Sustainability remains a top priority for most institutional and commercial facilities, and green roofs continue to attract attention from facility executives as an important element in enhancing the environmental friendliness of facilities.
Green roofs can help owners address a host of environmental, facility, corporate and social challenges, but ensuring the systems actually perform as intended and designed requires that owners tackle critical decisions that can make or break system performance.
“Owners tend to think it’s a simpler process than it really is,” says Louis Juhlmann, president of RCL Engineering, a roof consulting and structural engineering firm. “There is a lot to take into account, whether it's the structural capacity or figuring out the proper drainage. If you’re going to make it usable, what do you have to do from an access standpoint, like railings and ADA requirements? If it's just so people can look at it, you've still got to think about the maintenance of it.
“Green roofs are fantastic. I am glad that it's something that our construction and architecture industry is getting into. But I don't know that building owners always understand everything that goes into it or if their roof is even a candidate for a green roof.”
Green roof systems can appeal to facility executives for a range of reasons, from sustainability to energy savings. For owners considering installing a green roof on a facility, the first step often is to identify the goals that are specific to their project.
“We’re primarily interested in (a green roof) because of what it does at the urban level — reducing the heat island effect, muffling noise, encouraging pollinators,” says Jerry Caldari, a partner with Bromley Caldari, an architecture firm. “Those are the main reasons we like to do it. I also think they look great. They look fantastic.”
The type of facility in question also plays a role in an owner’s decision.
"There's a number of reasons why it's a smart choice to use a green roof, especially if you have like a like a single-story, large footprint building,” Caldari says. “The main thing being that a green roof will extend the life of the roof membrane because it protects it from ultraviolet rays.
“Additionally, a green roof will prevent a storm surge. If it's pouring rain, it'll retain the water after that surge happens and then drain the water off slowly. It helps the municipality, and it helps against the urban heat island effect. Urban buildings radiate heat in the summer, and having a green roof on top mitigates that.”
But for all the potential benefits of green roofs, not all buildings are candidates for hosting them.
"I get customers who call me all the time or potential customers who say they’re thinking about a green roof,” Juhlmann says. “Are they a good candidate? What are the benefits? What are the costs? If you're going to put a green roof on your building, you have to understand you're adding weight.
“The first thing you have to do is go through a structural analysis to see if you even can add the required weight. There are intensive systems that usually weigh less than 20 pounds per square foot. It’s very hard to get much below 10 (pounds per square foot) because you've got to consider what it's going to be when it’s wet. Intensive systems can go up from there.”
A related consideration is whether to install the green roof on top of the existing roof membrane and if so, determining the condition of that membrane.
“Are we going to have to replace the roof system that you currently have because your roof is currently at 15 years?” he asks. “Are we going to have to replace the entire roof system at the same time? That's absolutely key. It's a big mistake to just put a tray system or soil on top of your roof if it's an old roof and not in great shape.”
Owners next must decide on the type of green roof that is most appropriate for their facility. Extensive green roofs have shallow soil depths — 2 to 6 inches — and support low-maintenance vegetation. They also help manage stormwater and provide insulation, and often are the choice for light commercial and residential applications. Intensive green roofs feature deeper soil layers — 6 inches or more. They offer benefits related to aesthetics, air quality, noise reduction, and occupant access and use, and they are commonly found on commercial and institutional buildings.
System selection “depends upon what the goal of the of the installation is,” Caldari says. “For example, if you're simply trying to have a green roof to extend the life of the membrane and reduce your heating and cooling costs, then you would do an extensive system. They generally cost less, and they're a lower threshold for maintenance. Intensive roofs, which are thicker and deeper, we associate those with amenities."
Dan Hounsell is senior editor for the facilities market. He has more than 30 years of experience writing about facilities maintenance, engineering and management.
Ensuring Green Roofs Deliver
