The Baltimore area's largest green roof is high atop the Hilton Baltimore Convention Center Hotel, the only commercial building in downtown Baltimore that has a green roof. The 32,000-square-foot green roof is located on two buildings. Just for perspective, the roof will be slightly larger than the Baltimore Ravens' football field.
"Hensel Phelps has been working for some time to incorporate environmentally friendly and sustainable building techniques into the projects we build," explains Stan Carlat, Operations Manager of Hensel Phelps' Mid-Atlantic District Office, the contractor in charge of the Baltimore Hilton project. "The green roof on this project not only offers environmental benefits, it will also enhance the look of the hotel and the guest experience since the meadow-like environment will be visible from some areas of the hotel."
The hotel was designed by architects RTKL Associates Inc. and specifically, worked with Roofscapes Inc. of Philadelphia to design the green roof that was installed by the Furbish Company of Baltimore who specializes in sustainable building systems.
The team is rounded out by roofing companies Ralph J. Meyer Co. of Pittsburgh and local firm Rich Roofing, who installed the water-tight roofing system under the green roof, and Emory Knoll Farms of Street, Maryland, who will provide the plants that will populate the roof.
The first portion of the roof was completed in November 2007. Installation of the second part of the roof started in mid-March and is expected to be completed in early April.
Shallow roots, deep benefits
While some green roofs are modular systems which are constructed offsite and then installed, the Hilton's roof is being built in place four stories above the ground. It is an extensive living roof which means the depth of the roof and growing media is less than 6 inches. That makes it lighter in weight and lower maintenance than an intensive living roof which can be home to a wide variety of plants and even small trees. The roof will be planted with six types of Sedums, low growing, shallow-rooted succulents that are extremely drought tolerant.
"This roof will be a resilient, living machine," says Michael Furbish, President of Furbish Company. "The biggest advantage it will offer is storm water management. The roof helps control erosion and runoff because 65 percent of rainfall will remain on the roof, nourishing the plants. The rainfall that is shed from the roof comes off at a slower, deliberate pace very different from the destructive power of water falling in fast, powerful, concentrated streams that occur with a traditional roof. That means you're not stressing the ground storm water management systems and are also minimizing polluting runoff."
And this living machine does much more than manage runoff. The plants create a protective blanket over the base roof materials providing significant protection against UV damage and minimizing expansion and contraction of the roof caused by temperature variations which can cause structural damage. That in turn extends the service life of the roof to three to five times longer than the life of a roof without a green cover.
"You would usually have a 15 to 20 year warranty on the waterproof membrane on a roof," explains Furbish. "With a green roof, you have a roof that could very well last 80 years, which is an economic benefit for building owners."
Turning down the temperature on heat islands
While most people would guess that green roofs also help cut energy costs, they might be surprised to learn how they achieve those savings. It's not that the vegetation acts as an insulator, but rather it's a heat sink. According to the experts at Roofscapes Inc., the green roof slowly absorbs and holds energy from the sunlight and then releases it when the ambient air cools. It acts as a heat storage battery and reduces heating and cooling demands within the building.
The vegetative cover reduces roof surface temperatures to about two or three degrees below ambient air temperature. At the hottest part of a sunny summer day, the temperature of a dark colored roof is about 175 degrees Fahrenheit. A light colored roof heats up to 130 degrees Fahrenheit. But a living roof's temperature only rises to 95 degrees Fahrenheit.
Not only does this decreased temperature cut energy use within the building, it also mitigates what is known as the "heat island effect," a term used to describe the phenomenon in which urban air and surface temperatures are up to 10 degrees higher than temperatures in surrounding rural areas. Heat islands can increase health hazards including air pollution and deaths caused by heat waves. The plants on a green roof also help make the city a quieter place by reducing sound reflection and transmission.
Green roofs: a "growing" movement
As the drive to use more sustainable and environmentally sound building methods and materials continues to grow in the U.S., more designers and builders are including green roofs on their projects. In the near future, perhaps, green roofs will be as prevalent as they are in Germany, where the technology has been in use for more than 30 years and approximately 10 percent of all German roofs are green.
Michael Furbish foresees strong growth for living roofs and other sustainable technologies in the Baltimore/Washington area and across the country. "If we all just continue to follow approved building practices, we'd need four additional earths to supply the resources we'd use," he says. "That's not feasible. We cannot continue down our current path without jeopardizing our future, our children's futures and the futures of other cultures around the globe."