Green roof for Chicago City Hall

The flat roof of the historic city hall in downtown Chicago was transformed into a green roof garden.

the 11 storey town hall. The walls below it are over 100 years old, and familiar to European film fans at least as the notorious Blues Brothers paid their debts there at the last minute.

What has emerged on top of the town hall is a lightly contoured land­scape, planted on a shallow substrate with varieties of sedum and on a deeper one with trees and shrubs. It is possible to walk around the city hall roof on a curving path. Parts of the roof were removed for statical reasons before the planting, and provision of water for the roof plants was dealt with as part of this process. Rainwater from the penthouse (for technical units) which is built direct­ly on to city hall, and higher, is stored in several small tanks and taken to the plants when needed. If water is short, the municipal supply can be used.

City hall’s green roof has attracted a great deal of attention, and as part of the environment authority’s pilot project it has interested specialists, even out­side Chicago. In the city itself it is part of the extensive ‘City Roof Garden Pro­gram’, and the first example of the fact that planting on roofs is worthwhile, even from the point of view of economi­cal and sustainable water management.

Many people are familiar with the riveting view from the Empire State Building in Manhattan. Skyscrapers as far as the eye can see, the Hudson River blurs into the horizon, a freedom-loving, copper-clad lady 46 metres high be­comes tiny. But at some point the far distance becomes boring and the eye finds its way back and looks down at apparently trivial details: yellow taxis, for example, the winding paths taken by individuals, clouds of pigeons suddenly flying up, the upward and downward staggered patchwork quilt of the flat roofs. Flat roofs are very popular in big American cities. Unlike Central Europe, the airy space in these little rectangles is used for air-conditioning units and fresh water tanks, sometimes for junk. You would look pretty much in vain for a roof garden here. The USA is still a developing country when it comes to roof planting.

Fewer people know the view from the Sears Tower in Chicago, one of the high­est office buildings in the world, but if you look down from there it looks quite like the Manhattan roofscape. But this could change very soon, as this great city on Lake Michigan is one of the five major American cities taking part in the environment authority’s ‘Urban Heat Island Initiative’ pilot project. This is the United States’ attempt to reduce temperatures in the summer months, which are sometimes very high, and the smog levels in several cities. Roof plan­ting is one of the key elements of this programme, which is also intended to relieve the overloaded sewers when the rainfall is heavy.

There are very few roof-planting experts in the USA because of lack of experience. For this reason the Dreiseitl studio was invited to join an American planning team in 1999, and commis­sioned to produce a design for a roof area of about 3,600 square metres. And so now people look down from the top of the nearby skyscrapers on to the roof of

The garden is visible from the surrounding skyscrapers and has thus become a visual highlight.

Improving the city climate, greening of roof surfaces also significantly reduces rainwater run-off through detention, storage and evaporation of the rain­water.

A natural wildflower meadow right in the middle of Chicago has an aesthetic appeal which is rare in big cities.