Tall, slender stems of grass swish in the warm summer breezes, while coreopsis and coneflowers bob their heads, as if waving. They and dozens of other plants grow in a diverse garden planted on top of a garage in the middle of St. Louis, Missouri. If a person doesn't look up when walking or driving by, they'll miss seeing it.
Green Roof Revolution
St. Louis is one of the more recent cities to jump up on the “green roof” bandwagon. Chicago has long been known as a green city, with many green roof installations, including one atop City Hall. Seattle and Toronto are also leaders in the field, with large numbers of residential green roofs. In the Central Midwest, with its hotter climate, the challenges of growing green roofs are greater. St. Louis has a strong community of landscape architects and engineers who are helping to fuel the trend in Missouri with their innovative variations in green roof design.
What's So Great About Green Roofs, Anyway?
A green roof completely covers this flat-roofed home in Thousand Islands, Ontario, Canada. (Photo Credit: Shim-Sutcliffe Architects and greenroofs.com)
City residents, along with landscape architects, are pushing the trend as they realize they can maximize their spaces. Some homeowners add decks and walkways in addition to plantings, to create a visually appealing setting to entertain or relax. Others add rooftop patio planters, where they can grow extra garden produce in pots.
But the benefits don't stop with creating a spot to kick back with a bottle of wine after a long day and some extra veggies on the table. Falling rain takes time to percolate through a green roof, resulting in city sewers that aren't overwhelmed. Rainwater may take several days to drip through, and can then be collected in barrels to water the yard, saving money in watering costs. A green roof lessens the urban heat island effect, too. Green roofs, once established, become ecological habitats for birds, butterflies and other insects. In some cities, enterprising owners have installed rooftop beehives and harvest their own green roof honey. One of the major benefits, felt in the pocketbook, is that the insulation effect of green roofs reduces a homeowner's direct costs of both heating and cooling a residence.
Daisies, Daffodils or Dianthus?
“Primarily what is different from green roofs in St. Louis compared to green roofs in other parts of the country is the plant palette,” says Hunter Beckham, a St. Louis-based landscape architect with SWT Design (swtdesign.com), which has won awards for their green roof designs. “The structure and drainage of the roofs are the same, but different plants may be planted to stand up to our our higher temperatures and less humidity.”
Beckham often chooses herbs such as thyme, lavender and yarrow, but also selects from a wide variety of prairie plants, including coneflower, black-eyed Susan, Missouri primrose, and even prickly pear. Additional plants might include drought tolerant sedums, creeping juniper, Allium, Astilbe, Crocus and other bulbs, and several types of grasses, among many choices available. “Green roofs are not a 'natural' landscape,” he explains, “so it's not necessary to limit plant selection to local natives, though many natives work well in rooftop gardens.” He adds that some plants that are usually avoided are tall trees, which might overwhelm a green roof, both in size and in weight, and plants with a long tap root.
Green Roof Components
Before a green roof can be added on a house, garage or shed, an engineer needs to look at the structure and make sure it is strong enough to support the added weight of soil and plants. Otherwise, the first step may mean additional construction to strengthen the framing. “Structure is critical,” says Beckham.
Green roofs usually consist of five components. The first is a protection membrane, which prevents water from penetrating the roof. Additional components include drainage material, filter fabric, soil and plants.
Soil may be applied from 2 inches to 8 feet deep, though most installations have between 2 to 15 inches. The depth of the soil also helps determine which plants can be installed. “This is not typical garden soil,” explains Beckham. “Green roof soils are lightweight, and are designed so that larger aggregate particles rise to the top to withstand winds and prevent the soil from blowing away. The lower inches of soil contain more nutrients for the plants.”
Installation and Maintenance
For a typical residence, it takes a month or more to install a green roof, in a typical five-layered system. Green roofs can also be installed as a modular system, with the modules pre-grown. These can be installed in a weekend, as long as the structural work has been done in advance and permits have been obtained.
Extra watering is needed while the plants become established, which is usually a year, perhaps two. By the third year, plants should be well acclimated. If installed and maintained properly, a green roof should last 40 to 60 years, far longer than the life of shingles typically found on roofs.
Maintenance consists of plucking out weed seeds that blow in, such as elm and maple seeds, and occasionally replacing a plant that has become too big.
A Versatile Spot
Once a green roof is established, owners will think up many more ways to use the space. Some may install a small terrace with outdoor seating, while others add a golf putting range, set up a sunbathing area, or install a small water garden or bubbler fountain. Large roof spaces may even include a basketball or tennis court among the plantings. Whatever the use, green roofs are a trend that is becoming more prevalent. Perhaps there's one already in a neighborhood near you.
For more information, obtain one of the books on green roofs by Ed Snodgrass, such as The Green Roof Manual: A Professional Guide to Design, Installation and Maintenance.