It is difficult enough to plant a beauti­ful garden in a favorable location with good soil. The difficulties escalate, obvi­ously, in less suitable sites such as a rooftop.

The Hanging Gardens of Babylon⤒🔗

So it is not for nothing that the an­cient Hanging Gardens of Babylon are considered one of the Seven Wonders of the World. Ancient historians attribute these wonderful roof top gardens to Mesopotamia's famous King Nebuchad­nezzar (604-562 BC) who wanted to please his Persian wife Amytis. She apparently longed for the forested hills of her home­land. Her husband created elevated terraces "thickly planted with trees of every kind that, by their size or any other charm, could give pleasure to the beholder" (Diodorus Siculus, first century BC, cited in Clayton and Price).

According to various ancient sources, the extreme weight of these gardens was supported by twenty walls each more than twenty feet thick, with lengthy slabs of stone spanning ten to twenty foot wide cor­ridors between the walls. Arches allowed access to these corridors, but who would want to walk below with such a heavy roof? Over the roof stones, reeds impregnated with tar were laid down. Over that went two layers of bricks laid in cement and a layer of lead to prevent downward pene­tration of water or roots. On top of this ex­tremely heavy surface the laborers piled sufficient soil to support huge trees.

This horticultural marvel is said to have been located beside the Euphrates River. A mechanical system pumped water to the highest points in the garden, level with the top of the city walls. From there the water ran down, amply supplying the whole garden with moisture (see Peter Clayton and Martin Price. eds. 1988. The Seven Wonders of the Ancient World. Dorset Press, New York). Undoubtedly the an­cients knew how to build structures which could support massive loads. As modern engineers and architects are discovering, it is no easy feat to create rooftop plantings. We moderns, however, have the same ob­jective as the ancients — to create a pleas­ing environment in which to live.

Here and now←⤒🔗

City planners everywhere are familiar with the positive benefits of "green space," that is areas with actively growing plants, especially trees. The cooling effect of vege­tation comes largely from the process of evapotranspiration. Plants draw water from the soil and evaporate it from their leaf surfaces. The larger the total surface area of leaves in an area, the greater the cooling effect. These benefits come from the spe­cial properties of water. This liquid absorbs an astonishing amount of heat before evap­oration takes place. The absorbed heat is thus not available to heat the air.

One of the tragedies of modern cities is that expanding populations mean more buildings, more roads and parking lots and thus much less green space. Not only are roofs and pavement not green and cooling in their effects, but on the con­trary, they heat up dramatically in sunlight thus contributing to terrible summertime urban heat islands (areas of warmer city temperatures compared to the surround­ing rural areas).

There also is another beneficial fea­ture of green space which roofs and roads do not possess. Plants absorb and store large quantities of rainwater, whereas wa­ter striking roofs and pavement has nowhere to run except the storm sewers. In many urban areas flooding results when rapid runoff exceeds storm sewer capacity.

Unfortunately growing populations need homes and transportation routes. Thus cities seem trapped in ever worsening situations. Not surprisingly Europe, which is much more densely populated than North America, has led the way with solu­tions to these problems. They realize that if roofs are occupying precious space formerly devoted to natural plant communities, then the solution might be to cover the roofs with growing plants. Of course it is one thing to have a good idea and quite an­other to turn it into reality. Saturated soil is extremely heavy and few buildings are de­signed for that kind of load on top. More­over few plants grow successfully in such exposed and extreme conditions as a roof.

Details, details←⤒🔗

It was in Germany in the 1980s that green roof technology was first applied as a major solution to problems of the urban environment. During those early years there the industry saw an annual growth rate of 15-20% as a result of legislation and government incentives. Apparently the city of Stuttgart now requires green roofs on all flat-topped buildings. At Flori­ade in 2002 near Amsterdam, a section of the exposition devoted to lifestyles in­cluded several buildings with demonstra­tion green roofs and also vines covering some building walls. Canada and the United States, on the other hand, are at least ten years behind Europe.

