text: In ancient times green roofs consisted of cave-like structures or sod roofs covered with earth and plants commonly used for agriculture, dwelling, and ceremonial purposes. These early shelters provided protection from the elements, good insulation during the winter months, and a cool location in the summer. Unfortunately for modern conveniences, these were neither waterproof nor was there any system to keep out unwanted burrowing wildlife.
  Modern green roofs, which are made of a system of manufactured layers deliberately placed over roofs to support growing medium and vegetation, are a relatively new phenomenon.  However, green roofs or sod roofs in northern Scandinavia have been around for centuries.  The modern trend started when green roofs were developed in Germany in the 1960s, and has since spread to many countries.  Today, it is estimated that about 10% of all German roofs have been "greened".
  A number of European Countries have very active associations promoting green roofs, including Germany, Switzerland, the Netherlands, Norway, Italy, Austria, Hungary, Sweden, the UK, and Greece. Germany was the first country to start developing green roof systems and market them on a large scale. The City of Linz in Austria has been paying developers to install green roofs since 1983, and in Switzerland it has been a federal law since the late 1990s.  In the UK, their uptake has been slow, but a number of cities have developed policies to encourage their use, notably London and Sheffield.
  Green roofs are also becoming increasingly popular in North America, although they are not as common as in some parts of Europe. Numerous North American cities offer tax incentives to developers who integrate green roofs in their buildings. Toronto and San Francisco legally mandate new buildings to include green roofs
  Rooftop water purification is also being implemented in green roofs.  These forms of green roofs are actually treatment ponds built into the rooftops.  They are built either from a simple substrate (as being done in Dongtan) or with plant-based ponds. Plants used include calamus, Menyanthes trifoliata, Mentha aquatica, etc.)
  Several studies have been carried out in Germany since the 1970s. Berlin is one of the most important centers of green roof research in Germany. Particularly in the last 10 years, much more research has begun. About ten green roof research centers exist in the US and activities exist in about 40 countries. In a recent study on the impacts of green infrastructure, in particular green roofs in the Greater Manchester area, researchers found that adding green roofs can help keep temperatures down, particularly in urban areas: "adding green roofs to all buildings can have a dramatic effect on maximum surface temperatures, keeping temperatures below the 1961–1990 current form case for all time periods and emissions scenarios.  Roof greening makes the biggest difference…where the building proportion is high and the evaporative fraction is low.  Thus, the largest difference was made in the town centers".