Green Roofs are on the Rise

With benefits that include better runoff management and energy efficiency, green roofs are catching on across the globe.

In 2015, the French government passed a law mandating that all new commercial buildings must feature green roofs partially covered with either plants or solar panels.

The new law has drawn considerable attention since its passage, but France is hardly alone in its promotion of such roofs. Countries and municipalities around the globe have, for years now, been driving the adoption of green roofs via various mixes of legislation, subsidies and other incentives.

In fact, to find the world’s foremost user of green roofs, one should look not to France, but right next door—to Germany, says Brad Rowe, a professor in the Department of Horticulture at Michigan State University and head of the school’s Green Roof Research team.

“Germany is by far the leader,” Rowe says, noting that this is largely due to how the country typically structures its storm water service fees.

Dealing with runoff from rain storms is a major challenge, particularly for large municipalities with significant amounts of impervious surfaces such as paved streets and traditional roofs. Indeed, says Rowe, managing storm water runoff is the primary rationale for installing green roofs.

The way a locality charges for storm water services can be a main driver in property owners’ decisions to install green roofs. In many parts of Germany, Rowe says, storm water fees are assessed based on the amount of impervious surface a property contains, meaning that a building owner directly captures the economic benefit of installing a green roof.

On the other hand, in countries like the United States, where storm water fees are often incorporated into broader utility fees, a building owner’s decision to install a green roof is more of a general community benefit, less directly impacting that specific owner’s bottom line, according to Rowe.

Which is not to say that all U.S. localities have chosen to structure their systems this way. According to a 2014 report from law firm Arnold & Porter, more than 1,400 U.S. municipalities charge storm water fees, with most of them offering credits to building owners who install features including green roofs to help manage runoff.

And then there is the recent French approach of legally mandating green roofs, a tactic also used in places like Toronto, which in 2009 became the first city in North America to pass a law requiring green roofs as part of new development.

Whether it makes financial sense for owners or builders to include green roofs depends on the exact mix of laws and incentives governing their locality. In general, Rowe says, such roofs will be more expensive and time-consuming to install than standard roofs. In terms of retrofitting existing buildings, he notes that a key consideration is whether or not the structure is strong enough to hold the additional weight of a green roof.

“If you were going to put one on an existing building, you would definitely have to look at how much weight it would hold,” he says. “If it’s not strong enough to hold [a green roof], it’s probably not worth it to reinforce the building.”

That said, such roofs can provide a variety of benefits. In addition to helping manage runoff, they can produce energy savings and mitigate air and noise pollution, Rowe says.

In cities like New York, they’ve even become the setting for urban farms. “I think that for very local production on a small scale [such rooftop farms] are very feasible,” Rowe says, though, there are limits.

“You aren’t going to see grain combines up there,” he says.