Growing green in the sky with rooftop gardens

Growing green in the sky with rooftop gardens

26 May 2009

published by


Don and Ann Cottrell’s house is more “The Lord of the Rings” than Better Homes and Gardens. Similar to the dwellings in JRR Tolkein’s “Rings” trilogy, the Cottrell house blends in so well with its environment that it seems to disappear from certain angles – where only the chimney gives it away as a manmade structure.

“My son told his girlfriend that we lived in a hobbit house,” Ann Cottrell said.

The Cottrells built the house near San Diego State University in the late 1970s. The two retired SDSU professors’ house is dug into the hillside surrounding it. The top of the hill comes over the top of the house and forms the roof.

Ann Cottrell said because of how the entrance to the house is designed, many visitors don’t notice the plants on the roof.

“We have people who have been to our house many times, and then came from the other side and went ‘Oh my God, you’ve got dirt on your roof,’” she said.

Green roofs are taking root as a legitimate tool to repair damage to the environment. Several San Diego businesses are hoping to help the green roof movement grow. In the fall of 2007, the Cottrells had their entire roof redone to repair a leak and take advantage of new technologies by one of these companies, Building Green Futures. The roof was planted with drought-tolerant perennials, grasses and succulents.

A green roof typically involves several layers of waterproofing covered with materials for water retention and a root barrier. Usually about 4 inches of soil is put on top of this and planted – often with native or drought tolerant plants.

While most eco-roofs aren’t as extensive as the Cottrell’s, they have many of the same benefits.

Green fundAccording to the U.S. Environmental Protection Agency’s Web site, “On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50-90°F (27-50°C) hotter than the air, while shaded or moist surfaces – often in more rural surroundings-remain close to air temperatures.”

These superheated, dry surfaces that are so common in cities are called heat islands. Green roofs help reduce energy usage by breaking these up, not only through the insulation that comes from the growing plants and the material they grow in – but from the plants themselves. The plants cool themselves with transpiration – a process similar to sweating.

Hard city surfaces also create another problem: storm water runoff.

Green roofs hold 60 to 80 percent of this rain water, and can be fitted with cisterns to collect even more. The green roofs also act as a natural filter for the water and create habitats for birds, insects and even lizards.

And because the waterproofing and other roofing materials are sheltered from UV rays by plants, their lifespan is increased.

Another benefit is insulation and noise reduction.

“I just don’t understand why, in hilly, fire prone, hot San Diego, this hasn’t caught on,” Ann Cottrell said. “And I think one of the reasons is relatively few people are building individual homes. It’s mainly developers.”

A few lawmakers and city officials are working to encourage the growth of green roofs. For instance, Sen. Maria Cantwell, a Democrat from Washington state, introduced the federal Clean Energy Stimulus and Investment Assurance Act of 2009 on Jan 26. The bill, currently is in the U.S. Senate’s Committee on Finance. would give a 30 percent tax credit for green roof costs.

Locally, the Centre City Development Corporation approved the Downtown Community Plan in 2006, which included a bonus program for projects in downtown San Diego. It allows these projects to be denser if they incorporated green features, such as eco-roofs.

Brad Richter, CCDC assistant vice president of current planning, wrote in an e-mail that eight projects have received entitlements under the program, but only two are under construction.

But in San Diego green roofs can be more complicated than in rain-saturated Northern cities.

For one thing, the benefits of green roofs have to be weighed against water scarcity. Residents are facing a possible mandatory water reduction this summer. That is why these roofs are often paired with devices to catch rainwater, which can be used to water the roof as well as surrounding vegetation. They often also are created using native or drought-tolerant plants.

And growing plants is challenging in 3 to 4 inches of growth media – soil can’t be used on green roofs because it creates silt which clogs drainage channels. Growth media is usually a lightweight mix of organic material with mineral additives. It can include peat, humus, wood chips, sand, lava or expanded clay.

When that is compounded by San Diego’s arid climate, many plants die. But through extensive knowledge of biology – and some simple trial and error – eco-roof installers have discovered plants that can survive these challenging conditions. Ones that are especially useful are in the sedum genus, which include about 600 species of succulent plants.

Both Rosalind Haselbeck, co-owner of Building Green Futures, and James Mumford, president of Good Earth Plant Company, design these sky-high gardens.

Mumford uses the roof of his office like a green-roof laboratory and has 40 plus plants in 1,600 square feet. He is experimenting with technology that can make pitched roofs and even walls into habitats for plants.

“I think one of the most intriguing things about a green roof: It isn’t the answer to all our environmental issues, but it certainly cuts across many disciplines,” Mumford said.

Mumford said his 31-year-old company recently began working on green roof projects. It has done nine, which he describes as small and residential.

Mumford emphasizes that an engineer and the permitting process must be used on green roof projects. This helps ensure that the building can support the weight of the growing media and plants, including the water it holds when it is fully saturated.

The expense of green roofs depends on a variety of factors, including whether one is being retrofitted or is being designed as part of a new project, irrigation, the amount of growing media and the types of plants used. Mumford says generally, though, green roofs about double the costs of each square foot of roof.

Haselbeck said her company started building green roofs in the fall of 2006. It has finished six projects and is working on two more. The projects range from 200 square feet to 1,100 square feet.

Both of them see green roofs as a way for densely populated urban areas to reintroduce green spaces, which are so often squeezed out when cities expand.

“The only way that we can deal with the climate change and storm water issues in cities is to create more vegetative cover, and most of the time there is no room anywhere but the roof,” Haselbeck said.

Print Friendly, PDF & Email
WP-Backgrounds Lite by InoPlugs Web Design and Juwelier Schönmann 1010 Wien