• Jennifer Bousselet

Green Roofs Ramp Up with Passage of Denver Initiative

With the surprising victory on Election Day of Ordinance 300, a.k.a. the Denver Green Roof Initiative, green roofs have been thrust to the forefront of many Coloradoans’ minds. After spending nearly a decade at Colorado State University doing research and teaching about green roofs, I am amazed to see them become a political ‘hot topic’ in our state. However, there is good reason as they can be used to help lessen many societal level issues such as mitigating our urban heat islands and providing air filtration, while providing havens of biodiversity in the middle of cities. In fact, the restorative benefits that green roofs offer are what drew me to them 15 years ago.

Perhaps the most ambitious green roof in Colorado is the Mordecai Children's Garden above the parking garage at Denver Botanic Gardens. Photo: Michael Guidi
Perhaps the most ambitious green roof in Colorado is the Mordecai Children's Garden above the parking garage at Denver Botanic Gardens. Photo: Michael Guidi

Typically, the first question I get when the topic comes up in conversation is: “Do green roofs even work in Colorado?” Oh yes, yes, they do. More than 50 green roofs across the state attest to that fact. Rest assured that green roofs are well adapted to our climate if irrigation and maintenance are considered during the design phase. Careful plant selection, system component selection, and planning for irrigation as ‘insurance’ during dry seasons is primarily all it takes. Only a very small amount of irrigation, a fraction of what we use on our landscapes at grade, can ensure that plants on green roofs thrive. The 10-year old Denver Botanic Gardens green roof above Off Shoots Cafe is an excellent example as it receives 7 inches or less of supplemental irrigation per growing season.

(Europeans) have been installing green roofs in dense urban areas for decades to help solve the same issues we are facing now: managing air quality, urban temperatures, and slow peak stormwater flows.

Let’s clear up a few other common myths about green roofs while we are at it. First, green roofs don’t leak – at least not any more often than traditional roofs leak. In theory, every rooftop should have sound waterproofing and therefore not leak. But, life happens and sometimes waterproofing ends up being compromised. Anything that is sharp or has force can penetrate a membrane – hail, tools, and high heels to name a few. In fact, the soilless substrate and plants in green roof systems act as protectants for roof membranes. Rooftops with green roofs are protected from hail damage because the plants and substrate act as buffers. The hailstorm that shredded the Colorado Mills Mall last year hardly affected the green roofs near downtown Denver.

The sedum green roof atop the EPA building in Denver. Photo: Thomas Slabe
The sedum green roof atop the EPA building in Denver. Photo: Thomas Slabe

And if you ask any roofer what degrades roofing material over time, they will tell you that it is damage from ultraviolet light and the shrink-swell cycle of daily temperature fluctuations. Green roofs act like a ‘blanket’ on membranes to protect them from those two chronically damaging effects of our climate’s extremes.


If green roofs are well adapted to our climate, provide myriad environmental benefits, and in fact protect rooftops from our harsh environment, why aren’t they installed more? Good question. It’s the one I ask students in my online Green Roof Culture course at CSU on their final essay exam every year. After spending an entire semester looking at aspects of green roof systems and the industry worldwide, the students invariably come up with two common themes: price and education.


A small home green roof in Fort Collins built by Bousselot and her husband.
A small home green roof in Fort Collins built by Bousselot and her husband.

Gardening in general is not known as a cheap pastime, especially when you get into hardscaping and high value plants. I know, I have purchased many expensive conifers that didn’t make it through the winter season. Gardening gets even more expensive when you elevate it. Green roofs on average cost about $12-$25 per square foot for shallow systems (i.e., extensive green roofs) and $50 or more per square foot for deep systems (intensive green roofs or rooftop gardens). It is the rooftop garden sanctuaries that most people think of as green roofs – but they are expensive. And they are heavy. So, for good reason, the shallow systems are much more common.


When you stop and think about it, a lot of natural areas in Colorado mimic a shallow green roof; four to six inches of gravelly soil over bedrock is common in our foothills and even prairies. if plants can survive there, why not on top of artificial foothills – our buildings? That is the principle underlying my doctoral research at the EPA building in Denver ten years ago. Our results demonstrated that our hunch was correct. In fact, it is that vein of research that spurred my absolute fascination with our 3,000+ species of Colorado native plants and why I now work for the Colorado Native Plant Society.


Colorado native plants have evolved in our climate and shallow soils along with the nearly 1,000 species of native solitary bees. Boom – there is the formula for encouraging our declining populations of pollinators by using increasing green roof acreage as habitats. While we are at it we can even reclaim space for some of our threatened and endangered (many of them endemic too) native plants. There are hundreds of species in danger of extinction in our state alone. Rooftops are spaces where there is little chance for wildlife or hikers to destroy them!


On the educational side of the green roof question, many people assume that they are a ‘new’ technology. Au contraire, one of the ancient wonders of the world was the Hanging Gardens of Babylon – hardly new! The Vikings and early Europeans have historically used them on their dwellings. Even current day Europeans (1960s to the present) have been refining and developing the technology to fit modern era architecture. They have been installing green roofs in dense urban areas for decades to help solve the same issues we are facing now: managing air quality, urban temperatures, and slow peak stormwater flows.


If they are not ‘new’ then why are they rare? It goes back to cost. Green roofs used to be rare in Europe, as they are in the United States now, but once the industry hit a critical mass prices went down. In fact, it is nearly cheaper now to install a green roof in some parts of Europe than a traditional waterproof membrane because of the extended life of the membrane and reduced insurance claims. Leaks, hail damage, and fires are less common when you have vegetation protecting the rooftop.


If your interest is piqued in this emerging niche of Colorado horticulture, you can easily educate yourself. There are several excellent books about green roofs including one from a local expert, Karla Dakin, called The Professional Design Guide to Green Roofs. And you can read up on the newly adopted City of Denver ordinance at denvergreenroof.org. The ordinance only applies to large buildings in Denver, but we have something like 5,000 acres of these rooftops. It seems logical to me that the green industry in Colorado will have a new market this year. Drought tolerant, shallow rooted plants will be in demand.

Jennifer Bousselot is Special Asst Professor at CO State University in the Dept of Horticulture & Landscape Architecture. She has taught homeowner and college-level horticultural courses, and managed Master Gardener programs in two states over the past 15 years.