacts.

We think it is important not to trade one evil for another. We believe it is important not to create adverse offsite impacts merely to improve on-site conditions. We must think globally.

Reconsidering all those great benefits in the opening paragraph, we see they generally reside only in the far-right bubble on the life cycle diagram below.


A Path To Regenerative Buildings

The diagram above illustrates several different conceptual life-cycle scenarios, ranging from fully regenerative to maximally wasteful. We believe green roofs should strive to become fully regenerative, though we do not know of any examples yet. Meanwhile, we strive for incremental steps of lowering energy and chemical inputs, improving runoff water quality, and increasing recyclability.

For example, soil biology is a central part of the Purple-Roof approach. We think green roof plants should leave the nursery in a healthy condition, that green roof should support microorganisms, and that the best way to accomplish this is to encourage organic processes and minimize chemical use. This should improve the on-site environment, but this also should improve runoff water quality, and improve the offsite environment.

Most green roof media make extensive use of heat-expanded aggregates, the production of which is very energy-intensive. Purple-Roof uses less heat-expanded aggregate but does incorporate similarly energy-intensive mineral wool. These comparisons can become quite complex, as one material is heavy and requires much more transportation energy, and the other product is very light. We will not attempt to compare the carbon-impact of different solutions here; in fact, we think this is such a vital topic that we will follow up with details in at least one other blog post. This post is just to get the conversation started about life cycle assessments for green roofs. Any detailed comparisons should consider the full life cycle, which requires a lot of calculations.

Very generally, here are some aspects of green roof impacts to consider:

  • How well does the product perform its task? Sustainability is not about lowering performance to improve offsite benefits, but bringing these into balance, and improving both scenarios.
  • Transportation distances of heavier materials, such as green roof soil, aggregates, and accessories such as rooftop pavers.
  • Incorporation of high-embodied-energy materials such as heat-expanded media (expanded slate, expanded shale), and mineral wool.
  • Incorporation of recycled materials and demolition material. Generally, this can be good, but we have seen many waste products used in green media over the past few years, and often these products do not support healthy plants.
  • Dependency on irrigation. Permanent irrigation lowers the green roof’s ability to retain stormwater and may waste a natural resource. Alternatively, limited irrigation during occasional droughts may be a good use of resources.
  • Dependency on fertilizer. This can be very difficult to assess during the design phase, but a green roof plant expert should be able to assess the combination of climate, green roof soil selection, and plant selection to determine its general need for ongoing fertilizer application.
  • Use of toxic chemicals such as Phenol Formaldehyde. For example, the Purple-Roof specification calls for needled mineral wool, which requires no Formaldehyde binder. We think these toxic chemicals have no place in a green industry.
  • Excessive use of plastics. Or excessive use of any material, though plastics are the most likely culprit. For example, built-in-place green roofs usually incorporate only about 15-20% as much plastic (by weight) as a modular green roof.
  • Practical applications for re-use or recycling of materials. A material may be theoretically recyclable, but is it likely that the material would be recycled or disposed of in the future?
  • Excessively organic green roof soil. Though organic materials such as bark, peat, and compost sound very “green” and sustainable, excessive use of these materials can lead to green roof soils that deflate or have a very high nutrient runoff.

We think green roof life cycle assessments are very interesting, and we hope some of you do too. We are going to try to get our hands on some specific assessments so that we can compare them and share with you. Drop us a line if you’d like to collaborate on this.

There’s a lot more to consider! How do green roofs measure up? What can we do better?

We are currently teaming up with some of the brightest minds in life cycle assessment research and will shortly publish our article here on the site. Stay put!