A Closer Look at Green Roofs

Excerpt from interview with Andy Creath in the March 2013 issue of Energy Design Update:

To launch our series of articles exploring green roof research and data, Energy Design Update spoke with Andy Creath, LEED AP, of Green Roofs of Colorado, to gain a basic foundation on both the technology and industry.

Creath’s first piece of advice?

“The first thing to know about a green roof is that it is not always the best or least expensive single source solution for getting to benefits such as energy efficiency. However, the overall benefits of a green roof can offer multiple advantages. A green roof can yield benefits throughout the roof’s life.”

How about a brief history of green roofs?

Really, when you walk into a situation with green roofs, the builder is thinking why would I want to grow plants on a roof membrane? Why put my building in jeopardy? Why should I deal with additional weight, maintenance, and cost? But then you start thinking about what’s been in literature for a long time. Why do cultures dating as far back as the Hanging Gardens of Babylon create green roofs in hot, arid climates? Why did Scandinavian cultures embrace sod roofs?

In the early 1970s, the Germans started to ask these questions and pick up on green roofs; a lot of research went on. The modern green roof can trace its heritage to them. Green roofs have been done in Europe for 30 to 50 years now. The technology has been around for a while, and we use most of the same overall technology today. Materials have changed, the weight has lightened, but the overall design of a green roof has stayed the same.

What are some typical elements of green roof design?

A typical green roof begins with a membrane base layer, a root barrier, the drainage layer, the soil (referred to as media), and plants. A green roof needs to be designed to hold enough water for the plants and to direct excess water off the roof as fast as possible. We want the media to store water and lose any excess via the drainage layer and roofs drainage system. Media can be as thin as 2–3” in depth in an extensive roof plan, and up to 2’ to 3’ deep intensive plans featuring trees. A whole scope of designs come under the green roof label.

There are multiple different membranes you can use as a base: EPDM, PVC, TPO, rubberized asphalt, which is hot applied as a singular membrane, or polyurea foam that forms an impenetrable layer, and can be especially good for a remodel. The root barrier is simply a sheet of material that stops root growth from penetrating the membrane below. In design phase, you also need to pick particular plant systems that do not have roots like aspen trees or bamboo that are prone to penetrate materials. After the root barrier we have a drainage layer. This can be as simple as a clean aggregate, such as ¾ ” rock or pea gravel, that allows water to flow in or out. You may also incorporate a cover of filter fabric that stops particulates; this filter goes on top of the aggregate or other drainage material. On top of that, depending on the situation, you might have a water retention mat or cups to hold water and address stormwater issues by slowing down the flow of water off of the roof. Finally, within your green roof media, most designs incorporate either a drip irrigation or spray, which is especially vital in the first two years for establishing plants and times of drought.

Other design elements should include a vegetation free zone, so you can maintain access to wall areas, such as the transition between horizontal and vertical planes. These can be guarded off with aluminum edging, and covered with rock, and can even be used to frame the interior.

For the process of choosing plants, always take advantage of local knowledge and local plants (see Figure 14).

For the more arid west, the Denver Botanic Gardens is a great resource; they offer monitored growth areas featuring native plants in a xeriscape setting. Just do your research. In a broad sense, Sedums tend to do well across the country, until certain species are faced with very high temperatures. There is a group of plants already vetted for green roofs. Good strategies are finding plants that adjust, and tweaking technology to fit your climate. Maybe adding more moisture retention, or tweaking the soil composition to hold more water; that’s been the transition to bring green roofs to the dry west. Ask your client if they would allow several test plants to be planted on their roof.

When looking at residential applications, we are almost always looking at extensive or semi-intensive design, with media at 3” to 12” deep. That will encompass 98% of residential roofs. Several overarching themes have allowed the recent rise of green roofs. First, modern architecture has fallen back in love with flat roofs, so green roofs are more easily allowed within that design. Flat roofs also mean finding ways for aesthetic appeal, and green roofs can help. Surprisingly, a lot of motivation for green roofs is from the aesthetic. Unknowingly, the homeowner is getting multiple other benefits as well.

Any other design considerations for builders?

The biggest thing is how much weight do you have available for a green roof? In new construction, this question is not that big of a deal, especially in design phase with structural engineers – you can plan for it without major financial implications. With an extensive system, typically 30 to 40 pounds a square foot, saturated, could be your maximum weight. Where you see this question become crucial is within a renovation or remodeling situation. In cold climates, you already have a snow load you have to cover, and a green roof load is then calculated on top of the snow load. ASTM testing and saturated media weight testing is vital. This is what the engineers need. As with many energy efficiency elements, the first and foremost question is what to do in pre-planning. Structurally, a green roof always comes back to weight. You can design to fit parameters.

Pre-planning questions need to address what you want to use the space for. How will you make it accessible? Does there need to be irrigation, especially for the establishment period? What type of irrigation? Maintenance of a green roof is just like garden maintenance, it can be as simple as trimming the surface back twice yearly, or as complex as weekly upkeep. We always caution that no living thing is “no maintenance.” Especially if the homeowner values aesthetics, what elements would they like to see?

Some excellent resources for green roofs include the FLL, German Research Society for Landscape Development and Landscape Design (http://www.fll.de/ and http://www.epa.gov/region8/greenroof/pdf/IntroductiontotheGermanFLL2.pdf), ASTM Standards E2396-E2399 (http://www.astm.org/), and Green Roofs for Healthy Cities (http://www.greenroofs.org).

