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?
Small-scale roof greening on a porch.
Photo by Nigel Dunnett and Noël Kingsbury
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.
Aesthetic Value: Even where roofs are inaccessible but clearly visible, attractive planting can be beneficial.
Increased Roof Life: If an appropriate method of construction is used, green roofs will last longer than conventional ones.
Improved Energy Efficiency: The insulating effect of green roofs can save you money because they reduce heating and air conditioning costs.
Biodiversity Booster: Green roofs help create habitat and green-space networks in cities. Vegetated roofs can function as stepping stones for birds and insects, creating links between larger habitat patches.
Water Management: Around 75 percent of rainfall on towns and cities is lost directly as surface runoff. Because rooftops represent 40 to 50 percent of the impermeable surfaces in urban areas, planted roofs play an important role in integrated or sustainable urban drainage systems.
Pollution Reduction: Vegetation in urban areas can cleanse the air of fine particles, which settle onto leaf and stem surfaces as the air passes over the plants. Foliage can also absorb gaseous pollutants, sequestering the material in their tissues.
Noise Reduction: The substrate and plants on living roofs absorb sound.
Fire Protection: There’s some evidence from Europe that green roofs can help slow the spread of a fire to and from the building through the roof, particularly when the growing medium is saturated with water.
Can your home handle a green roof?
Structural Considerations: Retrofitting a green roof onto an existing building means it either must fit the roof’s existing carrying capacity or you must be prepared to upgrade the structure. When working with existing roofs, you may need to strengthen the roof with additional structural components such as columns, beams, and braces. You might also consider systems that attach the green roof to a wall or parapet or to small structures such as a garage or outbuilding.
Irrigation: There should be no need for irrigation except in the most arid climates if green roofs are carefully designed with an appropriate plant mix and substrate and once the plants have been properly established.
Components of a green roof
Weatherproof Membrane: An effective waterproof seal is an essential prerequisite for all green roofs.
Root Protection Barrier: If the membrane on the existing roof contains bitumen, asphalt, or any other organic material, it’s crucial that continuous separation is maintained between the membrane and the plant layer because the membrane will be susceptible to root penetration and the activity of micro- organisms. Root-protection membranes are usually made of PVC rolls laid out over the weatherproofed roof deck or surface.
Drainage Layer: If drainage is inadequate on a flat roof, damage to the roof membrane may ensue because of continuous contact with water or wet soil. Drainage layers include granular materials (gravel, stone chips, broken clay tiles, clinker, lava rock, pumice); porous mats, which absorb water; or lightweight plastic or polystyrene drainage modules.
Growing Medium or Substrate: The ideal substrate should be highly efficient at absorbing and retaining water while at the same time having free-draining properties. It should be able to absorb and supply nutrients, retain its volume over time, and provide anchorage for the plants. This is generally achieved by granular mineral materials that absorb water and create pore space, mixed with fine particles (in relatively small proportion) to which water will cling. Materials used as a basis for substrates include sand; lava; pumice; gravel; perlite; vermiculite; light expanded clay granules; rockwool; and crushed clay, brick, tiles, or concrete.
Plants: First and most important is that you choose plants that will thrive in the conditions expected to prevail at the site. Second, visual appearance may be important. Plant selection for roof greening is very different from choosing plants for a garden, largely because function outweighs aesthetics. Sedums reign supreme in the roof-greening world because they store water in their leaves and are shallow rooted. Look for the following features when selecting botanicals:
• Low mat-forming or cushion-forming growth
• Succulent leaves or other water-storage capacity
• Compact twiggy growth and small evergreen leaves held close to the stem on ground-hugging plants
• Grey or silver foliage
• Species known as geophytes that die down to bulbs or tubers during winter or during a hot, dry season (geophytes are often visually striking and can play an important secondary role in roof-greening vegetation)
• A seasonally dry natural habitat with shallow soil
• Shallow rooting
• Evergreen foliage
Adapted with permission from Planting Green Roofs and Living Walls by Nigel Dunnett and Noël Kingsbury (Timber Press, 2004).
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