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.