As currently practiced, green roofs in­volve several layers. On top of the roof in­sulation or other regular surface, a waterproof membrane with root repellent properties is laid down. On top of that goes a large porous layer through which water can move to drainage pipes. Above that, a specialised sheet is laid which allows water to percolate through to the porous layer. It too has chemicals added which prevent roots from penetrating the sheet and clog­ging the drainage layer below. Above the sheet a growth medium is placed. Normally we would use soil, but saturated soil is ex­tremely heavy. Thus the growth medium of­ten consists of mixtures which include sand or gravel, crushed brick, pumice, peat and organic matter or other nutrients. Into the growth medium the plants are inserted.

There are two basic types of green roof: those called extensive or eco-roofs, and those called intensive or roof gardens. Everything concerning eco-roofs is low. They have low weight, low capital cost, low plant diversity, and minimal mainte­nance. These may perhaps appear ugly to some people when growing conditions are less than ideal as in the winter. For these extensive plantings, the growth medium typically is from 5-15 cm (2-6 inches) deep. The increased weight is 73-170 kilograms per square metre or 16-35 pounds per square foot when fully saturated. Suitable plants include succulent Sedums or hardy grasses. After the first year only weeding once or twice per year is required at the same time as membrane inspections. Dur­ing the rest of the time the roof is viewed from a distance.

Intensive roof gardens represent the opposite of eco-roofs. First, since the growth medium is much deeper at 20-60 cm (8-24 inches), the roof must be able to hold up a saturated weight of 290-965 kilograms per square metre or 60-200 pounds per square foot. As far as selection of plant material is concerned, the sky is literally the limit as even trees are possible and quite common. These gardens are typically decorative, de­signed for people to visit, right up on the roof. That means all access and safety regu­lations must be followed. Since there is so much plant material involved, regular wa­tering is required, usually from an in-built irrigation system. It will come as no surprise in view of all these requirements, that pro­fessional expertise is essential at all stages of development. Every roof garden is different; there is no one size fits all. The exact design depends upon building shape, location and desired plant material. Typically a devel­oper will need to consult a structural engi­neer, an architect, a landscape architect, a mechanical engineer, a horticulturist and perhaps even an ecologist if the plan in­volves natural vegetation. Obviously none of this expertise comes cheaply, however in the long run the benefits often outweigh the initial investment cost.

True stories←⤒🔗

The stories of some North American buildings demonstrate the variety of appli­cations of green roofs in a market that is, as yet, scarcely aware of the phenomenon. Such is the story of Chicago City Hall. A 1999 study based on computer models sug­gested that if every building in Chicago were covered by a green roof the annual electrical energy savings for air condition­ing would amount to $100 million. Im­pressed by the thought of such substantial benefits, city council passed its first Energy Conservation Ordinance on June 3, 2001. All new or replaced roofs would now have to meet minimum standards such that they reflect sunlight rather than absorbing it and heating up. The best response would be to install a green roof. In this situation the city recognized the need to lead by example. In order to show that even old buildings can be adapted to new solutions, the city hired a team of professionals to design a suitable new topping for their landmark building, the hundred year old, flat topped City Hall.

This green roof was not designed to be visited by the public, but to be viewed from the 33 taller buildings in the immediate vicinity. Since it had to be attractive from afar and from above, a sunburst layout was chosen. Various sectors of the pattern were provided with lightweight growth medium at depths of 4, 6 or 18 inches. The selection of plants included native prairie and woodland grasses and forbs, hardy or­namental perennials and grasses, several species of native and ornamental shrubs and even two varieties of trees. Plants were situated in the design according to time of blooming and color so that a wave of color moved across the pattern as the season progressed. Differences in plant success in sectors planted to different depths should be obvious. Completed in 2001, monitoring for temperature and plant health began in 2002. Early in the program an air tempera­ture difference of 78 degrees F was recorded between a nearby traditional black tar topped roof and the new green roof.

