Room at the Top: Grow a Green Roof

Green roofs are all the rage in Europe, and they're catching on in progressive cities such as Chicago and Portland, Oregon. Should you plant a garden atop your house?

| March/April 2005

  • Small-scale roof greening on a porch.
    Photo by Nigel Dunnett and Noël Kingsbury
  • The green roofs on this house provide additional wildlife value to complement the meadow plantings at ground level.
    Photo by Nigel Dunnett and Noël Kingsbury
  • Green roofs can be installed on sloping roofs, although they will be subject to greater moisture stress than on flat or gently sloping roofs.
  • The green roofs on this house provide additional wildlife value to complement the meadow plantings at ground level.

Green roofs are one of the most innovative and quickly developing fields in the worlds of both horticulture and the built environment. A green roof is a green space created by adding layers of growing medium and plants on top of a 100 percent waterproof roofing system. This isn’t the same as a tra­ditional roof garden in which planting is done in freestanding containers located on an accessible roof terrace or deck. All green roofs are composed of at least two layers: the vegetation itself and the media or substrate within which the plants grow.

Integrating living plants with the built environment has many advantages. Greenery improves the visual and aesthetic aspects of urban areas, but more fundamentally it’s widely recognized as therapeutic—with a number of research studies illustrating this. Plants also provide habitat for urban wildlife. A great many animal species are happy to live in urban areas if they can find places to live.

All plants help ameliorate the effects of pollution by absorbing noise, trapping dust, recycling carbon dioxide, and absorbing and breaking down many gases. Plants help reduce the negative climactic effects of urbanization, for example, by absorbing some of the heat generated in city environments and absorbing the rainfall that runs off hard surfaces. The greatest advantage of planting on roofs and walls, however, is that it’s a way of getting greenery into places where conventional tree and shrub planting isn’t feasible.

Why build green roofs?

Amenity Value: Roofs are an enormously underutilized resource in urban and suburban areas. If a roof’s load-bearing capacity is sufficient, green roofs can provide recreational areas in neighborhoods where there’s little ground-level green space.

Food Production: Roof surfaces offer an opportunity for growing healthy food, particularly in high-density urban areas or where garden space may be small or restricted.



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