A Climate-Adapted Green Roof is Resilient!

by Brad Garner on Thursday, December 6, 2018

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A climate-adapted green roof is a green roof that performs resiliently in its specific location. This means that vegetation, soil, retention, and drainage all perform optimally. No one green roof profile can be expected to perform optimally in every climate and building microclimate. When choosing or designing a green roof profile, but sure the green roof is climate-adapted.

Dry climates, wet climates, cold climates, sweat climates!

Green roofs are often sold like manufactured products: One-size-fits-all. But unlike most manufactured products, green roofs interact intimately with natural forces: absorbing water, receiving sunlight, evaporating into the atmosphere, etc. Since each climate zone may have a unique combination of rainfall, sunlight, wind, temperature, and humidity, the green roof should be designed accordingly.


Annual Rainfall 2-Year, 24-Hour Storm Size Temperature Profile Warm Season Nightly Temperature Profile Primary Green Roof Challenges
Baltimore, MD 114cm (45 inches) ~9cm (3.5 inches) Temperate, four seasons, moderate winters Moderate summer nights Ability to handle repeat storms; ability to span droughts in summer
Cleveland, OH 99cm (39 inches) ~6cm (2.5 inches) Temperate, four seasons, cold winters Moderate summer nights Sustaining moisture levels
Denver, CO 30cm (12 inches) ~4cm (1.5 inches) Hot summers, long winters Cool summer nights Sustaining moisture levels; extreme drought tolerance
Atlanta, GA 234cm (92 inches) ~10cm (4 inches) Hot summers, short wet winters Hot summer nights Ability to handle repeat storms; ability to span droughts in summer; support non-Sedum plants
Singapore 234cm (92 inches) Tropical Moderate summer nights Ability to handle repeat storm events; high demand on proper drainage

In the examples above, there are two extremes: Denver and Singapore, one with hardly any rain, and one with some of the highest rainfall volume on the planet. These two locations obviously have different criteria.

But look again at Baltimore versus Cleveland. Most of us humans might consider Baltimore and Cleveland to have a similar climate, except Cleveland, usually gets more snow. But plants are more sensitive to some environmental differences than we humans are. Cleveland receives about 15cm (6 inches) less rain annually than Baltimore; further, the 2-year/24-hour design storm for Baltimore is about an inch higher than it is in Cleveland. This effectively means that in Baltimore, there is a higher need to manage larger storms and back-to-back storms, and in Cleveland, there is a greater need to sustain the green roof with lower available water.

Now let’s compare Baltimore and Atlanta. Atlanta certainly has hotter summers and milder winters; those facts influence our plant selection a little bit. Both locations have similar annual rainfall volumes and 2-year 24-hour design storms, so there are not many differences needed in profile selection due to that. But look at the differences in summer nighttime temperatures. Baltimore usually has fairly mild nighttime summer temperatures (say around 21°C [70F] on a 32-38°C [90-100F] day) but Atlanta often has hot nighttime summer temperatures (say over 27°C [80F] on a 32-38°C [90-100F] day). This turns out to be the most demanding climate difference.

Why do nighttime summer temperatures matter? In short, Sedum plants require relatively cool nighttime temperatures to respirate. Read more about it here. So in Atlanta, we will need to find plants other than Sedum, which probably means that the thickness and weight of the green roof profile is not determined as much by stormwater concerns as it is by plant survivability concerns, or that we need a profile that will manage water and surface temperatures in a way that provides nighttime cooling.

We hope this makes it clear why any single profile cannot be expected to perform optimally in all locations.

That’s so much to consider! How do I choose?

Choosing the right green roof for your climate can be challenging, especially if you are an architect or engineer who works in multiple climate zones. There are just so many variables.

One option is to learn which green roof profile works well in your specific area and just use that. But that presents another problem. What is meant by "work well"? Is that a stormwater criterion? Plant health criteria? Something else? What if your project requires different criteria from what usually "works well"?

Another option is to learn which layers provide which functions the best and learn to calculate different combinations of different layers for each climate zone. That can be done! But it's a lot of work and requires a lot of data collection. We use some big databases and quite a bit of code to do this.

Another option is to examine green roofs zone that are performing similar functions in a similar climate. This is one of the best ways! Except… there might not be many green roofs near you or many green roofs that have been installed for at least 10 years. Or, as was the case in Washington, DC around 2014, there might be green roofs all around you, but none of them meet the current stormwater criteria, so they aren’t good comparisons. Though evaluating historical green roofs is always a great idea, it is time-consuming, and not always practical.

The need to manage this complexity to design smarter is one of the main reasons Purple-Roof is offering a Green Roof Modeler tool. We didn’t call it a “hydrologic” modeler because it models more than hydrology, including climate-adaptedness.

The current beta version of the Modeler only includes forty (40) common profiles, and it estimates plant stress only using available water. We will add high-nighttime-temperature-stress soon. As we continue developing the Modeler, we want to make it increasingly helpful for you to select the most appropriate green roof for your climate by considering vegetation and soil, as well as hydrology.

Let's design, install, and grow climate-adapted green roofs together, for more resilient and greener cities.

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