The claim to fame of Toronto City Hall is the twin curved building towers. But this interesting complex now has an additional feature of interest. On the roof of a central one story building there now ex­ists a remarkable intensive garden that is open to the public. The former roof was scheduled to be replaced this year (2004). A consortium of business and environmental interests persuaded the city to develop this location into a demonstration green roof. The city contributed the amount they would have paid to replace the roof, and government grants and business provided the rest. The result, completed in 2000, is an interesting collection of green roof applica­tions, eight in all. Two are devoted to grow­ing food and two feature ecological themes. Of these, one consists of a patch of rare black oak savannah prairie. These plants were obtained from nearby High Park, long famous for its patch of a threatened natural ecosystem. The other ecological plot fea­tures a bird and butterfly garden with a mixture of native and ornamental plants.

Some buildings keep re-inventing themselves. The Merchandise Building in Toronto is a good example. It was in 1910 that the Robert Simpson Company built a strong but not particularly beautiful struc­ture to be used for wagon storage and the selling of harness equipment for horses. Six years later an eleven story mail order build­ing and warehouse complex was added. However, Simpson's went out of business more than thirty years ago and the building complex sat empty. Finally in 1997 a devel­oper proposed turning the derelict building into residential, retail and commercial space. One of the attractive features of this development is the green roof which was added to the plans in the final phases of the project. Completed in the year 2000, the green roof is located over the twelfth story and faces south. Thirty-seven hardy plant species native to Ontario include grasses, coneflowers, sunflowers and succulent Serums. Originally the developer had intended to place decorative planters around a rooftop dipping pool. To allow for the sub­stantial weight of the pool and sun terrace, the entire roof had been reinforced with steel girders. The planters, however, would have added too much additional load. An extensive green roof by comparison, would be much lighter. Unlike most such shallow plantings, this one is accessible to the pub­lic and has an in-built drip irrigation sys­tem. Thus the green roof allowed the developer to provide more recreational space with other heavier items than would otherwise have been possible.

Mountain Equipment Co-op is a re­tail business dealing with up-scale outdoor recreational equipment. Their corporate headquarters is in downtown Toronto. In 1998, as a demonstration of their environ­mental ethic, they converted a normal flat roof into an inaccessible green roof. The area was planted with a wild flower mix of sunflowers and perennials. This was so suc­cessful in attracting birds and insects that an even more environmentally friendly building elsewhere seemed called for. What better location could there be for such a development than in western Canada?

In 2002 the new Winnipeg Mountain Equipment Co-op was opened. It featured many environmentally friendly devices and soon was recognized for its innovative de­sign. An intensive roof garden grows on the roof. Water runoff from the roof and gray water (from washing) are stored in large vats in the cellar. A small 150-watt photo­voltaic panel on the roof powers a pump which draws water to the garden for irri­gation. The building also features two corn-posting toilets (all that the building code would allow). The nutrients from these toilets are added to the growth medium on the roof as nutritional supplements for the plants. Although the building features tall banks of windows, it is said that the cooling effect of the green roof precludes the need for air conditioning.

As more demonstration green roofs appear in North America, the public will be­come more familiar with the concept and the benefits. Besides cooling and water re­tention, green roofs may insulate a building for sound with a 40-decibel reduction pro­vided by 12 cm (five inches) of growth medium on the roof. As with all green plants, those on the roof absorb carbon dioxide and release oxygen to the air. In addition they trap dust particles on their leaves and these are later washed into the growth medium. These facilities attract lo­cal wildlife as well, particularly insects and birds. Not everyone however will be im­pressed with this benefit.

North Americans often act as if they know everything. Here however is a tech­nology which the rest of the world can teach Canada and the United States. The end result, it is to be hoped, will be cooler, moister, more greenly attractive cities.