The Hanging Gardens of Nineveh, which modern scholars believe to be the true location of one of the Seven Wonders of the Ancient World, were a work of unparalleled beauty in their time. The gardens, built in the seventh century B.C.E. by the Assyrian King Sennacherib, featured a complex irrigation system supporting plants growing out of stone arches and trees that seemed to hang in the air. Sennacherib called his property in Nineveh “a palace without a rival.”

That palace had one of the earliest documented green roofs, a manmade rooftop partially or completely covered in vegetation. These days, green roofs that rival the beauty of the one at Nineveh’s palace are regularly cropping up in cities across America, thanks to innovations in the industry that make green roofs sustainable economically and environmentally.

While civilizations throughout history have had their own versions of green roofs, the modern version began taking shape in Europe in the 1960s and ’70s. Studies on a range of topics—including root repelling agents, waterproof membranes, drainage and lightweight growing materials—helped to make green roofs more efficient and sustainable.

There are many environmental benefits to installing green roofs. Depending on a client’s goals, these rooftops can be customized to maximize rainwater retention, improve air quality or support biodiversity.

Green roof technology can significantly reduce stormwater runoff.

Most major cities have combined sewer systems where stormwater and sewage water flow through the same pipes. These systems can be overburdened during severe storms. By absorbing water into the vegetation and capturing runoff in roof membranes, green roof systems can help ease that burden. A green roof will hold rainwater until the roof’s membrane is saturated to capacity.

While the initial cost of installing green roofs is more expensive than traditional roofs, they’ll offer your clients energy savings in the long run. The Department of Energy projects that green roofs can reduce heating and air conditioning costs by roughly ten percent in natural gas consumption and effect a two percent drop in electricity demand for a typical building.

Green roofs also last two to three times longer than standard roofs, because they are protected from ultraviolet radiation and extreme fluctuations in temperature that otherwise cause roof membranes to deteriorate. The water absorption and waterproofing that green roofs provide also prolong the life of a rooftop.

While helping to keep the inside of buildings cooler in the summer and warmer in the winter, green roofs also reduce the ‘urban heat island effect’ of cities. Heat islands occur when manmade structures cause the temperature in cities to be significantly higher than surrounding rural areas. Traditional black tar roofs transfer heat from the sun, creating a warmer microclimate in the surrounding area.

The Environmental Protection Agency warns that heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution, greenhouse gas emissions, heat-related illness and mortality. Green installations can lower the ambient temperature on a roof by 10 to 12 degrees.

These roofs can also specifically be designed to offer habitats for rare or endangered species. Howard Freilich, owner of Blondie’s Treehouse in New York, New York, noted that green roofs have helped fight the decline in the bee population. “By having these different types of roof gardens, you’re now creating a habitat for bees and butterflies that migrate all over the country to have a source of food from one spot to the next spot,” Freilich said.

Green roofs fall into one of three classifications: extensive, semi-intensive and intensive. While individual companies differ on the exact definition of the three types, generally, extensive green-roof systems have three to six inches of depth, semi-intensive have between seven to 12 inches of depth and intensive green roofs are anything above 12 inches.

The extensive systems embody the more traditional understanding of a green roof, where the rooftop is covered in a single layer of plant growth. While this system greatly limits the options for plant species, it has lower maintenance, nutrient and irrigation requirements. Extensive green roofs are preferable from a sustainability perspective, since they are lightweight and can be installed on more rooftops. Greg Raymond, the North American business development manager for Sopranature in Chicago, Illinois, who has more than 30 years of experience in landscaping and the green industry, likes to call extensive systems the “workhorse of vegetative roof assemblies.”

For extensive system installations, prefabricated green roof vegetation can typically be purchased in interlocking modular two-foot by two-foot squares or as larger ‘vegetation blankets’ that can be rolled out across the roof for easy installation. These prefabricated modular units have a vegetation layer on top, followed by a substrate layer to retain water and anchor the vegetation, a drainage layer that evacuates excess water, a root barrier, a membrane protection layer, a waterproof membrane and a layer of insulation that sits directly on top of the existing roof.

Semi-intensive and intensive green roof systems are commonly referred to as ‘rooftop gardens.’ They have soil beds that are six inches or more, which allows for a variety of options like planters and raised planting beds. These are aimed at clients looking for custom work that offers aesthetic beauty in addition to the environmental benefits. These systems require more complex irrigation, fertilization and additional maintenance. They also put a heavier weight load on the roof.

With the risk of collapse, calculating the weight restrictions of a rooftop is crucial. Both Freilich and Raymond try to limit the total weight of an intensive green roof system to under 30 pounds per square foot when totally saturated. They also have to factor in additional burdens on the roof like mechanical units, snow and people.

“One of the important things is to keep the weight of the soil down,” Freilich said. Blondie’s does calculations on a building’s roof capacity to determine how much weight it can handle, then designs the system from there. They also use custom-made soil that avoids heavy clay, instead using more porous materials like peat moss and lava rocks.

One of the major benefits to intensive gardens is how endlessly customizable they are. In addition to the ecological benefits, they can provide clients with calming, beautiful spaces their employees can enjoy. Raymond noted that he’s seen an increase in rooftop garden installations in the healthcare industry. Many hospitals are installing rooftop gardens that their patients can see. “Obviously— there’s no secret—I think people feel better and probably heal better and faster if they’re able to view nature,” Raymond said.

One of the challenges of installing green roofs is getting the materials up to the roof. Freilich recalls a recent project where his team had an installation on the 35th floor of an office building. Around 150 cubic yards of soil—which is the equivalent of around 3,000 bags—had to be put on carts and wheeled into the building. Those types of installations are always tough on his staff. “It takes forever and it costs a fortune,” Freilich said.

If at all possible, companies will bring the supplies up to rooftops using a crane. The green roof tray systems can be put onto pallets, shrink-wrapped and craned up to the top of a building, which makes things much easier. Depending on the location, forklifts can sometimes be used. But if all else fails, crews have to carry the supplies up manually.

“We’ve conveyed material up to roofs as efficiently as with a crane and also as archaically as walking it up ten flights of stairs,” said Raymond.

While it can be a challenge to stage and install green roofs, the results speak for themselves. The natural beauty and the wide array of environmental benefits green roofs offer should make them an appealing option for your clients.