More and more buildings nationwide are getting makeovers, as owners install rooftop gardens. Since "green roofs" provide excellent insulation, they can keep room temperatures from rising in the summer.

As a result, electricity consumption can be cut. For this reason, green roofs are encouraged by the Ministry of Land, Infrastructure, Transport and Tourism and the Environment Ministry as effective measures to counter global warming and ease the urban heat island effect.

Most existing green roofs use a thin layer of soil to grow plants to keep the load on the roof to a minimum.

Usually, grass or a species of sedum, a succulent plant, is grown because both are relatively inexpensive. The idea is to cover rooftops with a green insulation sheet. It is true that the weight individual buildings can support varies. In the case of structures with slanted roofs, covering them with a thin layer of green is the only option.

But the drawback is that thin-layer green roofs are visually monotonous. Since they change little with the seasons, they are not very attractive.

There are other problems as well. The soil is so thin - only about 5 cm or so deep - it needs constant care and maintenance. Otherwise, the vegetation could wither in a few years. In short, thin-layered green roofs are vulnerable and tend to be short-lived.

Recently, thanks to advances in landscape technology, it is becoming possible to create full-fledged gardens on top of buildings that are just as pleasing to the eye as conventional gardens located on the ground.

The first step to create such a garden is to install a comprehensive drainage system by covering concrete roofs with waterproof sheets. Then lightweight soil specifically developed for green roofs is spread over the sheets. The depth of the soil is determined by the needs of the individual trees and plants, as well as how much weight the roof can withstand.

Usually, the thickness is between 15 cm and 30 cm. If there is enough space, roofs can accommodate Western-style gardens where visitors can take a leisurely stroll and admire the flowers. It is also possible to build traditional Japanese-style gardens.

If the building has security guards to keep an eye on trespassers, green roofs can function as "safe parks". Some department stores are aiming to attract customers with green roofs.

One of the lesser-known advantages of green roofs is the prevention of floods. When heavy rains hit locally, rooftop soil retains rainwater and thereby lessens the volume of water that flows into drainage canals, preventing them from flooding. The deeper the soil, the more water it can hold.

Based on these features, I recommend building full-scale gardens on rooftops. When central and local governments provide subsidies to applicants, they should give priority to projects that make the layer of soil as deep as possible.

The author is a landscape architect and professor of landscape science at the Tokyo University of Agriculture.

(China Daily July 11, 2008)