What’s to a Green Roof?

In response to growing concerns about climate change, green roofs – the placement of growing media and vegetation on top of buildings – are becoming an increasingly popular (and necessary) practice. In fact, the green roof industry has been growing at a rate of 5 to 15% since 2013 (Green Roofs for Healthy Cities, 2019).

The primary function of green roofs is to slow the generation of stormwater runoff by capturing falling rainwater during storm events and releasing it over an extended period of time. This process reduces the amount of water entering a community’s stormwater system, thus preventing urban flooding and combined sewer overflows. Green roofs also offer a host of co-benefits beyond stormwater management, including – but not limited to – cooling urban temperatures, insulating buildings, improving air quality, and providing wildlife habitat. While the climate action mitigation and adaptation benefits of green roofs are attractive enough, green roofs have also gained popularity due to their natural appeal and ability to provide public green space in dense urban environments.

However, despite their largely natural materials and aesthetics, green roof design can be surprisingly complex and require significant attention to detail during design processes. Questions one might find themselves asking while designing a green roof include:

• Which type of soil, plants, and structural materials will meet building codes?

• Will the green roof be able to hold enough water without exceeding the building’s support capacity?

• Which type of growing media will be able to support plant life while still effectively retaining water?

• Which types of plants will be able to survive the environmental extremes of green roof conditions?

• Will the selected variety of plants require any additional care, such as shading or fertilization?

More often than not, the answers to these questions change depending on the location and primary purpose of the green roof. Is the green roof intended to manage stormwater runoff, reduce a building’s operational costs, combat the urban heat island effect, some other reason, or a combination of reasons? Then, one can begin to explore answers to the questions like those listed above. Fortunately, there have been an abundance of research studies revolving around green roof design in an effort to help property owners, architects, and engineers quantify the benefits of green roofs and optimize their performance.

For example, a study in Shropshire, England explored which types of growing media make a green roof the most successful at managing stormwater runoff. As it turns out, both intra- and inter-particle pores (i.e. the spaces within a media type and between pieces of media) impact the water holding capacity and water retention of green roofs (Graceson et al., 2013). Therefore, green roof designers should not only pay close attention to the type of growing media they are using (e.g. perlite, pumice, crushed brick, etc.), but also the particle size of such material.

But, as mentioned before, stormwater management is not the only purpose of green roofs, and some building owners may be more interested in using a green roof to insulate their building and reduce heating and cooling costs. A study completed by the Environmental Protection Agency in Corvallis, OR (which I was lucky enough to be a part of) explored the impact of shading on green roof performance. The experimental green roof plots affixed with shading apparatuses experienced lower daytime temperatures and higher nighttime temperatures than those without shading (Bollman et al., 2021). Translated to green roof design, this finding suggests that green roofs intended to insulate buildings may perform better when covered by a shading apparatus.

Presumably the most noticeable feature of green roofs is the plant life. Because it can withstand both dry and wet conditions, Sedum is a plant commonly selected for green roofs. But, even differentiation between specific species of Sedum can impact the performance of a green roof. In a study in East Lansing, MI, researchers found that S. kamtschticum sequestered the most carbon overall, followed by S. spurium, S. album, and S. acre (Getter et al., 2009). Therefore, green roofs designed to combat climate change may benefit from close inspection and selection of specific plant species, like S. kamtschticum. To take this one step further, green roof design should reflect the conditions through which selected plant species will thrive. The same researchers from Shropshire also found that growing media with higher water holding capacities tended to better support plant growth and survival on green roofs (Graceson et al., 2014). In other words, the different components of green roofs impact one another and, therefore, must be selected and designed as one interconnected system.

These studies are just a few of the many, many projects dedicated to understanding the functionality and impact of green roofs, which designers can reference to ensure that their green roofs will perform at their peak potential. So, what’s to a green roof? Well, perhaps more than meets the eye. And, of course, as green roofs become even more commonplace, the wealth of knowledge from which we can draw on to customize and perfect green roof design will only continue to enrich and expand.

References

Bollman, M. A., DeSantis, G. E., Waschmann, R. S., & Mayer, P. M. (2021). Effects of shading and composition on green roof media temperature and moisture. Journal of Environmental Management, 281, Article 111882. 

Getter, K. L., Rowe, D. B., Robertson, G. P., Cregg, B. M., & Andresen, J. A. (2009). Carbon sequestration potential of extensive green roofs. Environmental Science and Technology, 43 (19), 7564-7570.

Graceson, A., Hare, M., Monaghan, J., & Hall, N. (2013). The water retention capabilities of growing media for green roofs. Ecological Engineering, 61, 328-334.

Graceson, A., Monaghan, J., Hall, N., & Hare, M. (2014). Plant growth responses to different growing media for green roofs. Ecological Engineering, 69, 196-200.

Green Roofs for Healthy Cities. (2019). 2019 Annual Green Roof Industry Survey.

Additional Resources

Green Roofs for Healthy Cities. (2019). Green roof and wall policy in North America: Regulations, incentives, and best practices (2019).

Greenroofs.com Projects Database. (2021).

U. S. Environmental Protection Agency. (2008). Reducing urban heat islands: Compendium of strategies. Draft.