“Unless there’s a density of green rooftops, say one every kilometer, then there’s not going to be much effect [in Shanghai],” said Dusty Gedge, president of the European Federation of Green Roof Associations.

“Also, rooftop gardens are only going to be part of the solution for cities. If anyone says they’re going to solve all your environmental problems, then they won’t,” he added. The problem is that all of those (considerable) benefits are often limited by the scope of integrated city planning policies.

Nannan Dong, a professor of landscape studies at Tongji University, points out that the new “vertical greening” rules that were approved in 2015 — and which mandate that 50 percent of new roofs get covered in plants — have come in too late. There are far fewer large-scale construction projects on the horizon than in previous years, and private developers are encouraged, but not obligated, to comply.

Dong explained: “For cities that have already been almost developed, you need to be ambitious. China is going in the right direction, but in many cities we have already urbanized. These policies needed to be in place 20 years ago.”

The challenges facing Shanghai can be applied across the world where politicians face regular criticism for not going far enough with environmental regulations, for fear of scaring off private developers with new building costs. Green roofs cost, on average, twice as much as a traditional roof—but they do last approximately twice as long, as they protect the surface beneath.

“They had the same problem in Germany,” noted Gedge. “They found out the hard way that you need to set the initial policy targets very high, because it’s very difficult to get it changed in ten years’ time when everyone’s got used to the idea.”

Across the world, from Los Angeles and Toronto, to Zurich and Copenhagen, major cities have been announcing the rollout of green-roof initiatives. By and large, European cities have legislated mandatory requirements, while U.S. city governments have chosen to incentivize developers via tax breaks. Most urban planning departments base their codes of practice on comprehensive German guidelines, which were first published in 1982.

However, despite such a clear template, not every city is in the position to build roof gardens safely.

The main engineering issue is weight. If a large amount of soil and plants are added on top of a structure that’s not designed to support that much weight, the results can be catastrophic. In 2013, 54 people died in Latvia when a supermarket collapsed under the weight of the topsoil being added to its roof—it constituted the country’s worst peacetime loss of life in 60 years.

In countries with well-enforced building regulatory systems, and cooler climates, such problems pose less of a risk — buildings that can withstand heavy snowfall can usually also bear the weight of a garden. But in fast-developing countries with warmer climates, there’s more danger.

“Accidents are very possible,” said Dong. “In China, developers are expected to do a risk assessment, but it’s very easy for people to cut corners and assume, ‘Oh, it’s a new building, it’ll be fine to hold a garden.’”

In order to retrofit buildings more quickly, and with a lower cost, architecture firms are experimenting with more nimble ways to build green roofs. Dutch architect Neville Mars has developed a low-cost roof mat, planted with seeds, that can be unrolled onto corrugated tin rooftops often found in city slums. These mats absorb much of the sun’s heat, making building interiors cooler, and transform into green coverage when it rains.