Green roof 101: Tips on installing a living roof
Instead of a plain shingled roof, many homeowners are opting for a green roof covered with living plants for the environmental and aesthetic benefits.
Green roofs are a fast-growing trend. The top three North American metro regions with the most square footage in green roofs, both residential and commercial, are Chicago, Toronto and Washington, D.C. In 2010, there were 4.3 million square feet of green roofs reported among the members of the North American industry association Green Roofs for Healthy Cities, versus 2.1 million reported in 2009.
The popularity of these roofs is based on a vast range of benefits, from their beauty, to stormwater management, reducing the urban heat island effect, improving air quality, energy efficiency, noise reduction, increasing biodiversity, and even offering an opportunity for urban agriculture for those who choose to grow food on a green roof.
Frequently discussed is the urban heat island effect, which the U.S. Environmental Protection Agency describes as urban areas that are hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be up to 5.4°F warmer than its surroundings. At night, the difference can be as high as 22°F (12°C). Heat islands can increase summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.
The daily dew and evaporation cycle created by the plants on a green roof and the replacement of black membrane roofs can cool cities during hot summer months and reduce this effect.
Steps to building a green roof
If a homeowner is interested in installing a green roof, there are several things to consider before beginning the process. One of the most important considerations is whether the roof will be added to an existing structure or new construction.
Green roofs are heavy due to the soil required and the water that the soil soaks up. So, if working with an existing building, the load-bearing capacity of a roof is the most important consideration. New buildings can easily be constructed to hold the weight of a green roof, said Monica Kuhn, a Toronto-based architect and Green Roof Professional (GRP) who has been designing green roofs for 18 years. The GRP certification is offered through Toronto-based Green Roofs for Healthy Cities.
Kuhn explained that when building a new house, "the increase in structural capacity is not a huge increase in price. It's incredibly minimal. When you're dealing with an existing house it depends on how much else you're doing in the house. If you're already gutting what's under the roof, it's pretty easy to beef up the structure because it's already exposed. And if you're not, it's pretty hard to do from the roof down unless you're building a second structure over top which will be responsible for bearing the load."
So a homeowner needs to first hire an engineer to determine the load bearing capacity of a roof, as well as the walls and any individual columns. "We've had situations where the roof itself wasn't capable of having a green roof, but there were spots on the roof where you could put a planter," Kuhn said.
Types of green roofs
There are three main types of green roofs — extensive, intensive and semi-intensive.
- Extensive — An extensive roof is considered low maintenance, and uses a modular system with trays that can be snapped together to fit onto a roof and include drainage layers, filter cloth, soil or another growing medium, and live plants prepared in moveable, interlocking grids. These roofs are typically 3-6 inches deep and are excellent for drought-tolerant plants. No irrigation is required.
- Intensive — The growing medium in an intensive roof is usually more than one foot deep and can support shrub and tree growth. No trays are used. Instead, each component of the roof, from the drainage layers, to growing medium, to plants, are installed separately. Irrigation is necessary in many cases and these roofs require maintenance, similar to a traditional ground-level garden.
- Semi-intensive — A semi-intensive roof is usually 6-12 inches deep and irrigation may or may not be necessary, depending on the region and the types of plants. Maintenance can be intensive because of the garden-like design of most semi-intensive roofs.
Pre-vegetated mats are an option for any roof system. These mats are like sod in that the plants are grown in advance and the mats are rolled out. Typically sedums are used, which are very drought hardy, said Emilio Ancaya, GRP, and owner of Living Roofs Inc. in Asheville, N.C.
"It's an instant green roof. The day you put it in, it looks like it's been there for two years," Ancaya said.
When installing a green system on any roof, whether existing or new, a waterproofing membrane must be in place to protect the structure from moisture. Most flat roofs already have a waterproofing membrane in place, while a pitched roof, such as on a home, is often covered in shingles, which are designed to shed water. These must be covered with a waterproofing membrane. If a building has an existing membrane, it will need to be tested for leaks.
"One of the benefits of green roofs is that you're protecting that existing waterproofing membrane on a flat roof so you're going to extend the life of it by two or three fold. The green roof is blocking the membrane from UV, and UV and temperature fluctuation are what breaks down a roof," Ancaya said.
A flood test, where drains are blocked and a flat roof is filled with water, is the best way to test for leaks, he said.
Next comes choosing the types of plants.
"Your plant list is pretty minimal. There are not many plants that survive on a roof. And soil is pretty coarse and it drains quickly. You are limited in your choices, although the plant palette can increase a lot with the deeper soil in intensive green roofs," Ancaya said.
Carefully choose roof placement
The placement of a green roof also matters, said Melissa Rappaport Schifman, LEED AP and Approved Contributing Expert for ProudGreenHome. When she installed a green roof at her own LEED Gold certified home, the original plans changed before the roof was put in place.
"We had planned on a green roof in one place and we added one in another place, and we wanted to do one more, but we would have had to add extra framing to do it, so that would have made it prohibitively expensive and not that good from a design perspective," she said.
Another important factor was being able to see the roof from within the house or the street. "You can see our roof from the second floor. We looked at doing it for our whole house, but if you can't see it you lose one of the benefits because it's pretty," she said.
An unexpected side benefit for Schifman was when hail damaged part of her exposed roof shingles, but not the area protected by the green roof. The roof also helps insulate the room below it and keeps the room that overlooks the green roof cooler as well. "It wasn't installed right when we moved in so we saw how it made a huge difference in our cooling costs," she said.
The demand for green roofs will continue to grow as communities realize the positive impact, said Steve Peck, founder and president of Green Roofs for Healthy Cities. "There's just this tremendous range of benefits and passion that people have for green roofs. It's not just in the U.S., it's global. There are all kinds of green roof stuff going on. We're seeing all sorts of public policy support because there are a lot of public benefits generated."
A slideshow of images of green roofs is available for viewing.
Photos courtesy of Green Roofs for Healthy Cities and Emilio Ancaya.
Companies: U.S. EPA
Teena Hammond Teena Hammond has published more than 2,000 articles in People and W magazines, Women's Wear Daily, and in dozens of newspapers and books. She also wrote a home improvement, remodeling and decor column that ran in Gannett newspapers nationwide. She's interested in all things green and would love to hear from you with your story ideas.