Green roofs offer energy savings, reduction of stormwater utility fees, and therapeutic benefits. But whether you can take advantage of these opportunities hinges on if your facility can even accommodate a green roof system.
Green roofs aren’t right for every building, and sowing the seeds of success involves several steps. The decision to undertake a project depends on an evaluation of your building, the incentives available in your area, and the overall goals of your system.
Follow these steps to see if your green roof dream will sprout into reality.
1) Seek Evaluations and Inspections
The first factor is how much weight your existing roof can support.
“Get a structural engineer to evaluate the system’s capacity,” recommends Peter MacDonagh, director of science and design for The Kestrel Design Group, Inc., a firm that designs green roofs. “Typical green roofs range from 15 to 35 pounds per square foot and have about 4 to 6 inches of media depth.”
These “extensive” green roofs are the most common and represent about 95% of green roofs in Europe, estimates Clayton Rugh, general manager and technical director of supplier Xero Flor America, LLC. “They are lighter weight, lower maintenance, and less expensive than intensive types,” Rugh says, adding that intensive green roofs can be many feet deep and hundreds of pounds per square foot. They are more commonly thought of as rooftop gardens, patios, or parks.
Your structural capacity dictates how much growing media you can support, and thus what plants you can grow. “A lot of times it goes the other way around – the design is done with trees and all sorts of pretty stuff, and then you learn the deck can’t hold it,” says Nathan Griswold, senior garden roof technical sales coordinator for supplier American Hydrotech, Inc.
If your existing roof can’t handle the load and you still have the desire and budget, structural upgrades or modifications can be done, Griswold says, noting that I-beams were added to a brownstone project in Brooklyn, NY, to support a green roof.
The next step is to inspect the membrane’s condition, says MacDonagh, recommending the use of a third-party roofing or membrane consultant. “Pay attention to penetrations and insulation. The entire system may need to be replaced first,” he says.
A roof that has been around for a few years has already begun deteriorating, so it’s not recommended to cover it with a green roof. Likewise, when a membrane still has usable life, taking on a green roof project isn’t the most feasible move.
“Age is important, because if you have an old roof, a green roof doesn’t make it young,” adds Rugh.PageBreak
2) Set Achievable Goals
Before starting a project, there are several questions to ask yourself.
“What are your objectives? What is your motivation? Is it aesthetics? Is it reducing energy demand or retaining stormwater on-site?” says Jordan Richie, director of education and accreditation with Green Roofs for Healthy Cities (GRHC), a nonprofit industry association. “After knowing this, manufacturers can work with you to find the system that best suits your building.”
The improvements a green roof offers will also factor into your cost-benefit analysis and help rationalize a sizable investment. Installed cost ranges from $12 per square foot to above $30, says Rugh.
At certain times of the year, green roofs cut energy use. During cooling season, they act as an insulator and decrease heat flow through the roof. “As the water in the system evaporates, it leaves the membrane cooler and your HVAC system works less,” explains Andy Creath, owner and founder of Green Roofs of Colorado, a green roof services firm.
Likewise, the same insulating effect of lessening heat loss reduces heating needs during winter. A Canadian study modeled savings from a 32,000-square-foot green roof on a one-story building in Toronto and found savings of 6% cooling and 10% heating usage, or about 21,000 kWh total, according to the EPA.
In certain areas of the country, installing a green roof can also allot you extra building or parking space because it offsets your area of impermeable pavement, says Griswold.
The main economic benefit of a green roof is increased roof life expectancy due to the protection it provides to the membrane, says Griswold. Membrane life can be extended two to three times, says MacDonagh, adding that the GRHC’s green roof calculator is an important resource to use for your particular situation.
“A green roof costs about the same as reroofing, so the ideal scenario is to couple a green roof project with reroofing,” Rugh suggests. (See case study #3 on page 34.) “Then about 15 years down the line, you basically get a free roof. It’s a two-for-one special.”PageBreak
3) Seize Incentives
More than 400 cities and utility districts nationwide utilize parcel-based stormwater billing practices based on a property’s impervious area.
“Roofs that don’t shed water instantly are a huge benefit to sewer infrastructures, so municipalities will reduce the sewage tax based on the ratio of your green roof surface,” says Rugh.
For example, the World Wide Fund for Nature (WWF) headquarters in Washington, D.C., earned almost $200,000 in tax abatement. (See case study below.) “Each municipality has varying levels of incentives. My suggestion is to research what is available locally. Manufacturers will know of some and can also help with your search,” Griswold says.
Certain sustainability initiatives even offer grants for qualifying projects. “Fee offsets are great, but on the other end, you can get cash to reimburse some of the upfront installation costs,” says Rugh.
For example, Schwab Rehabilitation Hospital in Chicago received $400,000 from the city’s Department of the Environment, which issued grants under its Urban Heat Island Reduction Initiative. (See case study on page 31.)
Another option that incentivizes the investment is financing. “Right now, borrowing is very inexpensive,” says MacDonagh. “If you go to a bank and prove that a green roof is a resilient financial setup for the building, that’s a compelling way to do the project.”
Just like trees, no two buildings are the same, so consider all of the benefits when making a decision.
“Find a system that has more than one purpose. Take a holistic approach. These are working landscapes that provide several services,” says MacDonagh. “Don’t think you need a silver bullet. You can use silver buckshot.”
Case Study #1: The Garden Walkway Benefit
Schwab Rehabilitation Hospital — Chicago
The fourth story rooftop of an existing structure
Offering horticulture therapy for patients and employees, the 6,500- square-foot green roof also features a walkway around the garden perimeter that is made of highly reflective, ENERGY STAR rated concrete pavers that reflect solar heat.
Because the hospital was also in the final stage of an adjacent $23 million multi-phased building addition and renovation during construction (2003), few funds were available. Landscape architects Douglas Hill Associates, Inc. discovered incentives from Chicago’s Department of the Environment, which issued a $400,000 grant under its Urban Heat Island Reduction Initiative.
Source: American Hydrotech, Inc.
The World Wide Fund for Nature (WWF) Headquarters – Washington, D.C.
Renovation of a large office
offering more than 250,000 square feet of workspace
The major overall goal was to reduce the first flush and/or peak flow impact of water during major storm activity, although the project also coincided with an aggressive renovation to earn LEED Platinum.
The 28,000-square-foot green roof treats and retains about 416,250 gallons of stormwater annually, meeting both municipal and LEED guidelines. As a result of this reduced stress on the city sewer infrastructure, the project earned tax abatement of almost $200,000.
Source: American Hydrotech, Inc.
Jacob K. Javits Convention Center – New York
An extensive reroofing and renovation project
The 292,000-square-foot green roof, which is the second largest on a single, free-standing building in the U.S., contributed to the goal of earning LEED Silver.
The Javits Center was due for reroofing, and both concrete pavers and green roofs were considered for a protective element over the new waterproof membrane. Pavers necessitated a structural upgrade, so a lifetime-extending, lightweight green roof was preferred. The roof also prevents about 6.8 million gallons of stormwater run-off annually.
Source: Xero Flor America, LLC
Silver City Townhomes – Milwaukee
Five structures housing 20 rent-to-own three- and four-bedroom townhomes
Located on land once used by vacant parking lots and old sports courts, the buildings earned a Milwaukee Award for Neighborhood Development Innovation (MANDI) for revitalizing the area.
The green roofs – totaling 11,577 square feet – were part of a citywide sustainability development. The energy-saving and environmentally friendly structures are likely to attract and retain long-term tenants and owners.
Source: Xero Flor America, LLC
Chris Curtland [email protected] is assistant editor of BUILDINGS.