IELTS Academic Reading Practice Test 5: Passage 1

For most of the history of modern architecture, the flat roof has been treated as wasted space, visited only by maintenance workers. Covering a roof with plants, however, is far from a new idea. In rural Norway, for example, houses were for centuries roofed with turf laid over sheets of birch bark. The bark kept out the rain, while the heavy turf held it in place and helped to keep the rooms below warm. What is new is the scale on which planted roofs are now being built.

The modern green roof largely took shape in Germany, where lightweight planting systems for ordinary buildings were developed in the 1960s and 1970s. Specialists today distinguish between two main types. Intensive roofs have a deep layer of soil and can support lawns, shrubs and even small trees. They are, in effect, rooftop gardens intended for people to use, but they are heavy, need regular care and can only be built on strong structures. Extensive roofs, by contrast, have a growing layer that is often less than ten centimetres deep and are planted with tough, low-growing species. They are not usually meant to be walked on, and once established they need little attention: one or two inspections a year are typically enough. Being lighter and cheaper, they are by far the more common type.

An extensive roof is in fact made up of several layers, each with its own job. The plants most often chosen are sedums, small succulents which store water in their fleshy leaves and can survive weeks without rain. They grow not in ordinary soil, which becomes too heavy when wet, but in a substrate made largely of lightweight mineral materials such as crushed brick, pumice or expanded clay, mixed with only a small amount of compost. Beneath the substrate lies a filter sheet, a thin fabric that lets water through but holds back fine particles which would otherwise be washed downwards and block the next layer. This is the drainage layer, commonly made of moulded plastic panels shaped rather like egg boxes. Their hollows retain a reserve of water that the plants can draw on in dry weather, while any surplus flows away to the roof's outlets. Underneath, a tough root barrier stops roots from forcing their way into the waterproof membrane, which in turn rests on the structural deck of the building.

The benefit most valued by city authorities concerns rainwater. In a city, most rain lands on hard surfaces and flows straight into drains, and during heavy storms the volume can overwhelm sewers and cause flooding. A green roof behaves rather like a sponge. Its layers absorb part of the rain, much of which later returns to the atmosphere through evaporation and through the leaves of the plants, and the water that does run off is released more slowly, reducing the peak flow reaching the drains. The amount held back varies greatly, however. In summer a roof may retain most of the rain that falls on it, but in winter, when the substrate is often already saturated and evaporation is slow, it holds back far less.

Green roofs also help to keep buildings and cities cool. On a sunny afternoon, a dark conventional roof can become far hotter than the surrounding air, and this heat passes both into the rooms below and into the street. A planted roof stays much cooler, partly because the vegetation shades the surface and partly because evaporation uses up energy that would otherwise heat it. In a single building, the result can be a noticeable reduction in the need for air conditioning on the top floor, although the effect on the floors further down is small. Across a whole city, many green roofs could help to reduce the 'urban heat island' effect.

There are other advantages. A waterproof membrane exposed to the weather is gradually damaged by ultraviolet light and by the daily cycle of heating and cooling; covered by plants, it is protected from both and can be expected to last considerably longer. Green roofs can also provide a habitat for wildlife, especially insects. In London, some 'brown roofs' have been covered with rubble from the building site and left to be colonised by whatever plants arrive naturally. The aim is to recreate the bare, stony ground favoured by the black redstart, a rare bird that in Britain nests mainly on derelict industrial land.

A growing number of governments now require green roofs. In 2009, Toronto became the first city in North America to adopt a bylaw making them compulsory on many new buildings, and in 2015 France passed a law stating that new commercial buildings must have either plants or solar panels on part of their roofs. Green roofs are not, however, suitable everywhere. They cost more to install than conventional roofs, and a substrate saturated with water is heavy, so older buildings may need to be strengthened before one can be added. Nor are extensive roofs free of maintenance: weeds and tree seedlings must be removed, and in long dry spells even sedums may need watering.