PRB Articles


Green-Roof Design

Green roofs are not new, but they are gaining in popularity in the United States. Over the past 30 years, there has been a steady increase in the application of this technology.

Rooftop-garden systems help mitigate problems traditionally associated with roofs, such as heat gain and loss as well as stormwater runoff.

During the summer months, for example, a rooftop absorbs and radiates heat from the sun, creating higher air-conditioning costs. In winter, heat escapes through the roof.

The plantings of a green roof act as an insulating barrier while absorbing stormwater and providing wildlife habitat.

“Green roofs were developed as a stormwater-management practice in Germany and then extended throughout much of Europe before being imported to the United States,” says Charlie Miller, owner of Roofmeadow, a design firm specializing in green-roof designs and strategies.

“Green roofs were developed as a way of forestalling improvements or enlargements to existing stormwater facilities. However, the ancillary benefit of creating a green roof is transforming the look of the environment.”

A green roof can be anything from a simple garden that produces fruits and vegetables to a full-scale operation with low-lying sedums and prairie-grass gardens.

Mastering Stormwater

In the Performing Arts District of Kansas City, Mo., one green roof project includes more than 2.5 acres of lush gardens atop a four-story parking garage. The parking areas are maintained by the city, and the parks and recreation department maintains the green roof.

During the planning phase, the decision was made to capture or reduce the amount of rainwater flowing into the stormwater infrastructure. One of the ways to accomplish this was to install two 44,000-gallon underground cisterns. The water collected is used to irrigate the rooftop plantings during the drier seasons.

“We worked with a plumbing engineer to design a green roof that captures all the rainwater that falls on the roof,” says Brian Davis, senior project landscape architect with Jeffrey L. Bruce and Company, the architectural firm that worked on the Kansas City project.

“The green roof, combined with the rainwater-collection abilities of the cisterns, makes this a huge stormwater-reduction project.”

Davis projects that the cistern system will pay for itself in about eight years due to the savings in not having to purchase domestic water for irrigation needs.

Of course, controlling water is one of the concerns with a green roof; this includes preventing leaks.

“We needed to make sure that the spaces beneath the green roof aren’t affected negatively in any way from the green roof,” Davis says. “We did this by having the best waterproofing that we could afford.”

What Makes Green Roofs Different?

One consideration of green-roof design is the creation of a landscape environment open to the broad expanse of sky. With a green roof, one can manipulate and do plantings differently than on the ground.

Since access can be allowed, denied, limited to specific areas, or limited to specific times, there is more flexibility in plant materials because the durability of the plantings is mitigated, allowing for the creation of a truly unique environment.

A green roof has specific soil depths depending on the type of plantings. Attention must be given to how the water moves through the green-roof landscape. How does the water pass through the plants, through the soils, and into the drainage pattern to eventually make its way to the drain?

Once the answers are known with a good understanding of the requirements for specific types of eco-plantings, there is then an opportunity to paint a green roof with a variety of different habitats.

“This gives you the ability to put a wetland right next to a dry meadow because you are creating the landscape from scratch,” Miller says.

“Green roofs give you the possibilities of mixing incredibly different ecological environments and blending them together to create a unique and inviting green-roof design.”

Already Built

For applications on buildings that already exist, knowing the load limitations is crucial. At the University of Virginia, a green roof has been installed as part of a historical-restoration project.

“During the design-development part of the project, we decided to go with LEED certification and use the flat rooftop area by adding a planting area for stormwater retention and filtration,” says Gerald Starsia, senior associate dean of administration.

“It was important to send a message to our students and alumni that sustainable design was important in the process of capital improvements and moving forward.”

Usage

Another consideration is whether the green roof is to be used in an active or passive manner; this will affect how the gardens are designed. For active-use areas, plan on pathways, alcoves, and seating areas to allow visitors to enjoy the view.

“Whether active or passive, there must be regular maintenance,” says Davis.

“There is a higher level of maintenance that needs to happen on a turf green roof than you would normally have to do on a ground location.”

Plantings maintained at the Kansas City site include ornamental shade trees such as red buds, honey locusts, and red maples. Grass plantings include Shenandoah switchgrass to tie in with the Performing Arts Center, as well as low-maintenance grasses such as fescue sod and Reveille bluegrass, which is also a low-water grass.

Regardless of the type selected, both require an alternative water supply to avoid spending the money for domestic water to keep the plants alive.

Green-Roof Experience

“Parks are about creating a refuge for people to be someplace else, and to be able to focus on the small details of life. To be surrounded by green and have their attention drawn to unique things,” Miller says.

“In parks, you're not creating a green roof for the stormwater benefits. You are creating it for the aesthetic and human benefits, such as the simple pleasure of being in a beautiful space.”

“Green roofs have a softening effect. We underestimate the benefit of that,” Starsia adds.

“If I had to do it over again, I would have done more of the roof in planted areas with a greater variety of eco-plantings.”

What Do You Want To See?

Saying you want a green roof is analogous to saying you want a house. It is important to know exactly what you want and for what purpose before you begin the design process.

Experts in the field recommend assembling a design team that has experience creating green roofs. The team should include an architect, structural engineer, plumber, landscape architect, and contractor.

“You want to work with a company that constructs and maintains green roofs, and can help with maintenance and troubleshooting. It is important that the company has a record of taking care of their clients over promoting a particular green-roof product,” says Miller.

“Unfortunately, there are a lot of companies that offer green-roof packages rather than starting from the point of helping their clients think, ‘I want to walk out and see...’”

Tammy York is a professional public relations consultant to outdoor recreation related businesses and parks. Her book 60 Hikes Within 60 Miles: Cincinnati is available via Amazon.com. She can be reached via email at tammy@landsharkcommunications.com.

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