Green roofs, eco-roofs, nature roofs, or roof greening systems: Good for the environment and economy

It doesn't matter what you call them, green roofs, eco-roofs, nature roofs, or roof greening systems are on trend to make a big difference in modern cities. They are breathing, live vegetative roofing solutions that reconcile the natural environment and the built landscape. Not only are green roofs aesthetically pleasing building toppers, they also offer substantial economic and ecological benefits.

The concept of vegetation-based roofing is an old one. The people of Iceland and northern Scandinavia used sod to cover their roofs as early as the 17^th century. The use of sod sprung from scarcity, the harsh northern landscape providing stone and turf with minimal wood resources. Occasionally sod was used in alternate layers with stone to form the walls of dwellings and communal buildings like churches.

Green roofs as we know them were invented in 1960s Germany. Today that country is considered a leader in green roof technology, innovation and usage, with an estimated 12% of flat roof space ‘greened’. The German green roofing industry is growing 10-15% per year, with projections for increased growth in the near future.

Green roofs fall into either the ‘intensive’ or ‘extensive’ system categories depending on the plant materials utilized and the intended use of the roof space. Intensive green roofs are park-like spaces generally limited to flat roofs and often accessible to the public. This roof uses a wide variety of plants but is characterized by its use of trees, bushes and shrubs that require a deeper layer of soil, typically more than 10 cm (4in). Intense maintenance is necessary and important. An example of an intensive roof can be found atop the Coast Plaza Hotel in Vancouver BC.

Extensive roofs, on the other hand, require limited maintenance and are often not accessible to the public. These systems utilize moss, grasses, succulents and herbs, plants that require relatively shallow soil reserves. An example of an extensive roof can be found atop the Vancouver Convention Centre.

Urban development has greatly encroached on forests and agricultural land, creating vast amounts of impassible space. Reclaiming this space is become increasingly important for both our physical and emotional well being. Green roofs provide us with one answer to this problem. These systems have many ecological and economic benefits including energy conservation, increased longevity of roofing materials, diminished urban heat island effect and storm water management.

In terms of energy conservation, green roofs provide a thermal layer which blocks the flow of heat into a building in summer and insulates in winter. This reduces energy bills year round.

Green roofs also have a longer life span than conventional roofing. This is due to protection from extreme weather exposure and ultraviolet light that cause traditional roofing materials to deteriorate.

The urban heat island effect causes metropolitan areas to be noticeably warmer than their surrounding rural zones. This phenomenon occurs in summer and winter. The main cause of this occurrence is the alteration of the landscape by urban development and its use of heat retaining materials. Waste heat created by energy consumption is a secondary source. Conventional roofing materials such as metal and wood reflect and absorb heat, trapping it in urban centers. Green roofs absorb solar heat in order to evaporate moisture from plants. Solar heat is also used as part of the photosynthesis process. The air above a green roof is substantially cooler than the air above a conventional roof, and can be pumped into the building as air conditioning in summer months.

Heat isn’t the only urban problem green roofs help assuage. Storm water runoff from roofs can increase erosion and contribute to flooding. It also increases the amount of water needing treatment before it is ready for human consumption. One of the major benefits of green roofs is that they absorb 60-100% of storm water they receive, absorbing and releasing it over several hours.

The initial cost of outfitting your home with a green roof is substantially more than conventional roofing. Prices vary from $14-$25 per square foot, making green roofs about 5-10 times more expensive. These costs may be offset in part by energy savings, but are not likely to be offset completely. Unless you’re rich, green roofs are not yet economically feasible for the average home owner.

It’s a much different story, however, for corporations and industrial building owners. Economies of scale make roofing an office tower or factory quite reasonable, potentially reducing the price to $4 per square foot.

As with wood framed structures, green roofs slightly elevate the risk of fire. To minimize this risk, non-flammable plants are preferable (such as succulents). Moss and grass can act as kindling in dry situations. For safety reasons, green roofs should have a vegetation free parameter of between 30 and 61 cm (12-24in). This will act as a fire break and allow for easy and safe access to the roof in the event of fire.