Native Plants for
Vegetated Roofs and Walls
A few words of caution and encouragement
Caution: Consider the roof membrane. Not all roofing materials are capable of receiving a vegetated roof. Shingles, for instance are not. They make a perfectly good roofing system, but it is a system of shedding water. You need to consider that a vegetated roof is going to have impounded water. It needs to be a water tight system, a material certified as a waterproofing membrane for constant immersion. There are exceptions, but you should confirm with a professional or with the roof membrane manufacturer that the membrane is suitable for use in your specific situation, including review of any drawings and installation instructions to assure a valid warranty. In addition, soil type and soil depth are major considerations. You should always confirm plant selection with a professional with vegetated roofing experience.
Encouragement: No project is either too large or too small to be consider for a vegetated roof. In Columbus, Ohio, the city has installed many bike racks with vegetated roofs, some are now more than 10 years old and performing great and new ones continue to be installed. In the Netherlands, they have canal barges and city busses with vegetated roofs. In Thailand, a 22,000 square foot roof is installed at Thammasaat University near Bangkok and is the largest urban rooftop farm in Asia. There are several urban rooftop farms in Washington, DC. The US Coast Guard headquarters in Anacostia on the old St. Elizabeth's site is a huge roof and was the second largest in the country at the time of its completion in 2010/11. The National Museum of the Marines at Quantico has a vegetated roof over much of it.
German Embassy, Washington, DC
Though vegetated roofs have been around a long time, their commercial use took a major upswing in the early 2000’s. Their benefits were long common knowledge but anecdotal, and local jurisdictions have had to offer incentives to developers to incorporate them. Since 2012, research and studies began to provide quantifiable benefits. And with technological advances since 2015, incentives are becoming a thing of the past. It is not just fast return on investment, but choosing the right vegetated system can be an up front cost savings. When used as a storm water retention and detention system, the large storm water tanks that take up site space or are located in the basement of a building can be completely eliminated. And of course when the developer can attract a rooftop farm, suddenly the roof is a leasable space and the cost of maintaining the vegetation is removed.
Several cities around the country including New York and Denver have passed requirements for new construction that a roof must be either vegetated or solar, or a combination of vegetation and solar. A common misunderstanding is that solar panels should be installed on a white roof. Though the white roof assists in keeping the building cooler by reflecting the heat, it causes the panels to run at higher temperatures which reduces the efficiency of the solar panels. A vegetated roof actually cools the panels increasing their efficiency. A simple test that has been performed showing the benefit of a vegetated roof and cooling requirements for a building. One example was a sunny day with an air temperature of 72 degrees F. A black roof had a temperature of 128F, a white roof was 110F, and a gravel roof was 100F. A very basic vegetated roof had a temperature at the soil of 71F, one degree cooler than the ambient air temperature. Consider the savings in the size of the cooling system, or the cost of running it when the temperature of the surface of the roof is 50F cooler than a traditional roofing system. Now put that together with the stormwater management potential for cost savings and increasing leasable space.
With the integration of native plants, the vegetated roof can move from simply providing stormwater management, significant cost savings, and improving energy efficiency to providing habitat. And not just with the plants, but through a comprehensive habitat design. A few landscape architects in the US have begun designing custom habitats, but in Europe the plant growers already have standard habitat packages consisting of not only native plants, but also sand areas and oak logs that decompose naturally providing nesting places, and gravel for heat sources for insects. The roof areas becomes more like a natural meadow, not just a green carpet. And a much more interesting and enjoyable amenity space for the building occupants.
Myths about vegetated roofs
You can’t put them on sloped roofs.
You can’t put them on wood roof decks.
You can’t put them on houses.
None of these myths are true And you can also use them in creative design for sculptural effects. The Spring 2020 opening of the Kennedy Center’s new “The Reach” area has just that. There is a very large vegetated area that begins as a flat roof and rises up as well as twists until it is vertical. This is visible as you cross the Theodore Roosevelt Bridge entering DC. If you visit the Kennedy Center, you can access this area and see it up close.
Kennedy Center, The Reach
Vegetated walls also have a long history. However, most associate this with vines growing on walls and cable assemblies or soil filled pockets. These are still used, but they are not the leading systems. Now they can range from art panels or movable partitions in office or residential spaces, or cover the exterior face of a building. In addition, they can be either fairly simple to maintain with watering by hand for small applications, or they can be fully computerized. This later is especially true for large scale interior projects and necessary for any exterior project. Typically the walls are gown in mineral wool panelized systems with water and nutrients delivered hydroponically. Exterior installations can come with fully automated water draining and recharging based on temperature changes and text or call a cell phone if there are problems.
Westlight Condominiums, Washington, DC
Well-tested native plants for roofs
The forbs and grasses in this spreadsheet have been proven to be reliable in testing by either Hydrotech or Emory Knoll Farms or both. Use the scroll bar on the right, or click here to see the whole table, which includes trees and shrubs.
Different native plants are suitable for different soil depths. Irrigation will be required during extended periods of drought for all of them.
Extensive Roofs: Depth of soil 4 to 5 inches.
Semi-Intensive Roofs: Depth of soil 5 to 8 Inches.
Intensive Roofs: Depth of soil 8 nches or more.
The system for retaining the soil may be trays or mats. Mats are typically grown with the lowest plants providing coverages. When harvested, the mats are rolled up and delivered to the project looking much like sod rolls, though with widths and lentos of about 10 to 12 feet. After unrolling on the roof, they can then be planted with the taller specimens or other plants that cannot tolerate the rolling process.
For more help
For additional objective and non-proprietary information, the best online references are the web site:
Both are located in Culpepper, Virginia. The Purple Roof web site has freely available articles with references for all technical sources as well as videos demonstrating various features. Green Roof Diagnostics is a testing facility where different roofing configurations can be tested in their storm water testing chamber.
Volunteer and expert William Pegues is willing to give you some quick advice. He has 44 years of professional experience in architectural specifications and materials research, specializing in vegetated roof, walls and stormwater management. You can contact him through email@example.com.
US Coast Guard Headquarters
Bike rack in Columbus OH
US Coast Guard Headquarters