Intensive Green Roofs: A Primer

An Intensive Green Roof (Click for high res version)

If you want the ultimate in a green roof then consider an intensive green roof. Not only can you enjoy the overall benefits of a green roof, but you can also use it as an additional living space. The uses for intensive green roofs are only limited by your imagination, and many people are finding they can create new, natural places of solace, places for interaction with nature, places to garden, and places to socialize and work.

You could have a vegetable and herb garden, or an expanse of wildflowers. Include a bench with some water accents like ponds and fountains. Even throw in a tree or two connected by stone walkways and bordered by low stone walls. These roof top oases feature lawns, annual and perennial plants, shrubs, trees and food gardens. Gazebos, seating, dining areas and greenhouses are popular additions.

Intensive green roofs match the meaning of the word-they are intensive in design, construction and price. They are not for every roof and structural considerations top the list when thinking about installing one. Intensive green roofs are being increasingly found on office buildings, hotels and apartment complexes and are even showing up on high-end single family homes.

With soil depths of 8 inches to 4 feet or more, intensive green roofs accommodate most any type of plant. They usually sit on flat roofs or roofs with very shallow slopes that are made of concrete. Since the planting medium, the plants and the necessary components can weigh as much as 150 pounds per square foot, the building often has to be designed for this type of green roof. But, for the right type of building it is also very possible to retrofit an existing roof that has a concrete deck, or adequate roof reinforcement, with an intensive roof. For roofs with concrete decks sometimes the only change needed is to upsize the concrete as was done at the Waterfront Hotel in Vancouver, British Columbia. There, the owners added a 2100 square foot herb garden in 1994 to replace a gravel path and ivy landscape that had been installed during the original construction in 1991.

Each intensive roof has unique components depending on the structure, climate and design. For the portions where plants will grow there is a typical cross section that begins with the structural support, or roof deck. On top of that you install a roofing membrane topped by a root barrier to prevent the roots from growing into the membrane. Next insulation goes down and is covered with the hydrologic control layer. It’s here that the excess water is drained away, some water is stored for plant uptake, and aeration is provided. This is often just a two-part assembly that includes a dimpled mat covered with a root barrier fabric. Next, load on the soil, or growing medium, to the appropriate depth for the plantings you will include, and start planting. The final step would be to install some type of irrigation system for the times when there is not enough rainfall to support the plants.

Intensive green roofs are definitely not for the faint of heart or for buildings that don’t have robust roofs, but where appropriate, they are adding brand new spaces where people can reconnect with the natural world.

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