Full Reading Test

ALook down on almost any modern city from a tall building and you will see a landscape of flat roofs covered in bitumen, gravel and metal. These surfaces absorb sunlight all day, and on a hot afternoon a dark roof can become far hotter than the air above it, releasing that heat slowly into the evening. Multiplied across thousands of buildings, this is one of the reasons why city centres are often several degrees warmer than the surrounding countryside, a pattern known as the urban heat island effect. For a growing number of architects and city planners, part of the answer is to cover these roofs with plants.

BThe idea itself is ancient. Turf roofs have been used for centuries in Scandinavia, where a thick layer of soil and grass helped to keep wooden houses warm through long winters. The modern green roof, however, is a much more engineered product. Its development owes most to Germany, where researchers and manufacturers in the 1960s and 1970s worked out how to build lightweight, layered systems that could be laid on ordinary flat roofs. A typical system today has a waterproof membrane at the bottom, then a barrier to stop roots from penetrating it, a drainage layer, a filter fabric and finally a specially designed growing medium in which the plants are rooted.

CGreen roofs are usually divided into two broad types. Extensive roofs have a thin growing layer, often less than 15 centimetres deep, and are planted with hardy, low-growing species such as sedums, which store water in their fleshy leaves and can survive long dry periods. They are light, need little maintenance and are rarely intended for people to walk on. Intensive roofs, by contrast, have much deeper soil and can support lawns, shrubs and even small trees. They are, in effect, roof gardens, and they require regular watering, feeding and pruning. Because a deep layer of wet soil is extremely heavy, intensive roofs usually have to be planned when a building is designed, whereas an extensive roof can often be added to an existing building without major structural work.

DThe benefits most often claimed for green roofs concern water. When heavy rain falls on a conventional roof, almost all of it runs straight off into gutters and drains. In older cities, where rainwater and sewage share the same pipes, a sudden downpour can overwhelm the system and send untreated waste into rivers. A green roof acts like a sponge: the growing medium and plants hold back a large share of the rain, then release it gradually or return it to the air through evaporation. The exact amount retained varies with the depth of the roof, the season and the intensity of the storm, but even shallow roofs noticeably reduce and delay the peak flow reaching the drains. In some cities, planners now treat green roofs as one part of a wider approach that also includes permeable paving and rain gardens, all intended to slow the movement of water through the urban landscape.

EThere are other advantages. The layer of soil and vegetation insulates the building beneath, reducing the energy needed for cooling in summer, and the membrane underneath is shielded from ultraviolet light and extreme temperature changes, so it tends to last considerably longer than an exposed one. Green roofs can also provide habitat for insects and birds, although ecologists point out that a roof planted with a single type of sedum offers far less to wildlife than one with a varied mix of native plants.

FDespite these advantages, green roofs remain the exception rather than the rule. The main obstacle is cost: a green roof is more expensive to install than a conventional one, and while the owner of the building may save on energy and roof repairs, many of the benefits, such as cooler streets and less pressure on drains, are shared by the whole city. For this reason some local governments have stepped in with regulations or financial incentives. Toronto, for example, became one of the first cities in North America to require green roofs on certain new buildings. Supporters argue that such policies are justified because the public gains so much; critics reply that the same money might achieve more if spent on street trees or larger drainage tanks. What is clear is that the flat roof, once seen as wasted space, is increasingly regarded as a resource.