The decision process really looks like asking the following questions: why put a green roof on my house? Then, you have the policy side, the potential benefits to the entire city, and the possibility to address multiple issues.

What can we expect, generally, as benefits from a green roof?

From the residential side, roof life is one of the biggest factors; anytime you have a membrane down on a rooftop, UV radiation, and a high disparity between the high and low temperatures of the day means that membrane is constantly stretching, contracting, and shifting. That movement breaks down the membrane and dramatically impacts its lifespan. A rooftop in the summertime is getting up to 150°, and down to close to freezing at night. It could be a 100-degree swing in a 24-hour period. With that type of temperature gradient, we could have major issues. A conventional roof life could be 15 to 20 years. However, a green roof could be 35 to 40 years, with some climates seeing 90-year life spans from a green roof. How is this possible? The green roof provides protection, offering a break to the roof membrane in terms of temperature and impact. As Thomas Slabe pointed out in his observations and research at the EPA green roof, this is an important observation because the extent of variation in temperature seen is the likely reason for differences in the serviceable lifespan of a roof, which may be 2 ½ times longer for a green roof than that of a conventional roof. Slabe’s results indicated that the temperature variation at the control roof membrane they studied was 205% greater than that at the green roof membrane. A great amount of material expansion and contraction, along with material fatigue and failure, accompany temperature variations.

Not only does this mean a longer lifespan and monetary benefit to the homeowner, green roof applications also mean less waste. Using figures from a US EPA OSWER report (EPA. OSWER, 2003) on solid construction wastes for the year 2003, it is likely that 70,000 tons of solid waste can be avoided annually for every 1% of existing conventional roof surface area in the U.S. that is converted to a green roof.

In a single residential scale, you get small, individual benefits. Yet, in several areas, a green roof becomes an important efficiency driver in the overall city development scale.

Green roofs can also promote biodiversity by attracting birds, bees, and other fauna that may be displaced by development. A series of green roofs grants the ability to have greenways all the way through urban corridors. That also leads into increased area for food production, especially in cities.

There are several energy efficiency benefits that green roofs can yield to their structure. While actual efficiency savings are hard to calculate, given the fluctuating R-value of the roof, most literature estimates 5% to 20% savings. In the summer, with water evaporation, a green roof can act similarly to an evaporative cooler, as water comes out of the green roof, ambient temperatures go down on the rooftop, and heats up the building less (Figure 15). This allows air conditioning to work significantly less for the space just below. On a larger scale, this combats the urban heat island effect, which works on bringing down the overall temperatures across the urban zone, leading to a wider local benefit.

Green roofs allow you to work in tangent on multiple, overall goals, which is why we’re seeing promotion of green roofs in wider design, in policy, and support for their deployment in mass scale. EPA research in Denver also recorded potential winter benefits as well, as the roof remained at 32° longer than a conventional roof, which could mean the building stays warmer and allows heating systems to work less. There’s an overall case that needs to be further researched: what can I downsize to take advantage of the benefits of the green roof? Heating, ventilation, and air conditioning systems (HVAC)? Maybe.

Done right, they offer a multiplication of benefits. Green roofs offer a cooler, more long-lived roof, cooler buildings, and a viable stormwater plan. Buildings pay for stormwater permits, based on how much water comes off the site. The question becomes how do you keep water on your site by slowing it down, so contaminated water doesn’t flow in rivers, or create overflows at a wastewater facility. This has been one of the biggest benefits of green roofs: they absorb the first 1” of moisture from a storm, allowing everything to slow down. Slowing down the water exiting from your roof and running it through plants, soil, and the drainage layer also means any water that does leave is much cleaner than runoff from a conventional roof. For a good beginning guide to the principles of green roofs and stormwater, see Urban Drainage and Flood Control District’s Best Management Practices for green roofs. LEED recognizes this effect, and offers stormwater quality and quantity points to green roofs (refer to LEED Points for Green Roofs below). This goes back to the large-scale process – green roofs constructed over a large area are very efficient at controlling and conditioning water. The cities of Chicago and Toronto actually feel it’s cheaper to give out money to promote green roofs, rather than update and expand their treatment plants. In research, the Denver Botanic Garden installed a 14,000-square-foot green roof on their new parking structure, and will be taking stormwater numbers there. The EPA has also been taking stormwater and temperature gradient data on their green roof for last the five years, versus monitored data from a control roof.

What are your thoughts on combining a green roof with solar systems?

Despite the purported controversy, that’s another reason to look at a green roof, as we’re seeing a lot of growth in solar. These technologies can combine; solar panels work better at lower temperatures, so you get higher performance on a green roof. Some plants can grow better underneath the panels, as the panels offer shade during parts of the day.

LEED Points for Green Roofs

• Site Development, Protect or Restore Open Space and Maximize Open Space: SS 5.1, SS 5.2
• Stormwater Design-Quantity and Quality Control: SS 6.1, SS 6.2
• Urban Heat Island Effect, Roof : SS 7.2
• Water Efficient Landscaping: WE 1.1
• Innovative Wastewater Technologies: WE 2
• Optimize Energy Efficiency: EA 1
• Regional Materials: MR 5.1
• Recycled Materials: MR 4.1
• Innovation in Design